HomeMy WebLinkAbout20202029.tiffWeld County
Brenda Dones, es, County Assessor
Jason Marini, Deputy Assessor
Valu • tion Report
of
Residential Improved Property
For
County Board of Equalization
BACHOFER ROSS
Petitioner
vs.
Weld County Assessor's Office
Responden t
Docket Number: 2020-2029
Parcel Number: 130930000048
Schedule Number: R5270886
Appeal Number: 2008226216
Date: 2020-07-27
Time: 2:00 PM
Board: 1
Prepared By
Duane M Robson
Assessor's Office Senior Appraiser
Assessor's Indicated Value
RESIDENTIAL $227,000
TOTAL: $227,000
Page 1 of 11
Subject Photo
General Description of Subject Site and Improvements
The subject property is located at 7525 Highway 85 in Weld County. The legal description of the
property is 16219 PT L6 SW4NE4 30 2 66 Lupton Meadows .
The subject property is a 6.361 acre partially wooded parcel of triangular shape and fairly flat
topography. It is located a half -mile south of Weld County Road 18 and a quarter -mile west of US
Highway 85. The South Platte River borders the northwest portion of the parcel. Per FEMA flood map
# 0812302102E dated 1/20/2016, approximately 2.2 acres are in the floodway (Zone AE) and 2.6
acres are located within the 100 -year floodplain (Zone AE). The single-family residence is also
partially (33%) located in the 100 -year floodplain.
The subject house was constructed in 1994. It has 1,575 square feet of finished living area above
grade. There are 3 bedrooms and 2 bathrooms. The Assessor has classified the structure as Ranch 1
story home of fair quality construction. The 1,575 square foot walkout basement includes 945
finished square feet. There is no garage. There is a 1,360 square foot average quality utility building
built in 2000.
The assessor completed an exterior inspection on November 21, 2019.
Page 2 of 11
Aerial View of Property
Location of Floodway and Floodplain
Page 3 of 11
Market Approach Summary
The subject property has been classified as Residential for assessment purposes. Residential property
value shall be determined solely by consideration of the Market Approach to Value {39-1-103(5)(a),
CRS
}•
As required by 39-1-104(10.2), CRS, the market value is determined by utilizing data from the period of
one and one-half years immediately prior to June 3 0th, 2018. If sufficient comparable valuation data is
not available within the eighteen -month time period, the assessor shall use market data from the five
year period immediately prior to June 3 0th, 2018. When appropriate, all sales are to be time adjusted to
the appraisal date of June 30th, 2018.
Although the appraisal date is June 30, 2018, the physical characteristics are reflective of the property as
of January 1, 2020.
The comparable sales in this report were selected using county records and the Multiple Listing Services.
The Weld County Assessor's Office has verified that the comparable sales are arms -length transactions
based on review of the Real Property Transfer Declaration, telephone or personal confirmation
interviews and physical inspections to confirm property characteristics at the time of sale.
DEFINITION OF MARKET VALUE: The most probable price, as of a specified date, in cash, or in terms
equivalent to cash, or in other precisely revealed terms, for which the specified property rights should
sell after reasonable exposure in a competitive market under all conditions requisite to a fair sale, with
the buyer and seller each acting prudently, knowledgeably, and for self interest, and assuming that
neither is under undue duress. {The Appraisal Institute}
The market value of the property as of June 3 0th, 2018 is:
RESIDENTIAL $227,000
TOTAL: $227,000
Assessor's Indicated Value
Page 4of11
Time Trend
By law, sale prices are time adjusted to the appraisal date of 06/30/2018. An adjustment for time is
made if the sale prices of properties have appreciated/depreciated due to inflation/deflation during
the 24 -month period of 07/01/2016 to 06/30/2018. We refer to the adjustment as a 'time'
adjustment. If market conditions have not changed, no adjustment is required even though
considerable time may have elapsed. {C.R.S. 39-1-104...said level of value shall be adjusted to the
final day of the data -gathering period.) -
Sales and resales of the same or similar properties are a good indication of the changes in market
conditions over time. In addition, simple linear regression where sales are graphed vs. the month
the sale occurred provides insight into the trend of the market This trend is tracked as a monthly
rate of change. Since one of the most important aspects of market value is location, time trends
should be determined by location, also known as economic area or neighborhood. The use of the
property (vacant land, residential, commercial) should also be considered.
An analysis of improved sales in rural Fort Lupton neighborhoods concluded that there was no
discernable time trend for the 24 months preceding 06/30/2018. The same analysis was
performed on 48 sales during the period from 01/01/2014 to 06/30/2018. This analysis revealed
that sales in 2014 required a time trend to 06/30/2018 of 0.58% per month.
Page 5 of 11
F. J +�+diW ICI It. \ri.'P
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Comparable 4
Page 7 of 11
Market
Grid C:omparab
lie s
Subject
Camp #1
Adjustment
Comp fti
A'ijusinient
Comp #3
Adjustment
Sale Date
12/22/2O116
6122/2012
O1/O9/2O14
Sale Price
$412,500
$475, d
$230„GOO
Time Adj Sale Price
$412.50O
5472500
$3O5,O23
Parcel Number
13O93OOOO048
131123000040
13111 OO13
146936OOOO3O
Account Number
R52.70886
85422236
R5392286
ROO26788
stye et ,Adldress
7525 CR 85
8540 CR ?1
1E:774 C'nunty Road 22
636 CR 23.5
Land Size
6.3-61 Aces
$1510,657
2 Acres $25,857
$64.20.0
9.75 Acres .$.185,4O9
-$34,752
2.32 Acres S92,27"9
$513,370
(Neighborhood
42D9 -U3
42094)0
4216-00
SO
42034K]
$0
Builtas Description
Ranch 1 Story
Ranch 1 Story
11/2 Story
$O
Two Story
Quality,
Fair
Average
-$45,5OO
Fair
Fair
Condition
Typical
Typical
Typical
Typical
Year Built
195,4
1983
$3,3000
1973
56,300
1977
$5,10101
Residential &q Ft
1,575
1,752
-$9,350
1,4'04
$8,SSO
2,22
--$33,a50
laths
2
2
2.
3
-56,OOO
Basement Sq Ft
1,57'.5
1,372.
$4,O6+0'
SM
$13,$2D
780
S15,5t0D
Basement Finish
945
0
$14,175
824
5O
O
$14,175
Garage
None
2 -Car
-52O)OOO
None
2 -Car
-$2Cr 00
Outbuildings
1 O rtbuilding
2 Small Outbuildings
SO
1 Outbuilding
No Outbuilding
$7,01 ►O
1OO Yr Floodplain
2.6 Acres 33% Imp
O Acres O% Imp
:50
O Acres 0% Imp
$ D
1.32. Acres O% imp
SO
FIc+crThay
2.2 Acres
O Acres
-551,OOO
0 Acres
,c Si Iu"0'0
�� .r, r_- t.1
-$;51 O:10
6 Adjustment
-$43„515
$ Adjustment
-$57;OS2
S Adjustment
-1510,297
Gross % Adjustment
52.4%
24.2%
Cross r Adjustment
65.3%
Gross % Adjustment
Net 96 Adjustment
-10.5%
@let 96 Adjustment
-12.1%
Net % Adjustment
-y.4%
Final Market Value
6227,OOO
Adjusted Sale Price
$362,9,85
Adjusted Sale Price
$415041.2
Adjusted Sale Price
5294.725
Final Market 'value 1r SF
$144
Adjusted Sale Price / SF
$2M
Adjusted Sale Price / SF
$264
Adjusted Sale Price / SF
5107
Market Grid Comparables
Subject
Comp#4
Adjustment:
Comp #5
Adjustment
Comp r6
Adjusimefit
Sale Date
10/30/`_014
Sale Price
$4.21,5013
Tin) P Ad' Sale Price
$540,413
Parcel Number
130930000042
146925100025
AL -count Number
85270886
R1C04€95
Street Address
7525 at 85
..... 7 6 Junius St
Land Size
6361 u.c res
L02 Acres $86,273
.$6$,384
Neighborhood
4209-00
4-203-00
$0
Built -As Description
Ranch 1 Story
^,e n _h 1 Story,
Quality
Fair
Average
-$54, 85C
'Condition
Typical
Typical
Year Built
1994
2002
50
Residential Si Ft
1,575
2,1661
-554,.300
Baths
2
2
Basement Sq Ft
0
$31,533
1,515
Basement Finish
945
0
514,17E
Garage
None
3 -Car
-530,000
Dflthuildif s
1 Dutbuild:117
3 Outbuilding."
_53O,00C
10O Yr floodplaird
2_I3 A Ellms T33.f:_ 1-,7,
2.02 Acres 100% Imp
SO
FIoo'Jfora 'a
7 . Acres
0 Acres
-$51 0CC
Adjustment
-4120:,081
Gross % Adjustment
61 0%
Net % AtIj u stment
-72
final Market Value
S227rCCO
Adjusted Sale Price
$420,.32.2
Final Mark€t 'Value ;i SF
1:144
Adjusted Sale Price / SF
$267
Timeline of Value
2019 Original value - $287,570
2019 value resulting from Assessor's level appeal - $276,000
2019 value resulting from CBOE appeal - $227,000
2019 value resulting from BAA appeal - $227,000
Page 11 of I I
NOTICE OF DETERMINATION
Brenda Dones
Weld County Assessor
1400 N 17th Ave
Greeley, CO 80631
RECEIVED
JUL 0 9 2020
WELD COUNTY
COMMISSIONERS
Date of Notice: 6/26/2020
Telephone: (970) 400-3650
Fax: (970) 304-6433
Office Hours: 8:00AM -- 5:00PM
ACCOUNT NO.
TAX YEAR
TAX AREA
LEGAL DESCRIPTION/ PHYSICAL LOCATION
R5270886
2020
2240
w
z
0
>-
I
w
a
0
cc
n.
BACHOFER ROSS (BN)
7525 US HIGHWAY 85
FORT LUPTON, CO 80621-8809
RESIDENTIAL
PROPERTY CLASSIFICATION
16219 PT L6 SW4NE4 30 2 66 LUPTON MEADOW
S DIV #3 LYING E OF C/L OF S PLATTE RIVE
R SAID C/L DESC AS BEG ON S LINE L6 A DI
STANCE OF 610' E OF SW COR L6 N33D54'E 1
90' N59D29'E 470' N50D29'E 262.17' TO E
LINE L6
7525 HIGHWAY 85 WELD
ASS SSOR'S VALUATI0
ACTUAL VALUE PRIOR TO
REVIEW
227,000
ACTUAL"VALUE AFTER
REVIEW
227,000
TOTAL
227,000
227,000
The Assessor has carefully studied all available information, giving particular attention to the
specifics included on your protest. The Assessor's determination of value after review is based on
the following:
BA01 - The 2019 level of value for the reassessment cycle (2019 and 2020) was adjudicated by the
BAA.
If you disagree with the Assessor's decision, you have the right to appeal to the County
Board of Equalization for further consideration, § 39-8-106(1)(a), C.R.S.
The deadline for filing real property appeals is July 15.
The Assessor establishes property values. The local taxing authorities (county, school district, city,
fire protection, and other special districts) set mill levies. The mill levy requested by each taxing
authority is based on a projected budget and the property tax revenue required to adequately fund
the services it provides to its taxpayers. The local taxing authorities hold budget hearings in the fall.
If you are concerned about mill levies, we recommend that you attend these budget hearings.
Please refer to last year's tax bill or ask your Assessor for a listing of the local taxing authorities.
Please refer to the reverse side of this notice for additional information.
Agent (If Applicable):
15-DPT-AR
PR 207-08/13
R5270886
2020-2029
ASOIOCo
APPEAL PROCEDURES
County Board of Equalization Hearings will be held from
July 27th through August 3`d at 1150 O Street.
To appeal the Assessor's decision, complete the Petition to the County Board of Equalization shown
below, and mail, file online, or deliver a copy of both sides of this form to:
Weld County Board of Equalization
1150 O Street, P.O. Box 758
Greeley, CO 80631
Telephone: (970) 356-4000 ext, 4225
Online: www.co.weld.co.us/appsl/cboe/
To preserve your appeal rights, your Petition to the County Board of Equalization must be
postmarked or delivered on or before July 15 for real property — after such date, your right to
appeal is lost. You may be required to prove that you filed a timely appeal; therefore, we
recommend that all correspondence be mailed with proof of mailing.
You will be notified of the date and time scheduled for your hearing. The County Board of
Equalization must mail a written decision to you within five business days following the date of the
decision. The County Board of Equalization must conclude hearings and render decisions by August
5, § 39-8-107{2) C.R.S.-- of you do not receive a decision from the County Board of Equalization and
you wish to continue your appeal, you must file an appeal with the Board of Assessment Appeals by
September 10, § 39-2-125(1)(e), C.R.S.
If you are dissatisfied with the County Board of Equalization's decision and you wish to continue your
appeal, you must appeal within 30 days of the date of the County Board's written decision to ONE of
the following:
Board of Assessment Appeals District Court
1313 Sherman Street, Room 315 Contact the District Court in the County
Denver, CO 80203 where the property is located. See your
(303) 866-5880 local telephone book for the address and
www.dola.colorado.gov/baa telephone number.
Binding Arbitration
For a list of arbitrators, contact the County Commissioners at the address listed for the County Board
of Equalization.
If the date for filing any report, schedule, claim, tax return, statement, remittance, or other document
falls upon a Saturday, Sunday, or legal holiday, it shall be deemed to have been timely filed if filed
on the next business day, § 39-1-120(3), C.R.S.
PETITION TO COUNTY BOARD OF EQUALIZATION
ate of the property's value as of June 30, 2018? (Your opinion of value in terms of a
unt is requi d veal property pursuant tp § 39-8-1 6(1.5), C.R.S.)
tr& ► 1vea,kW/46 iy 1q (pier?-91-0orY
What is the basis for your estimate of value or ydur reason for requesting a review? (Please attach
additional sheets as necessa and any supporting documentation, i.e., comparable sales, rent roll, original
inst fed cost, appraisal, etc) �/^� 5 02- e
, `h G1 F 71-e trfeifd'� �6'o f5qt) �� 6``c��1 L S C, � r%
6 fe+T 5d
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I, the unbersi ned •wner or agent of the roper"ide ti pdve, affirm that the statements contained herein
9 9 y ��
nn any ttac nts.hereto are true and complete. / /
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Date l
15-D PT -AR
PR 207-08/13
R5270886
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2020 Weld County Board of Equalization Appeal Schedule R5270886 - Yahoo Mail Page 1 of 1
2020 Weld County Board of Equalization Appeal Schedule R5270886 thebachofer@yah.../Inbos
An Jason Marini <jmarini@weldgov come Jul 20 at 4:23 PM
To: thebachofer@yahoo.com <thebachofer@yahoo com>
Cc: Chloe Rempel <crempel@weldgov.com>
Hello Mr. Bachofer,
I am writing to you today in response to your 2020 County Board of Equalization (CBOE) appeal on your property at 7525 US Highway 85. We have reviewed your petition
and we do not have any support for a value lower than your 2019 value of $227,000 The $227,000 value was set at your 2019 CBOE hearing and upheld by the 2019
Colorado Board of Assessment Appeals [BAA}.
You have the right to another hearing with the CBOE to appeal your 2020 value; however, your 2020 appeal at the assessor level stated that your reason for requesting a
review was "to get a state Board of Appeals hearing." You have the option to skip the 2020 CBOE hearing and file on directly to the 2020 Colorado BAA by requesting an
"administrative denial" at the CBOE level This can be accomplished by contacting Chloe Rempel at the Clerk to the Board office and requesting an "administrative denial".
Chloe's contact information is :, •e i I.; or 970-400-4213, If you take this option, then the CBOE hearing will be canceled; you will get notification from the
CBOE of the "administrative deny" decision; and then you can petition directly to the Colorado BAA
I will be happy to discuss this with you further if you have any questions.
Thank you for the opportunity to be of service.
Jason Marini
Chief Deputy Assessor
Weld County Assessor's Office
970-400-3591
Confidentiality Notice: This electronic transmission and any attached documents or other writings are intended only for the person or entity to which it is addressed and may contain
information that is privileged, confidential or otherwise protected from disclosure If you have received this communication in error, please immediately notify sender by return e-mail and
destroy the communication Any disclosure, copying, distribution or the taking of any action concerning the contents of this communication or any attachments by anyone other than the
named recipient is strictly prohibited
https ://mail.yahoo. c orn/b/folders/ l /mess ages/AEyg4Rhj xAN9XxYZYwPc SM-45 Rs? . src=... 7/21/2020
REAL ESTA'FE APPRAISAL
75251.1S Highway 85
Fort Lupton. C'ulorrtdn
June 30, 2017
JOHN DeRti NGS, MAl Al-GKS
40 Kearney Street
Denver, CO 80220
t
Qualifications of
JOHN F. DeRUNGS, MAI, A}-GRS
40 Kearney Street
Denver CO 80220
Certified General Appraiser in Coloutdo (I,iecnse #CG 1316697)
DESIGNATION:
EMPLOYMENT:
EDUCATION;
Awarded MAi by the Appraisal Institute (A1) in 2002
Awarded Al-GRS (AI • General Review Specialist) in 2014
Independent Fee Appraiser • 1996 — present
Real estate valuation of commercial properties including; office
buildings. Shopping centers, industrial facilities. development
land and special use properties
Associate Appraiser • Van Court and Company • 1990 - 1996
Areas of concentration included eminent domain, estate
planning, public and private acquisition and mortgage financing
Master of Business Administration in Finance
University of Colorado at Denver • 1989
Bachelor of Landscape Archileelure
University of Oregon • 1981
Appraisal Related Courses
Uniform App Standards for Federal Land Acquisitions • sehed, 9/2017
Appraisal Standards and Professional Practice • 2016
Appraisal Review General • 2014
Appraising Cell Towers • 2013
'I he Appraiser as an Expert Witness • 2012
Condemnation Appraising • 2010
Partnership and Common Tenancy Valuation ■ 2008
Litigation Appraising • 2004
Separating Real and Personal Property from Intundihle Assets • 2004
REPRESENTATIVE CLIENTS
Alpert Homes
US Army Corps of Engineers
Bank of Colorado
Bear Creek Development
Boulder County Open Space
Burlington Northern Santa Fe RR
Chapman and Roth PC
City of Thornton
City of Westminster
Cherry Creek Schools
Dan Culhane PC
Denver Urban Renewal Authority
Uenver Water Department
Douglas Public Library District
Douglas County School District
Cit Commercial Finance
J & B Building Corporation
Jefferson County School District
Lampert Realty
McCarty Land and Water
South Suburban Park District
Slate of Colorado
Trust for Public Land
US Bank
JOHN F. neR(JN(S. MA! Al-GRS
JOHN F. DERuNG , Mau, AIGRS
40 KEARNEY STREET
DENVER. CO 80220
JOHN.DERUNOS@AVCVALUE.COM
303-789-3315
lime 30. 2017
Jeffrey 13. Cullers
Herres & l lerrera. EEC
3600 So. College Avenue, Suite 204
Fort Collins, Colorado 80525
RE. Real Estate Appraisal
7525 US Highway 85
Fort Lupton. Colorado
Dear Mr. Cullers'
At your request, I have moues a physical inspection and estimate of both the current as -is market
value of the captioned properly and its value under the hypothetical condition as ills were not
adversely affected by periodic flooding. A complete deseriptirnn of the property appraised is included in
the report that follows. along with my value analyses, conclusions and supporting documentation. It is
my understanding; that this appraisal will be used for litigation support
MI pertinent data gathered in my investigation is included. I hereby certify that I have personally
inspected the subject property and have considered all of the component parts of market value as defined.
I further certify that I base no present or contemplated interest in the property beyond this appraisal of
Market Value. This appraisal has been completed in compliance with the Uniform Standards of
Professional Appraisal Practice (USPAP) as promulgated by the Appraisal Standards Board of the
Appraisal Foundation. As indicated in my qualifications. I hold the MAI and Al -ORS designations lino
the Appraisal institute and have performed this appraisal to those standards endorsed by that professional
organization,
In my opinion, the current market values of the described property. as of May 10, 2017, at the
time of my most recent inspection, was as follows
Market Value As -is: $145,000
Market Value under the hypothetical condition
As if it were not adversely affected by periodic flooding: $500,000
Respectfully submitted,
John F. UeRungs, MAI AI-CiRS
Colorado Certified Appraiser COO13 I6679
-2-
XX IN F. DcRUNGS, MAI Al-GRS
TAKE OF CONTENTS
Title Pap:
Letter of Transmittal 2
Executive Summary .................4
Appraisal Value type...,...,.,...,,, 5
Intended Use/Intended User of the Appraisal 5
Property IdcntineuEion .,.,,....,,., 5
Property Rights Appraised , ,,,,,,,,,,,,,,,,,,,,,,,,,,,, 6
Scope of the Appraisal .. 6
Date of 6
Contingent and 1.imiting Conditions,— ........................ 7
Description of Real l:.statc Appraised ....,..,.,, ................... .................................................... 8
Area Description ......,., 8
Neighborhood Description i2
Subject Properly Description
Ownership and Tex Infurrnatkua ,...,
P'roperty' I liscor) ............. ........,,......,,........._...............,..,_.,...,.,..,.,..,.....,.,....,..,...,.._.....,.,,,...a,....,...,,...... 24
flighcst and best Use . 21
Summary of Analysis and Valuation _.................................... 24
Methods orValuasion........... .....,..........,....,............... . ,...........,...,...... ,,,.., 24
Valuation - As -Is t;l.and only) ,...,,,..,... ?f,
Valuation - as Wit were Unaffected by 1p looding......,........,..,....................................................... 33
Certificate. Signature and Late 40
Qualifiemions of -the Appraiser 41
Subject Photographs
JOHN F. DeRIJNGS, MAI AI-CiRS
EXECUTIVE SUMMARY
OWNER OF RECORD: Ross Bacliofer
EFFECTIVE DATE OF VALUATION: May 10.2017
DATE OF REPORT: June 30, 2017
LOCATION: About one mile southwest of the Weld County Road 18
intersection with US Highway 85 along the east bank of
the South Platte River adjoining the Fort Lupton city
limit
LAND AREA:
ZONING:
FLOOD HAZARD AREA:
5 gross acres more or less
Agriculture in Weld County Zoning Ordinance
According to FFMA Map 08123C2IO2E. revised
January 20, 2016. about 3,75 acres is within a floodway
or I O00year flood hazard area.
IMPROVEMENTS; One story 1.575 -SF house with a full walk -out basement
completed in 1994 per assessor
HIGHEST AND BEST USE: Single home site
MARKET VALUE CONCLUSION:
As -la:
As if it was not adversely affected by periodic flooding
using a hypothetical condition:
-4
S145,000
$4011,0110
JOI IN F, 17cI iJNGS, MAI AI-GRS
APPRAISAL. VALUE TYPE
1'he purpose of this appraisal is to est intate the Market Vahre of the subject property, as of the
effective elate. as defined below.
Market Value /6 the most probable pried which a prober/• should bring in is competitive
and open market tinder all conditions requisite to a, fair sale, the buyer and .salter each
acting prudently and knowledgeably, and assuming the price is not of eected by mitre
stimulus. lrirlilic it In this definition is the consummation of a .sale a5 of a specified date
and the 1►cas.sing rf lltk. froru seller to hut•r•r under conditions whereby.
f. Buyer and seller are t} plc'ally motivated;
?. Both parties ore well it► fr►raned or well cub+iser1, and acting in
what they c•crnsider their own hest interests.:
3. • t r•c°asonab , time is allutrecl fire rr rpusure in the open market:
a. Payment is mode in terms elf erode in US. dollars or in terms s f
,linwu iul arrangements comparable thereto: and
5. Me price represents the normal coitsieleraiioirfir the, property
sob/ 113;4(rcA-a flr'.vpecial or em'eatire• fir:anci►t, or sales concessions
gromed Ill' curt Roe asccic•iatrii with the soh'.
This definitions is taken from the Glossary Section of the Unrlorm 57ardards of Professional
Appraisal Practice OAP AP). published by the Appraisal Foundation,
INTENDED USE/INTENDED USER OF THE APPRAISAL
It is my understanding that this appraisal IA ill be used for litigation support by the current owner
and his counsel. I do not intend any other use of this report,
PROPERTY IDENTIFICATION
The property under consideration is about one mile southwest of the Weld County Road 18
intersection with 1JS I lighway 85 along the east bank of the South Platte River adjoining. the Fort 1.uptau
city limit. Its street address is 7525 US Highway 85, Fort Lupton, CO 80621. A partial legal description
of the property is
"Part of Lot 6, Lupton Meadows Land Company Division #3,
Crusty of Weld, State of Colorado"
- 5 -
JOHN F. DeRUN(.JS, MAi Al-GRS
PROPERTY RIGI ITS APPRAISED
The property rights appraised in this study are the fee simple estate. No other separate
consideration has been given to a division of interests, the interests of tenants in possession, or the
interests ormortgaage holders. if any, unless specifically otherwise noted,
SCOPE OF THE APPRAISAL
The scope of the real estate appraisal includes the tilllowing;
1. A physical inspection of the property,
2. A search oldie public records relative to the properly. This search encompasses, among other
items. tax and assessment information. easement, and other private, as well as public, deed
restrictions. zoning and history of the property.
3, A summary of neighborhood and regional area characteristics, as well as an analysis of supply
and demand within the subject's market segment,
4. Analysis or physically possible uses. legally permissible: uses, and all feasible uses in order to
estimate the highest and best use of the property under consideration.
S. Research of the public records using third party services for comparable sales and listings,
including telephone verification, where possible, of all the sales and listings with the buyrer, seller,
or their representative. Each property was inspected and deed verification was conducted.
Comparison or [tae comparable properties to the subject properties was made with consideration
of such differences as legal encumbrances, financing terms. conditions of sale. market conditions,
location. physical characteristics. availability of utilities. zoning, and highest and best use.
6. The preparation of a narrative appraisal report in compliance with the Standards of Professional
Practice (.1JSPAPI promulgated by the Appraisal Standards Board of the Appraisal Foundation, as
well as the Standards of Professional Practice of the Appraisal Institute.
DATE OF VALUATION
The effective date of this appraisal is May 10, 2017 at the time of my most recent inspection►.
Market data was assembled during the month of May 2017. The report was prepared during the week of
June 4. 2017,
- 6 -
JOHN F. DeRUNUS• MAI Al-GR
CONTINGENT AND L1MFI'1NG CONDITIONS
IONS
This appraisal is subject to the following contingent and limiting conditions.
t I. Title to the property is assumed to he good and marketable, and the legal dcl;cription furnished k
assumed to be correct.
?. Sketches in this report are intended to be 'visual aids and should not be conrstrucd as surveys or
engineering reports.
3. All information in this study has been obtained from reliable sources. The writer cannot.
however, guarantee or be responsible for the accuracy of information furnished by others.
4. Possession of this report, or a copy thereof does not imply the right of publication or use for any
purpose by any other than that of the addressee, without the written eonsxnt of the appraiser.
5. I have made a physical inspection of the property corder study. While the inspection revealed ato
obvious physical hazards, it was insufficient to discover the existence of hazardous materials, A
prospective purchaser or investor is advised to seek professional assistance in investigating the
soil contlitious and checking for the presence of hazardous materials. I reserve the right to
modify the value conclusion it'adverse physical features are discovered,
6. This appraisal is of real estate only: that is, land, building improvements and site improvements,
and excludes the value ufany personal property:
7. I believe that the land and building areas used herein represents the must accurate information
available. Since differences in the square footage oldie property improvements could have an
impact on my value conclusions. I reserve the right to amend the report accordingly should more
exact measurements become available.
K. I have not been engaged and am not required to give testimony or be in attendance in court or
provide such testimony by reason of the appraisal. unless- prior arrangements have been made in
writing,
-7-
RAIN F. DeRUNCS, MAI Al-GRS
DESCRIPTION OF REAL E'ST'ATE APPRAISED
AREA DESCRIPTION
The subject property is located within Weld County- in the path of northerly suburban growth,
benelitting from the advauttageous regional ace ess available from the metro area's beltway,
Highway E-470. Weld County is located in north -central Colorado and conttuns almost 4,000
square miles of land area. Greeley is the county seat and principal city, and contains 34% of the
county's population_ The area's economy has hislorieelly been concentrated in the Agri -industry
and food processing businesses. hut bias diversified substantially over the past 20 years.
Fhe oil and gas industry. advanced technology, manufacturing. tourism and the service sector
have all played a role in the trdnsforntation of the area's economic base. In addition, the wealth of
research and development activities which stem from the educational facilities in and near the
area have promoted this change. These, institutions include the I iniversity of Norlhent Colorado
at Greeley, and the University of Colorado at Boulder and Colorado State University in Fort
Collins in nearby Boulder and Larinier Counties. Aims Community College and other smaller
colleges complement these major institutions.
A significant share of the stele's projected population growth is expected to be in Weld County
according to the state demographer. The population of the northern Front Range, defined
geographically as Lorimer and Weld Counties could see its population double by 2040 with much
of that growth occurring in southwestern Weld County, In particular, the county's population is
concentrated in an 800 -square mile area in this southwestern part of the county where the subject
property is located, Commuter traffic to the Denver area has skyrocketed since the 1990's and the
opening of Deriver International Airport fostered that increase. The 2016 county estimate of
294,932 people places Weld County at the top of the ftastcst growing counties in Colorado and
fourth in the nation when compared to census counts in 20111 of 252.837 and in 2000 of 1110.936.
The City of Greeley parallels that increase. With an increase of 10% since the census estimate of
92,881 in 2010. it is also one of the top cities in the state for growth.
Historically_ agriculture and related agri-businesses have played a major role in Weld County's
prosperity. Warm summer days. cool evenings, and low humidity are common. The average
annual rainfall is less than 12 inches, but substantial snow accumulations in the mountains to the
west provide an abundant supply of water through an cxtensise system of reservoirs, canals. and
-8-
SUB -REGIONS & COMMUNITIES
The sheer size arid diversity of Weld County allows 11 to meet almost any type of site requirement (torn North suburban
Denver 1•25 needs in 220 Weld to the booming opportunities of the 1-25 and Hwy 34 corridors in Control!West t Weld, to
the wide open spaces of farmland/prairie in East anad No t)Weld,
SOUTH WELD Cowan
with a population estimated to reach 70,867 by 2017, the
southern region of the county has experienced some of the
fastest growth over recent years Offering 0 I attractive smaU
town/mral quality of life with abundant new homes, expo ant
schools and brealhtnk.iny mountain views the region rs
strategically located just north of metro Denver and east of
Boulder, which has fueled booming residential growth With an
estimated 21,781 households and $278,7 million in retail sates,
the region is poised to see continued growth The median
household income average is $63,530, the median home value
average is $202E386 and it offers a wide -range or ed ucaboreal
opportunities, sport venuos and recreation. Commercia&
industrial sites arc available and large tracts of undeveloped land
provido opportunity for future growth. East/west access is
provided by Hwy 52 which connects to I-76 and 1-25. North/
south access is provided by 1-25 and Hwy 85 which connect to 1;
70 in Denver and I- in Wyoming. DIA is 20-30 minuteS away
�s_�_�. - _ ,ir
iYq Moab
North A East Wald County
Central/West Weld County
South Weld County
` lNor;i l• WELD COUNTY
Home to the Pawnees National Grasslands,
this sub region is the most rural in the
County with on estimated municipal
population of less 1,000 However, its
become one of the most Important energy
regions ror both oil/gas development and
wind farms. Ws also an important 1arrrrig
and live -stock grant area East/west
access through the region is provided by
Hwy 14 which intersects with Hwy 85 and
CE1_,,i,TRAL/WEST WELD COUNTY
With a population estimated to reach 178,904 by 2017, this sub
region is. the most populace of the County With 58,880
households and $119.7 billion in sal , it's the retail core and
offers an attractive mix of lifestyles from unite -size cities to small
rural communities nestled in amongst sore of tt"to most
productive farm land in the country, The median household
income average is $52,367 and the median house value average
�s $167,'I73 The region offers spectacular mountain views, en
excellent quality of lifewith ptart of outdoor recreation low
traffic, excellent schools and easy access to two state
universities Developed commercial/ industrial sites abound and
the region has seen numerous employers locate/ expand which
has produced a vibrant and growing business sector. The region
has excollont oostMest access via US ktwy, 14 and US Hwy 34
which connect to 1-25 and 1-76. US Hwy 85 provides north/south
access connecting to 1=70 in Denver and 1-80 in Wyoming Main/
short !hate halt access Is ample and DIA is about an hour away,
JUAN F. [) RIJNiiS, MAI AI-JPS
irrigation ditches. While urbanization has taken its toll, about 80% of the County's 2,56 million
acres is still devoted to farming and raising livestock, The County ranks in the top ten, the only
county outside nfCaliforniu, in the nation and I irst in the State in the value of agricultural
products produced. The hulk of the County's agricultural economy is centered in livestock
production. It is estimated that about 5% of die employment in Weld County is related to
agriculture,
Industry statistics shwa that personal income and employment in Weld County have changed
significantly over the last two decades. from a strongly agricultural economy to at diverse
economy with expanding manufacturing and service industries. in particular, advanced
technology business has introduced new, .clean" industry to the area and contributed to income
growth within the manufacturing sector. 1 he natural advantages of climate, the mountains
nearby. and the availability of recreational opportunities have become important factors in
attracting and %lainutg highly trained technical and scientific personnel.
Al the sante time, the county is establishing as reputation as a center for the oil and gas industry
advancing technologies and manufacturing, although those industries are still recovering from the
latest downturn in late 2014. '1 he County produces about 85% of the oil and gas in the .-late and
associated service industries have expanded.
From 2010 to 2014, Weld County's employment growth had boomed at a 5% average annual
growth rate, outpacing Colorado and pulling it among the fastest growing economics in the US.
The largest percent of wage and salary income is generated by the manufacturing sector at 11% of
gross regional product but the energy sector at 10% is not far behind. The 2015 decline in oil
prices and subsequent job loss was shoe -lived hut employment increases arc still one-half of what
they were in the early part of the decade.
In the last live years. the value of construction projects in Weld county has also increased
significantly due to sustained growth in population and employment. Currently, the 1-25 and US
Highway 34 area has the largest concentration of commercial construction underway in the state,
After recording, one of the worst periods ever in the late 2000s, the housing market has steadily
improved with new home building permit activity up an average growth rate of 20% per year
since then.
to -
JOHN F. l)eR(NGS, MM Al-GRS
Increased economic production has drawn new manufacturing operations, like two Vestal wind
power component factories, leading to a cut in its unemployment rate hy ballot -what it was just
lour years ago. The major private noii-retail employers in ilic county are listed below.
COMPANY
1. IBS USA
2. Banner f lealth
3. Vestal
4. State Farm Insurance Company
5. 'Fetetech
6. I is l iburton Energy
7. Anadarko Petroleum
S. Select Energy
9. Noble Fncrgy
10. l.eprino foods
Major highways servicing the area include US Highway 85, which links Greeley to Denver
through Brighton and Adams County. The Colorado Department of Highways continues to
upgrade that highway in several locations. US Highway 34 runs east -west and links Greeley with
the towns of fort Collins, Fort Morgan, Loveland and Sterling and also provides access to
Interstate Highway 25, which lies 16 miles to the west. interstate Highway 25 runs north -south
along the eastern slope elf Colorado's Front Range. The City of Greeley has a small municipal
airport but is only about 40 miles, or a one hour drive. away from Denver International Airport
and I lighway E-474 beltway travel reduces that significantly. There is in -city access to mainline
rail and bus service. Regularly scheduled city bus route provide public transportation an the
Central Business District_ educational facilities, and major shopping areas.
The County's economic growth has become more closely lied to that of the metropolitan Denver
area and its continuing importance us a cotter of regional commerce, I expect Weld County to
continue to outperform most other parts or the United States. However. its prospects for
continued prosperity are also fostered by the stabilizing effect of the large percentage of
governmental employment, a wealth of natural resources and amenities of climate. culture. and
mountain surroundings which attract tourists. students. and new residents from around the world.
-II
JOHN F, DeRUNGS. M,AI 111-68S
NEIGHBORHOOD DESCRiPTi{)N
The neighborhood within which the subject property lies is comprised of rural lands in southwest
Weld County. Fort Lupton, the largest nearby community is located where [!S I lighway 85 and
State Highway 52 cross. With almost 7,850 residents. it is also the most prominent community in
this purl of the County and has grown from census estimates of 7,426 in 2011) and 6,786 people in
2000. Fort Lupton's future growth boundary extends in a southerly direction to County Road K
where it meets the City of Brighton's expansion across the Adams County line.
Other towns include Platteville. along US I lighway 85 al Highway 66. and Hudson, located at the
interchange of State Highway 52 and Interstate I lighway 76, Like Fort Lupton. these towns
historically acted as service centers for surrounding farms and raneltcs. However. a recent
population increase is generally attributable to the rise in new residents who commute between
this part of Weld County and the northern suburbs of the Denver Metropolitan urea.
The primary transporlation route serving this neighborhood is US Highway 85, generally a 4 -lane
highway that extends south to Denver and north to Greeley and eventually to Cheyenne.
Wyoming. The Union Pacific railroad also adjoins US Highway 85 to the east. Access control
measures such as new frontage roads and interchanges have been built in recent years rather than
widening the highway to 6 lanes. Colorado I lighway 52. a 2 -lane paved road connects Fort
Lupton to Inlersurte Highway 76. nine miles east at I Judson. It also provides access to Interstate
I lighway 25, about 8 miles west of Fort Lupton. These highways provide for growing commuter
traffic flow to northern Colorado communities in both Weld and Winter Counties. General
aviation facilities are available at Denver International Airport, approximately 30 miles to the
southeast.
The topography of the area is mostly flat with gently rolling hills. The South Platte Riti er flows
north just west Of file communities of Fort Lupton and Platteville. A series of tributaries and
irrigation canals arc found in this area Sonic arc designated 100 -year flood plains by F'bMA.
Several small reservoirs are scattered throughout the arca, however, by far, the most significant
nearby feature is Milton Reservoir, occupying over five square miles.
A study conducted by the Colorado Geological Survey in 1984 indicates that various locations
throughout the neighborhood could be subject to subsiclema.: resulting from past years' coal
mining activity. Lands within such subsidence zones typically require sub -surface stabilization
- 12 -
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To The 2007 Fort Lupton Comprehensive Plan
JOAN F. DeRUNGS, M t AIsGGRS
before development i; &towed. Geological resources in the area include oil, gas and coal
deposits, and active wells dot the area+
Aiihoutzh businesses and residences exist on subdivided lots within the limits of surrounding
communities. surr0 u ndiln 8 neighborhoods are comprised of agricultural land of varying quality.
the best land consists of 'litigated and river bottom lands west of US Highway 85 in association
with the South Platte River. Higher land to the east and west of I I ighwuy 85 tends to be arid and
more suitable for dry farming and grazing. `Elie open character of rural lands has recently
attracted new residents who have built new homes on fttrmktnds. Their livelihood is Men
unrelated to a ricultire. Most surrounding uses employ private utility systems. The Central
Weld County Water District has built main tines over portions at Southwest Weld County. While
rural development can be served, the district's li=nes luck the capacity for development at higher
than one %srr it per 35 acres in many locations ithout rtr rM exemptions.
I believe that the prospect lbr continued growth in area communities over the next I 0 to 20 years
is very good. The neighbothood is located within vas> commuting distance of the e northern
suburbs of the Metropolitan Denver Grua as well as notthear Colorado communities like Greeley.
Because ()Meld County's efforts to preserve farmlands, demand For home sites outside of
incorporated areas is likely to exceed the available supply leading to an upward trend in land
values.
SUBJECT PROD' RT Y DESCRIPTION
Site
As silovvo on the following page. the subject site is located about one mite southwest of the Weld
County Road IS intersection with US Highway 85 along the east bank ofthe South Platte River
That puts the central pan of Fort Lupton which provides local services, within a few minutes drive
south of this location, But where it lies within unincorporated Weld County, it is just outside the
town limit vkihicli adjoins it along its east boundary, the principal feature at this northwest edge,
or Fort Lupton is the South Platte River anti awe gravel mining and water storage areas
associated with it. More specifically, the property lies at the end n i a private roadisspay that
extends mile west of I JS l lighway 85 along the midpoint of the section line on which Weld
County 16 and 18 borders. 'Ibis unpaved road serves the. iii i st ing residence.
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JOHN I'. DeRUNGS, MM Al -ORS
topography of the site is characterized by a series of river `lynches" that terrace down to the
river's p t sent course in a northwesterly direction. That puts about % of the lowest portion of the
property in a designated flotxtplain and floodway according to Et MA Map 08723C2 l02
revised January 20, 2016 and shown on the county's w e s iic.. in addition, rbatscd on the owner's
reports and photos taken by others. there is reason to believe that n portion/or the properly outside
and adjoining the hvarcl area has also periodically flooded for several years. A drainage canal
known as the Platteville Ditch crosses the property near its southeast corner and is bridged by a
wwd structure on the access drive.
While I have not been provided a soli repon for this property, the prevalence ofactive gravel
mining nearby . uggests the top soil l eve I is comparatively thin.: n:. Subsoil coutl itions in this pan of
Weld County do not typically limit construction if appropriate foundations and structural design
are used.
The subject property is roughly a triangle with over 900 lineal feet of river frontage and an
average depth of 660 along the ren mi n ing sides. A total land urea of about live gross acres is
indicated. I did not receive title documents and the subject properly may be encumbered by
rights or claims for minerals., entry and surface, use but I have no reason to believe thal those
agreements contain provisions that would adversely ailect the value or the property,
Under the'Weld County Zoning Ordinance, the subject tract is currently lit zoned A (agricultural
district). This designation allows principally agricultural uses but with the provision for one
single Iam i iy dwelling, oil and gas production facilities and certain public uses. Some higher
intensity activities" such as grain elevators, recreational stniciures and mining of all types req u i r
a special permit.
The totting ordinance requires a minimum lot size of 0 acres, f lonelier. Weld County's
Subdivision Ordinance also provides a second lot of less than 3$ acres. using an exemption
process. What; originally designed to promote tht continued family ownership of farms. zoning
officials report that most of those exemptions ptions ant approved regardless of the applicant's is goal,
Most often, the approval process focuses on the ideal size 01' rho second lot, As a typica 1
property's agricultural productivity increases, efforts to retain as much land in production as is
possible usually result:3 in a second lot of a few acres or Tess. By contrast. Lots of joust fewer than
35 acres have been appRwed where agricultural productivity is low.
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JOI IN F. DeRUNGS. MAI Al-GRS
As pan of the city's future growth area, a corridor along the river that appears to include subject
is designated for park and open space in the newly amended (2017) land use map of the city's
comprehensive plan. Mineral lands arc also shown adjoining the river corridor particularly to the
west where active gravel mining is underway. Properties to the north, south and east (and across
US Highway 85) are currently in agricultural use. mainly irrigated fans, dairies and grazing
lands. Some surrounding farm lands have been subdivided using the exemption process to
provide sites for new homes. Despite this location's distance from services, the availability of
rural land has attracted new residents whose livelihood is not dependent on agriculture,
Meeanwhilee, while designated by Fort Lupton for more intensive mixed use, there is still plenty of
properties between here and established parts of the city that are likely to develop first betare they
reach subject's surrounding area so that it will likely be many years before the complete transition
from strictly agriculture takes place.
-l8-
JOHN F. OcRIJN(S, MAI AI-GRS
Impruvements
Central portions of the subject site were improved about 25 years ago with a 1,575 -SF ranch -style
residence with a full walkout basement. Each floor measures L575 square feet, or 3.150 SF of
gross building area. In describing this building, I have relied on my personal inspection. The
building is of wood frame construction clad with metal siding and supported by a concrete slab
and poured -in -place concrete foundation. A pitched roof is coffered with asphalt shingles and has
metal gutters. Ample fixed fenestration is available on both levels including circular accent
windows on the south elevation. French doors are used to reach the deck and from the walkout
basement. Iwo 10 -foot wide wood decks with railings that were added along the entire cast and
west elevations (Willis building are accessed from exterior stairways providing generous outdoor
space, Large wood planters and a concrete landing are found just outside the walkout basement
cue Iraace
The main entrance on the east elevation opens first into a living room with nearby coat closet and
then into the "great" room used as a kitchen and dining area. The Belly equipped kitchen features
wood cabinetry, major appliances. (except for a disposal), and affixed island with tile flooring.
Hard wood floors are uncovered in the dining room and carpeting is used in Iwo hArooms and a
full bathroom that arc reached down a nearby corridor. One of these is a master bedroom with a
second bathroom. A carpeted stairway to the basement opens 10 a family room along die west
side of the basement where large windows mid French door provide ample daylighting. Two
carpeted bedrooms with storage closets are also found. The remaining 20% of the basement
along the south wall is only partially finished and is used ax a laundry and utility area.
A 1,200.SF frame storage garage with metal cladding un a concrete slab and apron faces the
house from the north. It has an oversized door and electrical service only. Several other smaller
agricultural outbuildings are Ibund nearby,
At the time of my inspection, the building appeared to be of sound construction, full> functional
and in good physical condition typical of a 25 -year old home, I understand that replacement and
repair of finish in the basement has been necessitated by periodic flooding. My inspection
showed no obvious defects to the building's structure but the owner's photos showed some
surface cracking eel' Ilse concrete slab. Cho long term affects flue to this inundation are not known.
JOlIN F, QeRUNGS, MA1 AI-tiftS
My study of the surrounding arca shows that where subdivision has occurred, it generally
produces one to three acre sites making the typical percentage of building coverage for properties
of this type quite low (under 1 %). While at live acres the subject could readily produce two lots
of this size. the impact of the 100 -year floodplain confines the area that can be built on to, just
over an acre. Consequently. I don`t believe that excess land exists in the form of a second
building site that could be readily marketable based on the seclusion that buyers desire at this
location and due to the central location of the existing residence.
OWNERSHIP AND TAX INFORMATION
The Weld County Assessor's records show the ownership of this property in the name of Ross
13acltater of 7525 US 85 Port l.upton_ Colorado. ft is identified on the County tax rolls by Schedule No.
309-30-9-00-04S. However because the property is valued as an agricultural use, this assessment does
not provide an indication of value under rite market value definition used in this assignment. Real estate
taxes are paid in full with no special or prior taxes owed.
PROPERTY I ['STORY
I understand that Mr. l3achofer acquired the property from his son in 2001 and has rented it since
then. No recent arrrrs.length transfers that would provide an indication of its current value have occurred,
nor was the property bring actively marketed for side on the date of r,alue.
. 20.
JOHN I'. DeRUNGS, MAI A1•GRS
I iIGHEST AND BEST USE
The Appraisal Institute defines highest and best use as follows:
That reasonable and probable use that supports the highest present value,
as defined, as of the effective date of the appraisal, Alternatively. that
use, from among reasonably probable and legal alternative uses. found to
be physically possible, appropriately supported, financially feasible, and
which results in highest land value.
11wc definition above applies specifically to the highest and best use of the land. It is to he
recognized that, in cases where a site has existing improvements on it, its highest and best use may very
well be determined to be different front the existing use. The existing use will eonlinue, however, unless
and until land value in its highest and best use exceeds the total value of the properly in its existing use.
The four essential criteria for use under this concept were considered in the sequence shown
below:
1. Physically possible uses were considered in tarns of the size, shape, land area, and topography.
Also considered were the availability of public utilities and age. condition, and functional utility
of tile improvements.
Legally permissible uses were considered. These result from such limitations as those imposed
by private deed resirictirins_ zoning, building codes. and environmental regulations.
3, Financially feasible uses were those uses that meet the conditions imposed by the two previous
criteria and that may be expected to produce a positive financial return.
4. Maximally productive use is than use which will provide the highest rate of return or value to the
land.
The following tests must he met in estimating the highest and best use: the use must be legal: the
use must be probable. not speculative or conjectural: the use must include a profitable demand! and the
use must bring to the land the highest net return for the longest period of time
Highest and Best Use as if Vacant
In review. the subject site is a roughly triangular five gross acre parcel situated along the South
Platte River just outside the Fort Lupton limit at the north edge of ihe community in
unincorporated Weld County. While readily accessible from US Highway $5, about '4 mile to
flaw cast_ surrounding uses tend to be agricultural on much larger tracts and sometimes improved
with rimehettes except for the active gravel mining operation which lies along the west side of the
JOHN F. DeRUNGS, MAl AI.GRS
river, Indeed, while subject is zoned agriculture because of its proximity to the river, its top soil
tends to be thin and inadequate to produce many crops. At the same time, economics of scale in
gravel mining make k unlikely it would be independently used for those resources. Except for
electricity from United Power, central utilities are not generally available and so private well,
septic and propane use is a common way to serve residences.
Over one-third oldie property along its northwest edge is in a designated floodway of the South
Platte River as shown on FIRM maps. However, periodic flooding in recent years suggests that
the area of inundation hos moved beyond that designated area in some places to where the 100 -
year floodplain has been mapped and includes central portions of the site where the residence is
currently located. Rut because that still leaves the southeast one -quarter of the property
unaffected, it is still best suited to its current use as a residence because of the appeal of this
riverfronl location near liS 85.
As described, the Weld County's Subdivision Ordinance provides for a second lot, in sonic cases,
using an exemption process. The approval process would likely address whether a minimum lot
si,,c of 2', acres when using private utilities can be achieved while still providing a second
building site out of the floodploin that meets other requirements for use of a well and septic
system, While zoning officials report that most exemptions are approved. the County can
exercise discretion in the way this exemption is decided.
Meanwhile. 1 believe the marketing of two lots one mile upriver from snhject is instniclive in this
case. Despite the scarcity of uvnilaiblc sites, that listing period has now extended for almost two
years and has led the owner to offer the property as one four acre site. instead of one 2‘A and one
I',' acre site, While the offered price may have prolonged the process, it seems to suggest that
subject is probably more marketable as a single site than as two, in part because it's most
favorable attribute. (that of privacy), would also be fostered,
Feasible uses of the site are determined not only by physical and legal constraints but also by
current market conditions, The current residential market poses a unique combination of
challenges for many conventional hnntchuilders. '1 he influx of oil and gas business and new
manufacturing growth has fueled healthy demand for housing. According to recent reports few
other metropolitan areas have seen better rcw.ncs". in sale prices. But rising construction costs
and a contraction of homebuilders during the lust reee lion has made it tough to deliver enough
22
JOhIN F. DeRUNGS, MM Af•tiRS
new homes. New building permit activity is still one -1184.0f the levels ul'Wiwi years ago and so
population increase has outstripped the supply of new homes causing prices to rise and impacting
affordability, In stay opinion. the highest and best use of the subject site, as if vacant and
available for development, is four its continued residential use to take advantage of this river from
location on that over an acre portion unaffected by the flood Ward area,
Highest and Best Use as Improved
As described, the site is currently improved with a single family hoarse and outbuildings built
about twenty years ago, they appear to have been well -maintained and are in good condition.
PEMA maps show those improvements to be at the edge of the 100 -year floodplain and under
the hypothetical condition as if it were not adversely affected by periodic Hooding, they
represent the highest and best use of the property as improved.
In its ac -Is condition however periodic flooding likely impacts not just the continued use of the
walk -out basement level hut the rest of the structure as well, Repair and replacement of interior
finish is costly but the more serious prospect of structural damage to the tbundation and floor
from subsurface water pressure could be the source of surface cracking of the slab observed in the
owner's photos. I have not been supplied with engineering studies that would form the basis of a
more definite cost to cure, But beyond its actual affect on the home is the perceived impact of
inundation which puts up significant barriers to the marketability oldie improvements, The
prospect of periodic flooding to a structure not designed for insulation raises the likely prospect
that it will have to be rated and a new home built outside of the affected area, Accordingly, the
value of the property then is land only, as any salvage value of materials from the demolished
structure is likely to he offset by the cost of its removal_
2 -
JOHN F. UettUNriS, MAt Al-GRS
SUMMARY OF ANALYSIS AND VALUATION
METHODS OF VALUATION
lit the valuation of real estate, there are three commonly accepted approaches to value. These
include the Cost Approach, the Market Data Approach, and the Income Approach.
The Cost Approach establishes current market value of the land. as if unimproved, To
this is added current reproduction costs new of all building improvements. Icss accrued
depreciation. For appraisal purposes, accrued depreciation is defined as the difference
between current cost and present value.
The Sales Comparison Approach compares and relates the property under study to
other similarly -improved properties that have sold in the general area in recent years.
This approach has the greatest application when detailed market data regarding the sales
of a number of similar properties is available. Units of comparison arc developed from
the comparable sale properties and applied to the property being appraised.
The Income Approach to value converts net income attributable to the real estate into an
indication of property value by the use of a eapitulilation rate. After establishing the
gross potential income. deductions are made for vacancy, operating expenses, and return
to non -real estate items. this results in an estimate of net income to the real estate which
is capitalized with an overall capitalization rate providing for both the mortgaged and
equity potions of the investment.
In this assignment, only the Sales Comparison Approach has been applied to reach a market value
conclusion. While the property is currently leased. obtaining a return at' investmem is seldom the
goal in buying this property type and finding relevant lease rates with which to develop an
Income Approach is difficult and does not generally produce a usable indication.
the Sales Comparison approach is the most common technique fur valuing land and houses and it
is the preferred method whets comparable sales are available. With this technique, sales of
similar parcels of land or similar homes are analyzed and compared to the property being
appraised to form an opinion as to the reasonable and probable market value of the property being
-2d
JOHN 1'. DoltLI NCS. MM M.1 RS
appraised. the comparison process is based on an analysis of the similarity or dissimilarity of the
property. The Safes {'comparison Approach may be used to value land that is actually vacant or
land that is being considered as though vacant for appraisal purposes.
In this case. I concluded that highest and best use oldie as -is property is akin to vacant land since
periodic flooding of'the property has dramatically shortened its useful life and makes ii virtually
unmarketable. file basis of its as -is value is then by comparison with other sales of land. Under
the hypothetical condition as if the property were not adversely affected by periodic
flooding, I have investigated recent sales of similar homes like subject, I gave special emphasis
to available sales of riverfront property and where re a walk out basement design was used.
-25-
JOHN F. I)eRl1NGS. MM AI=URS
VALUATION - AS -1S (Land only)
LAND SALES DATA
In the coming pages are a summary and map of six recent sales of land lovatcd in the outskirts of
Fort Lupton. In analyzing these and other sales located in Weld County, a number of
generalizations can be made, These include the following:
LOCATION
l.txaion is the primary factor in the valuation of virtually all development land. As applied to this
appraisal, the maximum land values (per acre) are found with those tracts having frontage on arterial
streets, corner orientation. proximity to high -quality existing development, and good auras to highways.
SITE OF TRACT
Another iinpurtunt factor is the size ofthc tract being considered as related to market data. As a general
rule. the smaller the parcel, the higher the unit value (per acre) and, conversely, the larger the tract, the
lower the unit value, This is primarily because the smaller tracts are in greater demand and within the
paeans of many investors and users. while the larger tracts have a wholesale aspect requiring greater
financing capacity. equity inv'estmenr, expense, and time to develop.
TERMS AND CONDITIONS OF SAI,.F
Salo prices listed for comparable sales and listings are strongly influenced by the terns and conditions of
sale offered by the seller, and by the motivation ot'both the buyer and the seller. When liberal terms are
offered, such as low down payment, low rate sri'intc►est. and payment over an extended period uftltnc,
prices tend to be inflated. When the seller requires all cash, prices tend to be depressed or discounted.
DATE OF SALE.
The date of comparable sales is also important in estimating present land values. This is not only because
of cycles in land values within the local economy. hut also because of the intlattumnry trend Wetting the
value of llw dollar during the last forty years,
ZONING AND par E TI Al. USE
The highest land values, on a unit basis, are typically found under the higher -density commercial uses.
followed in succession by multifamily residentiallofficefindustriul uses. single-family use, suburban
residential, and, finally. agricultural use, While the existing zoning elassif cation may not absolutely
dictate future use °Idle land, it has a strong hearing on both sale prices and land values. A purchaser whet
must rezone land to a higher or different use enlist consider not only the probability of such rezoning hut
--
JOHN F. t)eRIYNGS, MM Al-GRS
also the time told expense required,
PHYSICAL FEATURES OF Tiff, LAND
in considering current land values, a number of physical factors also influence value. these include
topography or slope, provision for surface drainage, floodplain, soil conditions. ground cover. view. and
amount of usable land area. among others. When all of these conditions arc favorable. the purchaser might
reasonably he expected to pay a premium, and when one or more are unfavorable, an offer to purchase will
most probably be discounted,
AVAILABILITY OF UTILITY' SERVICES
Since impending development is dependent on the availability of utility services. this factor also strongly
influences the price paid for Iund. Tracts that can be served with natural gas. electricity, sanitary sewer and
domestic water will command a higher price over otherwise comparable tracts which cannot he served or
where the costs of obtaining service are excessive
On the following page is a summary of six sales and a current listing of properly near subject As
shown by the sales map. these sale; arc located on or near WCR 23 about one mile east of
subject. Thal location overlooks the valley and provides some western mountain views making it
an appealing area for limited new home building that has occurred here in recent years on large
lots, Unfortunately. I found no recent indicators from recent sates of South Platte River frontage
or of sites that were similarly affected by a flood Inward area although a current listing of land is
available just one mile upriver front subject.
One-half of these sales occurred within 36 months of the effective date or value. Based on
market trends published by iRI:S, (an on-line residential real estate service that tracks sales by, zip
code), improving market conditions have yielded an ris'ertgc increase in home sale prices during
this period of about %% per month. Assuming that a parallel increase in land value occurred the
adjusted sale prices then supply indications are from $78.750 to $201,1100. The price -per -gross -
acre is them from about $12,500 to S24,000 rounded, although that is based on a comparatively
wide range of lois from 3.40 to 9,4 I acres.
JOIIN F, I3eRUNOS, MAI AI4 RS
COMPARABLE LAND SALES
SALE I LOCATION
GRANTOR/
GRANTEE
SALE DATE
COUNTY
DATA
ACTUAL TIME
SALE ADJUSTED
PRICE PRICE
ADJUSTED
GROSS
ACRE OF
LAND/
PRICE
PER ACRE
S's or MICR 18
west of tiVCR 23
IthC'X13-1227
lot C-
7 WIN of WC:R 23
~Huth of WCR
I8
RI:('X I3-127
I.ul A
Golden Buena
Cire.ilrr Front
Range flames
Cioltleu !locus
AggreNatrsi
(irontcr rront
Runge 1Imo
3 'WS of Welt 23 D&C Neale
'r: mile wuth nl' Ncy'va
VCR IS Itothig.ucz.
RIi('X I6-89
412015
49113864
W15114
4051580
$70.000 578.750
¶70.9410
.7%
S7II,201)
140/
$23,160
3.401
$223,880
COMMENTS
Row lots; with mountain
and reservoir t iew sold at
voluinc discount For new
!louses built hy buyer
3131;17
4291302
$ I90,999 $201.,00() 9,411
$21,36O
+8.5%
4 WIN or WC'R 23 Pioneer Lund:
!, mile south of ,1 & N tiddler
111'('1( 11t
RI; 51111 I.at A
8/29112
3869679
590,4100 $115.200
514.400
11I°o
5 W/S of WCR 23
% li]IIe south
11'CR 1)t
R[? 34)1•I I.ut A
'Pioneer Land?
Justin Cole
4.7.136
3378734
6 9'1S of WC'1t 23 Pioneer loud/
'A mile south :CO (lordlier
of WCR 111 I lcnnes
RI{ 5010Lot A
&I
378995')
$199.900 $ 199.000 9,171
521.700
-0-
Lot wen ueIt and septic
hut winatural ga and
deed re 4rieting home
size to MOW than 2.01111•
SI: and nun -modular
design
Lul with mountain views
I,t,l with pond and
mountain vievtxun which
nc1t home was built in
21x)7 at the Iasi market
perk
5811.000) S 107.211() I 8.58,
S 12.495
I nr with mountain views
at:quint by builder who
sold nevi how in 20112
L
About !'_ mule hte eeders
northwest of the Ratty
11'i'It 14V.
intersection with
US 85 along the
west bank of the
So. Platte River
LISTING
1—CC7(11
rvvo wcIl permitted lots
with river frontage that
arc raised out of
Ilriodplain and u xes:ed
)elan gravel road one
mile upstream front
subject offered together
JOHN DcRUNUS MM. AJ-GRS 6117
nts cmbay* pemansdYrkcaulpurt c rare -r{ qfrr‘: L'
of er use Se US Win Ili N diring 'it.1i 4 vcsz ink A' K.4:i +` i4 n ne i•.K rim
r I beAr PS D NagaAVcrr
JOHN l , DeRUNGS, MAI Al -ORS
Land Sales I and 2
A homebuilder acquired two home sites within about six months of each other near the
intersection of Weld County Roads 18 and 23. Mountain views to the west are available
here on this exposed ridge overlooking thc river valley about a mile east of subject.
They report that they are an exclusive buyer of sites from this seller w ho has gravel
mining operations in this part of Weld County and obtain these sites through county
approval in a rural exemption process, They brought adjusted lot prices of $78,750 and
$111,200 for 3,4 -acre sites in a raw condition without scnvices. Una per gross acre basis,
that yields indications at $23. 160 and $23,880. New homes which arc later described in
this report were built and sold on these lots.
Land Sale 3
13y far, the most recent (March 2( 17) sale lies on the west side of WCR 23 about V, mile
south of WCR 18. The seller's agent reports that this buyer paid the full price of
$199.999 within a very short time of its offering raising the prospect that a higher price
might have been obtained. After it was created by a rural exemption, these 9,41 acres
were offered subject to restrict ions on home size (over 2,000 SF) and construction (non -
modular). Natural gas is available in this county road for extension but no utility services
were in place. Its indication after adjustment is $201,000 or at the rate of $21.360 per
acre.
Land Saks 4, 5 and 6
Another owner created three lots and offered them for sale in the last dozen years along a
'4 mile stretch of Weld County Road 23, just to the south. Near the peak of prices before
the last recession. just over nine acres was sold in 2006 for 5199.000 or almost the same
price that Land Sale 3 very recently went fbi _ That suggests that the reported shortage of
available sites here has pushed prices at least to pre -recession lever and no adjustment
was applied. A new home was subsequently built on Land Sale 5.
By comparison, a home builder paid $80,000 for about 8'A acres in August 201 l before
recovery in prices had begun or at 60% of current levels, Eight acres nearby brought
$90.000 a year later in August 2012 as some improvement had begun. But those
indications at about half of what more recent sales show suggest that the time adjustment
probably understates the rise in values at the beginning of the recovery.
- 30 -
JOHN F. DeRUNGS, MM hl-(ittS
Listing
This offering of vacant land is just north an modern log home built in the 19908 above
the level of the nearby flood hazard area. It is reached from WCR 141AA via an unpaved
gravel road just one mile upstream from subject. Comprised of two lots at 2.5 and 1.6
acres. it was first offered about two years ago with the home Ior $130,000 each, or
$260,000 total, with a well permit in place and an available water tap that would cost
$26.500. That offered price then was increased to $187,500 each. or $375.000. and with
more recent documentation regarding the extent of that flood area, the seller is very
recently quoting $425,000 for this vacant properly as a single site.
That asking price approaches what brand new homes have recently sold for nearby and is
well above the pattern of other recent sales show making it unlikely that it will sell
anytime soon. I note that asking prices tend to exceed the eventual negotiated price after
customary negotiation.
LAND SALES ANALYSIS
l inlike the subject site. none of these sale sites had private utility systems in place ie septic
systems and wells. Listing agents I spoke to estimate the costs of obtaining service ranges from
about $25,000 to $45.000 t'nr permits, drilling and equipment. or $35,000 average. Accordingly.
my conclusion has been adjusted by that amount to account for those systems already in place on
the subject site.
Sales 1, 2 and 3 are the most recent sales transferring within about 32 months of the effective date
of value from Septemhet 2014 to March 2017. Sale I and 2 sole( for $70,000 each and Sale 3 sold
far $199,000, leased on market trends published by IRK, (an on-line residential real estate
service that tracks sales by lip code), improving market conditions have yielded an average
incfvasc in sale prices of homes during this period of about '.' °fin per month. I have applied a
parallel increase in land prices as well during that period yielding sale prices of 579.100, $81.200
and $201.000. Expressed on a psf basis indications of $23,160, $23.880 and $21.360 per gross
acre provide a comparatively narrow range despite the difference of about six acres in land area.
As indicated, Sale 5 was included only to show that the market for land has reached pre -
recessionary levels. Sale 4 and 6 show the extent to which that recent market uptick has occurred
but since it understates the adjustment since about five years ago. it is the least useful indicatory.
3t-
JOHN F. DeR1IN('rS, MM Al-GRS
AS -IS (LAND VALUE) CONCLUSION*
After time adjustment, unit prices ranged from $12,495 to $23,880 per gross acre with a mean of
$19.500 per acre and a median of $21.530 per acre. But using the best comparables, (nos. 1.2
and .3) produces a mean of $22,800 per acre and a median of $23,160 per acre.
In my opinion, the indicated market value of the subject property should be based on a unit value
of $22.000 per acre, yielding an estimated value at $110,000, (before adjustment for utility
systems) calculated as shown below:
5.0 gross acres x $22,000 per acre — $1 10,000
That eunelusiun presumes that an exposure period of six to nine is necessary, `f'hat period is
based on those circumstances reported for tlee comparables and prevailing market trends that
show demand for vacant land is on the upswing.
Finally.I have adjusted this price of raw" land for the added presence of private well and septic
systems in place reported to currently cost an average of $35,000 to furnish. That brings my final
conclusion to $145,000 as show n below.
Land Value: S I 10,000
Plus Private well and septic
Indicated As -is Market Value: $145,000
(Land Value)
- 32 -
JOHN I'. DeIU.sNGS, MM Al -CRS
VALUATION - AS -l1" 1T WERE UNAFFECTED BY FLOODING
IIOME SALES DATA
As considered in the home sales summary that appears on the following page, 1 have selected the
seven most comparable sales that have occurred in this part of Weld County northwest of Fort
Lupton And with one exception all are similarly within a e/4 mile of the South Platte River. I
included one home sale located on the cast side of Fort Lupton because it was the only one that
featured a walk out basement design like subjects that has recently sold even by searching in a
wider area. Otherwise, these are mainly modern one-story (some with an upper level} ranch
homes built in the last thirty years on three acres or note and cob Iwo lacked basements. I fecund
no sales of homes with basement flood damage at this location but that is no surprise as it tends to
support local realtor claims that such properties arc not marketable. I have also found no active
listings ol`any homes for sale but attribute that to the general tack of inventory that characterizes
the current market. Auer deducting tiny concessions paid b1' the butter. unadjusted sale prices
shown ranged widely front $275.000 to $492.500.
These sales mainly occurred within 24 months of the effective date of value from June 2015 to
October 2016. As described in the previous section. improving market conditions have yielded
an average increase in sale prices during this period of about 'A% per month. Atter adjusting
even older stiles for t ime by that rate. these sale prices then supply indications that narrow the
range somewhat from $32,000 to $546,675. Less the basement area, the price -per -square (PSI')
is from about $151) to over $270 PSF. hut based on a comparatively wide range of from I,60R to
2.514 SF in home sires.
Because that puts the subject improvements at 1.575 SF just below the lower limit of the range, I
have relied on indications from overall adjusted sale prices rather than unit prices in my
valuation. l lowever, because of its design with a walk out basement. the perceived amount of
living area is somewhat increased. Consequently, comparison using unit prices still supplies a
test of reasonableness to my conclusion using overall sales prices.
-13•
JOI1N F. DeRUNGS, MM Al -ORS
COMPARABLE HOME SALES
SALE
LOCAT ION
SALE DATE ACTUAL TIME
GRANTOR/ COUNTY SALE ADJUSTED
GRANTEE DATA PRICE PRICE
ADJUSTED
ABOVE
GRADE COMMENTS
LEVELSF/
PRICE I'SF
1 10624 WC. -11,
18
2 7699 WCR 23
3 11566 WCR
I8
4
11586 WCR
Is
Greater FR
1 lomcti�
S&1) Srhhnser
10/4t I6 $445.000
4242661
$460,575
+3,5%
1,805!
$255.20
Greater Ilk
Homes:
b&S Salcerio
V&K
Sorenson/
I3&R Mcrriu
9'1&'I5 $428,500
.1139432
111,30;15 $356300
4161743
$471350 1334/
$271.80
. 10%
$386,80{1 1.608/
$240.55
8.5%
Brand new 3 13K 2BA
ranch home with 2 -car
garage and unfinished
full basement on 3.4
acres
Brand new 3 BR 214
BA ranch home with
2 -car garage and
unfinished full
basement on 3.4 acres
60 -year old 11f: -story.
4 BR 2BA updated
1arni house w/o
hasemcnt or garage on
5 acres
I.C; Everist'
F&L Dodge
5 12341 WCR
11%
6;29115 $492,500
4120618
J&n Sinner/ 10:09115 $475,000
Nun 4149773
Crawford
$516,675
2,514/
$217435
$520.125 1.9.3ry:
$269.20
49.5%
6 7493 US 85
J Carithemf 9/4'12 $275.000
'I'L Crawford 3870647
7 17781 WCR
14
AR Perez/ 6'31I 4 $137,000
Craig Dowell 4021838
Buyer gutted 30 -year
old ranch home with 2.
car gage and
basement on 8.8 acres
overlooking future
lake
20-yr old 3 BR 2BA
ranch home with 3 -car
garage and no
basement on 5 acres of
rover frrontatw
$352,000
o 28%
2.307/
$152.60
80 -year old two-story
home with basement
on 2.6 acres near
subject
$513,475
+17.5%
2,025/
$253,56
70-yr old 3BIt 2!6.. A
ranch home witlt walk
out bu3ement and 3 -car
garage on 5 acres
JOHN DLRUNGS MM. Al-CiRS 6'17
P Mi4s4Vinu tt1atmr4L1►k flaw Sur ,Arl11".iSmut
filiallma CA Oat ten ° : S Ca tit n of Mil f!t
ti!
DVS WO PI harm gr pup mot
I O.Md1I 0 kiln coy UMW ; sr
WO S.1 IV 'WA '4 erp.S h ON
44 WAD Ctur, raitIOR
JOHN I. DeRUNGS. MA1 Al-GRS
Sale 1 and 2
The highest adjusted unit prices arc for these two brand new homes described as Sale I at
$460.,575 and 5255.20 PSI and Sole: 2 at $471.350 and $271.80 PSF. They sold in the
last year or two about one mile west of subject situated well above the river valley giving
them mountain views. Both were on 3.4 acre sites that are part of a rural subdivision ()la
farm on the southwest corner of Weld County Roads 18 and 23. The homes an only 10
to 15% larger than subject at 1.805 SF and 1,714 SF and have attached two -car garages
and full, although unfinished. basements. As brand new homes. they supply the best
indicator of the upper limit of the indicated value range found on a per -square foot basis.
Sale 3 and 4
About 1/3 mile east of WCR 23 on the south side ofWCR 18 are two homes that sold in
mid 2015 at a location overlooking the valley and closer to subject. Sale 3 at an adjusted
sale price of $386.800 and $740.55 PSI is what was paid for a home that (at 1,608 above
grade SF) is most similar to subject by sine. That sale price was after a $3,500
concession was paid by the buyer for closing costs. The agent reported receiving
multiple offers for it and so it sold "as -is" at the list price.
Its 4 bedroom 2 bath layout is closely comparable but while it featured an upper level, it
had no basement or storage garage. On five acres of land, portions of this remodeled
farm house were 60 years old and although it had been updated with a new roof, I IVAC
and water purification system, it had not been fully remodeled. Overall. that suggests
that subject should bring more than those indications on an overall sale price and square
footage basis, "['he subject offers more livable square footage with the walk -out basement
area and is a newer home with a detached storage garage.
Adjoining Sale 3 In the east is at ranch home with a basement ofmore comparable viruage
(to subject) but at 2.514 S} on 8.8 acres, it is much larger. It has an appealing location
overlooking what will be water storage in this currently active gravel mute. While it did
not appear in the MI.S, Don long with Mel -'ceder Realty reported that he sold it for the
gravel mine operator at this location. White it brought $492,500 he reports that it was
poorly constructed and the interior had to he bully gutted by the new owner. That still
puts Sale 6's overall adjusted sale price of $546,675 ai the upper limit (because o1' its
sire), but it yields a unit value of $217.45 PSF or at the lower end of adjusted unit prices.
-3t-
JOHN F DeR11;J(OS_ MA1 Al-GRS
Sale 5
This sale property is particularly notable for its location just one mile upstream from
subject which shares its quiet and secluded betting fronting the South Platte River.
Where is it reached from WeR l4Y via an unpaved gravel road. a home vvas built in the
1990s ( like subject) above the level of the nearby tlocxl hazard area.
According to the listing agent, it was part of a large holding of land originally acquired
by i_('i liverist es pan of their gravel mining operation. To his knowledge, it has never
flooded even during the 2013 event which inundated other parts of Weld County. It was
later subdivided and three lots. one of which was improved with this home. were then re-
sold to one of its associates in 2013 in a non -arms -length transaction.
The more recent October 2015 transaction involves the southerly five acre lot only on
which That 1.932 -SF three bedroom two bath log home sits. Its main level is similar in
size to subject but it has an upper portion and no basement. Superior features include a
three -ear attached garage, central air conditioning and district (CWCWD) water but it is
of comparable vintage, The home itself was olrered Mr $495.000 and it sold for
$475.000 within two months to buyers who valued its secluded location. Atter
adjustment, an indication for Sale 5 is at $520, 125 and $269.20 PSI'. Altogether, it
received the most weight in my conclusion because of those obvious similarities by
location. size and age.
Sale 6
Sale 6 at $352,000 and $152.60 had the only readily accessible location reached just off
I ligliway 85. like subject. White just a short distance away. because this sale is almost
live years old, it ocetirred before the market's recovery began_ Major portions of the
home are also about eighty yours old putting it well below other indications and at the
lower limit of the range. Despite its proximity, I have included it here for information
only because of its proximity' to subject.
Sale 7
The only home that has recently (6/14) sold with a walkout basement like subjects lies
about five miles east of eeniral Fort Lupton. White it was also about twenty years old
and set on live acres. it offers sigiaiiicantly more square footage (at over 2,000) and had a
37.
JOHN l', UeRUNGS, MAI AI -(;RS
three ear garage, Those superior features appear to play more of a role in the adjusted
sales price indications of $5 13,475_ or $253.56 PSF than its walk -out basement. design.
HOME SALES ANALYSIS
The sale of a modern home with the seclusion of South Platte River frontage at this location is
quite rare. That led nwte to give primary weight to Sale 5 at $520,125 in my conclusion. While
quite different by construction. the main level square footage is similar and additional living area
on a partial second story is akin to that back (west) portion of subject's walk -out basement that
receives ample natural light. It also featured an attached three car garage but has only one small
outbuilding,
White Sale 7 is notable as the only recent sale of a similar vintage home ►vith a walk -out
basement design like subjects, I believe that at $513.475. it exceeds what subject can bring.
Its main level alone at over 2.000 SF is 30% larger and it also had an attached 3 -car garage. 'that
more than offsets an inferior location east of rt. Lupton and well away from the South Platte
River
Subject's value is also tempered by indications for brand new housing on similarly -sized sites
shown as Sales I and 2, But while they offer mountain views, these are exposed locations with
little privacy of? this county road. Both had main levels that are 10-15% larger than subjects and
attached garages, but meanwhile their strictly below grade basements were sold unfinished.
Overall, as among the most recoil sales, their adjusted sales prices at $400,175 and Sf171,350
received seconder weight in my conclusion.
Indications from the remaining sales are outliers, Near the low end of the range is an updated
familtousc described as Sale 3. At below $400,000, it dates back 60 years and had no basement
or garage. Next door to it, the largest property considered in this analysis (at over 2,500 SI= on
8.8 acres) brought the high end of the range. F.vcn though it was subsequently gutted, its adjusted
price at $546,675 is much higher and justified by that large size.
- 38
Jul -1N F. lleRIINGS. MAI AI•GRS
HOME VALUE CONCLUSION - AS -IF IT WERE UNAFFECTED BV FLOODING
Applying the aforementioned weight to Sales 1.2 and 5 brings an indication nf$500.000 rounded
as shown below putting subject's estimated value within 3 to 8% of most oldie adjusted sale
prices shown herr. Using the adjusted unit prices from these three sales at from $263.80 to
$271.80 PSr this conclusion suggests that subject's perceived livable area is about 20% higher. or
325 SF more than the actual 1,575 SF found on the main level. That appears to be reasonable
given the day -lighting provided by the walkout design to the from of the basement level and so
passes the 'test ofreasonableness". I have also considered an adjustment for the out buildings of
various remaining utility. But because some of these types of structure were also found in the
sales comparables. the impact on overall value in this case is not necessarily measurable.
Weighting of Sales by Sales Price;
Sale 1 - 17% x $460.575 -- $ 78.300
Salo 2. 17%x$r17I,350$ 80,130
Sale S - 66% N $520.125
Indicated Value; $501.710
Rounded: $500,000
That conclusion presumes that an exposure period of three to six months is necessary. Thai
period is based on those circumstances reported for the comparables and prevailing market trends
that show demand for homes is strong.
39
II
JOHN F. DeRUNGS. MAI M-GRS
CER'1'1FICATION
I certify that, to the hest of my knowledge and belief
-the statements of fact contained in this report are true and correct_
-the reported analyses, opinions, and conclusions are limited only by the reported assumptions and
limiting conditions. and are my personal, impartial, and unbiased professional analyses. opinions, and
conclusions.
-I have no present or prospective interest in the property that is the subject of this report, and i have
no personal interest or hia ,s ►vith respect to the parties involved.
-I have not performed services a, an appraiser regarding the property that is the subject of this report
within the three-year period immediately preceding acceptance of this assignment.
-I have no present or prospective interest in the property appraised, and no bias with respect to any
property that is the subject of this report or to the parties involved with this assignment.
-rny engagement in this assignment was not contingent upon developing or reporting predetermined
results.
- my compensation tin -completing this assignment is not contingent upon the development or
reporting of a predetermined value or direction in value that favors the cause oltlte client, the amount
of the value estimate, the attrrirrme:nt of a stipulated result, or the occurrence of a subsequent event
directly related to the intended use of this appraisal.
- flue appraisal was made and prepared in conthrmit`° with the Appraisal Foundation's ( Worm
Stirrtrkrrds' of PrnfeiisinaaalAfrpraisa! Practice.
the use of this report is subject to the requirements of The Appraisal Institute relating to review by its
duly authorized representatives_
-as of the date ()fetus report. I. John F, DeRungs, MAL Al -ORS have completed the requirements
under the continuing education pmgranr of The Appraisal Institute.
-1 have made a personal inspection of the property that is the subject ofthis report
-no one provided sig,nihie;rrnl professional assistance to the individual signing this report,
June 30.2017
Respectfully submitted.
of F.())
John F. DeRungs, MAI. Al -CRS
Colorado Licensed Appraiser CG13 16679
- 40
Luoking east along unpaved drive used io reach the propert} from the highway
Ditch crossing of unpaved iacccss road to reach the southeast comer of the property
view of -improved poRi 01 property set along the west ,icotth ditch
A".
® )
(R
y o ."
. -
» % t
I, ' �
�
m !11
.0\
No kast dakon of idn«
Front entrance reached from wood deck on east elevation
Southwest elevation of residenceshows walkout tusenteul design
Walkout basement entranrL
Kitchen
living room
Main levxl bedroom
Bathroom
Good fenestration on west wall of walkout basement
Bedrooms and laundry room open off basement (amity room
HispiC
vir
Typical finish in basement bedroom
Portion of unfinished basement along west side used as laundry room and storage
Storage garage with concrete apron
I J46` • . .
d�l'v r
W S. 1). fir,
Interior vieu of storage garage
;
Outbuildings and residence beyond
Outbuilding
Outbuilding
GL&A
October 6, 2015
Ross Bachofer
P.O. Box 652
LaSalle, Colorado 80645
GANSER LUJAN 8r ASSOCIATES
Hydrogeolagical and Environmental Consultants
RE: EEC RAS Model Results of the Hydrologic Impact of LG Everist Mine on
Bachofer Property; 7525 Highway 85, Fort Lupton, Colorado
Ganser Lujan and Associates (GL&A) has quantitatively evaluated, using the HEC RAS
model, hydrologic flood impacts on the Bachofer Property caused by aggregate mining at
the adjacent LG Everist Mine site. The Bachofer property is located on Lot 6 in the SW
1/4 of the NE 1/4 Section 30, T2N, and R66W adjacent to and along the east side of the
South Platte River directly across from the open -pit mine site. The property is situated
between the river and Highway 85 and 1/2 mile south of Weld County Road (WCR) 18.
The property is triangularly shaped and wedged between the river on the west and the
Plateville Irrigation and Milling Company Ditch to the east. The house sits on a flat area
on the eastern edge of the trees as shown in Figure 1 below.
Summary
Flooding of the lower level of the Bachofer residence has occurred repeatedly since 2004
when an underground shiny wall, soil berms, and soil mounds were constructed in and
around the open -pit cells at the mine site. Since that time, flood events on the Bachofer
property have occurred fairly regularly with flows greater than approximately 6,400 cubic
feet per second (cfs) (GL&A, 2013). In the past, prior to installation of the slurry walls
and berms and creation of soil mounds in the mined areas, the flows reached over 10,000
cfs and no flooding occurred. Discharges of 10,000 cfs and less that have caused
flooding since 2004 are significantly below the 100 year flow event of 29,000 cfs as
determined by the US Army Corps of Engineers for the Flood Insurance Study for the
Town of Fort Lupton (U.S. DH&UD FIA, 1978). The lower level of the house sits
approximately 1 foot above the 100 year flood plain level of 4874.0 feet as determined by
Weld County for obtaining a Flood Hazard Permit. As such, the homeowner was not
required to have a permit because his land was above the 100 year floodplain level.
There was also no history of flooding on this property prior to that time.
The results of these model simulations appear to support the historical and current
observations that construction activities at the Everist mine have not only altered the
hydrologic regime of the South Platte River but have also changed the morphology and
topography of the original floodplain on the west side of the river causing a constriction
and restriction of flood flows. As a result, flood flows, which in the past could spread
and dissipate across the pre -mining flood plain with no effect on the Bachofer property
on the east side, now flood his land and house.
6692 W. 96th Place, Westminster, CO 80021, 710-272-2306, Falcaitne. rrirtr mr.•rist. et
GANSER LUJAN & ASSOCIATES
Hydrogeological and Environmental Consultants
In addition, the present model results by GL&A differ significantly from previous
modeling results conducted by Deere and Ault Consultants, Inc.(D&A, 2008) that were
submitted to Weld County Public Works Department. These previous results by D&A
showed negligible impact of mining operations on the water level surface from the 100
year event when compared to pre -mining conditions. Modeling results from GL&A
indicate a significant difference (approximately 2 feet) in surface water elevations
between pre -mining and existing mining surface topography. In addition, model
simulations using the 5 -year event (7,900 cfs) are close to the 100 year event determined
by the Flood Insurance Study (FIS) for the Town of Fort Lupton (U.S. DH&UD FIA,
1978).
As part of this modeling work to demonstrate the causal link between mining
development and subsequent flooding of the Bachofer property, GL&A performed the
following tasks:
+ Simulated surface water flow through a series of 7 cross -sections that extend east -
west across the South Platte River from the Everist property to the eastern side of
river approximately 1500 feet upgradient and 3000 feet downgradient of Bachofer
property.
• Developed cross -sections from existing topographical surveys presented in a
Deere and Ault report submitted to Weld County dated December 9, 2008. Data
from this period was used to develop cross -sections representative of current
mining topography. Pre -mining data were obtained from the USGS digital
elevation model with a 2 -foot contour resolution based on the 1950 Fort Lupton
7.5 minute quadrangle map.
• Simulated peak discharges of 7,900, 10,000 and 29,000 cfs for the 5, 10, and 100
year events, respectively.
• Prepared a summary report describing results of the HEC-RAS model and rebuttal
of Deere and Ault report.
Bachofer Property
The Bachofer property is located on Lot 6 in the SW 1/4 of the NE 1/4 Section 30, T2N,
and R66W adjacent to and along the east side of the South Platte River. The property is
situated between the river and Highway 85 and 1/2 mile south of WCR 18. The property
(Figure 1) is triangularly shaped and primarily wedged between the river on the west and
the Plateville Irrigation and Milling Company Ditch to the east. The house sits on a flat
area on the eastern edge of the trees as shown in Figure 1 below. This flat area forms the
post -Piney Creek alluvial terrace deposit and is comprised of interbedded sand and
gravel. The area is heavily vegetated with cottonwood, willow, and occasional elm trees,
tall bushes, and grass that extend from the house to the bank of the river.
- 2 -
GAN ER LUJAN irk ASSOCIATES
Hydrogeological and Environmental Consultants
Figure 1
According to Mr. Bachofer, the lower level of the house has repeatedly flooded since
August 2004, when the flows at the Fort Lupton Gauge reach 6,400 cubic feet per second
(cfs), whereas in the past the flows reached over 10,000 cfs and no flooding occurred
(QL&A, 2013). The elevation ofthe door on the bottom level floor has been surveyed by
a licensed surveyor and is at 4875 feet above mean sea level elevation (amsl). The 100
year flood plain level determined by Weld County for obtaining a Flood Hazard Permit
on 5/19/1987 was 4874.0 feet and as such Mr. Bachofer was not required to have a permit
because his land was above the 100 year floodplain level. There was also no history of
flooding on this property prior to this time.
HEC-RAS Modeling
Surface water modeling of the South Platte River adjacent to the Bachofer property was
performed using the IIEC-RAS model to simulate the hydrologic impact of aggregate
mining on flows in the river and its direct impact on the Bachofer property. The HEC-
RAS model, developed by the US Army Corps of Engineers, is designed to perform one-
dimensional hydraulic calculations on a full network of natural and constructed channels
(U.S. Army Corps of Engineers, 2010).
GANSER LUJAN & ASSOCIATES
Hydrogeological and Environmental Consultants
Study Area and River Reach
The study area and river reach along the South Platte River are shown on Figures 2 and 3
and extend approximately 4,500 feet upstream along the channel from WCR 18 and
approximately 1,500 feet upstream past the Bachofer property. A total of 7 cross -
sections (presented on Figures 4 and 5) extend from approximately the Lupton Bottom
Ditch on the west side of the study area to the east side of the river. The eastern extent of
the cross -sections varies from 300 to 1800 feet beyond the South Platte River channel.
Model Flow Rates
The model was used to conduct a flood plain analysis for the 5-, 10-, and 100 -year design
events. The 100 -year flood event (29,000 cfs) was used as the baseline flood event as
previously modeled by Deere and Ault in their Flood Hazard Development Permit #355
submitted to Weld County December 9, 2008 (D&A, 2008). This peak discharge value
was used in the Flood Insurance Study (FIS) of the Fort Lupton area to generate the
existence and severity of flood hazards. The results of the study delineated the extent of
the 100 year event. As part of the FIS, hydrologic flood plain analyses for recurrence
intervals of 10, 50, 100, and 500 years were also performed. These recurrence intervals
correspond to 10,000, 22,000, 29,000 and 52,000 cfs, respectively.
In order to determine the 5 -year event for this modeling study, a flood frequency curve
was created with the recurrence intervals and discharge values from the FIS data. The
plot of the flood frequency curve is shown below with a best fit regression line.
Discharge for the 5 -year event was calculated from the regression equation on the plot
below to be 7,900 cfs. This peak discharge value is slightly above the value recorded at
the Fort Lupton Station , which currently floods the Bachofer property.
Flood Frequency Curve for South Platte River by Fort Lupton
100,000
110000
1,000
1
y 0.421x+3.602
-.992
10 100
Recurence Interval (years)
1000
-4-
GANSER LUJAN & ASSOCIATES
Hydrogeological and Environmental Consultants
Pre -Mining Topography,
In order to accurately conduct a flood plain analysis prior to the existence of the Everist
Mine, it was necessary to obtain topographic data from a period before mining activities
commenced in 2004. These data need to reflect an undisturbed topographic surface
before any construction or earthwork began.
Pre -mining topographic data for this model were obtained from the 7.5 -minute digital
elevation model (DEM) data files from the USGS national elevation dataset (NED).
These data are digital representations of cartographic information in a raster form. DEMs
consist of a sampled array of elevations for a number of ground positions at regularly
spaced intervals. These digital cartographic/geographic data files are produced by the
U.S. Geological Survey (USGS) as part of the National Mapping Program, DEM data for
7.5 -minute units correspond to the USGS 7.5 -minute topographic quadrangle map series
and is based on 30- by 30 -meter data spacing with the Universal Transverse Mercator
UTM projection. Each 7,5- by 7.5 -minute block provides the same coverage as the
standard USGS 7.5 -minute map series.
The NED is a seamless raster product primarily derived from USGS 10- and 30 -meter
(DEMs), and, increasingly, from higher resolution data sources such as light detection
and ranging (lidar), interferometric synthetic aperture radar (ifsar), and high -resolution
imagery. The 3-D raster data file is from the same period of time that the 7.5 minute Fort
Lupton quadrangle map was generated and was used to create a 2-D contour map with 2 -
foot contours by interpolation algorithms using Autocad+r1 Civil 3-D.
The pre -mine topographic map and cross -sections used as input data in the model are
shown on Figures 2, 4 and 5. The cross -sections depict the pre -mining topography with a
dashed line and existing topography with a solid line.
Existing Topography
The 2008 aerial survey topographic map submitted in the Deere and Ault report (2008)
was used as the current topographic surface of the study area. The map is presented on
Figure 3. Ineffective flow areas were used to represent natural and man-made
depressions.
Manning's Roughness Coefficient
Manning's roughness coefficient "n" was calculated from USGS curve rating data
measured at the Fort Lupton (U.S.G.S., 2013) gaging station located '/2 mile to the north.
The curve rating data were obtained over a 10 -year period from 2003 until 2013 and
contain nearly 100 measurements. These measurements include channel velocity and
mean depth. The average channel velocity and average mean depth were calculated from
these data. Manning's "n" was back calculated from Manning's equation given by:
2 1
1.49R3S=
n
-5-
GANSER LUJAN & ASSOCIATES
Hydrogeological and Environmental Consultants
Where: v= velocity (ft/s)
R= mean depth (ft)
S— slope (dim)
The average velocity from the rating curve is 2.41 ft/s and the average mean depth is 1.40
ft. The slope was calculated to be 0.0016 using the total linear distance of the river
channel and the difference in elevation between the up and downstream boundaries of the
model. Solving the equation for n, n = 0.031.
This "n" value is lower than the one (0.035) used in the Deere and Ault model but is
closer to the value (0.030) used in the FIS study. However, the "n" used in the GL&A
model for the overbank values is 0.040 and was considered conservative compared to the
overbank values used in the FIS model that ranged from 0.045 to 0.120. A lower
roughness coefficient gives a lower water surface elevation. Given the gravelly nature of
the overbank material and abundant vegetation along the banks of the river channel that
includes large cottonwoods, an "n" value of 0.040 appears more appropriate than 0.035
that was used in the D&A model.
Model Boundary Conditions
Boundary conditions are necessary to establish a starting water surface elevation at the
upstream and downstream ends of the river system. In a subcritical flow system,
boundary conditions are only required at the downstream end of the river. It was
assumed that subcritical conditions exist in the river reach for this project. The slope of
the river along this reach is low enough (0.0016) to preclude high velocity flows and
supercritical flow. Under these model conditions, it was assumed that the river flow
regime is at subcritical conditions and the critical depth was calculated by the model
during model simulations.
Model Results
A summary of model results are presented in Table 1 and complete model output results
are presented in Tables 2 and 3. HEC-RAS cross -sections of model results are shown in
Figures 6 and 7. These results indicate that existing mining conditions in 2008 (based on
the 2008 aerial survey conducted by L.G. Everist) result in a maximum change of
approximately 2 feet in the water surface elevation compared to pre -mining conditions
for the 100 -year flow event. At cross-section 3193.73, which transects the Bachofer
property, there is 1.86 -foot change in water surface between pre -mining (1978) and
post -mining (2008) topographic surfaces for the 100 -year flood event.
-6-
GANSER LUJAN & ASSOCIATES
Hydrogeological and Environmental Consultants
Table 1
HEC-RAS Modeling Results for 5, 10, and 100 Year Flows
Pre -Minim Topography
2008 Minim Topography
Change in
Water
Cross -Section
Profile
Q Total
Water Surface
Energy Grade
Water Surface
Energy Gmde
Surface
Elevation
Elevation
Elevation
Elevation
Elevation
(cfs)
(ft)
(ft)
(ft)
(ft)
(ft)
4700.78
PF 1
7900
4873.27
4873.34
4874.25
4874.27
_ 0,98
4700.78
PF 2
10000
4873 48
4873.56
4874.58
4874.61
1.1
4700.78
PF 3
29000
4874.85
4874.94
4876.21
4876.26
1 36
4233.11
PF 1
7900
4873
4873.04
4874.14
4874,16
1.14
4233.11
PF 2
10000
4873.22
4873.25
4874.49
4874.5
1.27
4233,11
PF 3
29000
4874.57
4874.63
4876.1
4876.13
1.53
3193 73
PF 1
7900
4872.43
4872 46
4873.91
4873.94
1.48
3193.73
PF 2
10000
4872 66
4872.69
4874.26
4874.29
1.6
3193.73
PF 3
29000
4873.95
4874.01
4875.81
4875.86
1, 86
2354.37
PF 1
7900
4871.88
4871.92
4873.6
4873.65
1.72
2354.37
PF 2
10000
4872.1
4872.14
4873.97
4874.01
1.87
2354.37
PF 3
29000
4873.38
4873.44
4875.42
4875.49
2 04
1388.01
PF 1
7900
4871.01
4871.11
4871.61
4872.41
0.6
1388.01
PF 2
10000
4871.35
4871.42
4872.17
4872.94
0.82
1388.01
PF 3
29000
4872.54
4872.63
4874.09
4874.48
1.55
634.85
PF 1
7900
4869.84
4870.2
4870.57
4870.91 _
0.73
634.85
PF 2
10000
4870.11
4870.57
4871.3
4871.62
1.19
634.85
PF 3
29000
4871.63
4871.85
4873.08
4873.39
1.45
188.69
PF 1
7900
4868.84
4869.32
4868.02
4869.74
-0.82 _
188.69
PF 2
10000
4869.04
4869.54
4868.72
4870.57
-0.32
188.69
PF 3
29000
4869.91
4870.79
4871,88
4872.64
1.97
The water surface elevation corresponding to this change, as shown on Table, 1 is
4875.81. As previously discussed in GL&A report dated October 4, 2013, the 100 -year
flood plain level determined by Weld County for obtaining a Flood Hazard Permit on
5/19/1987 for the Bachofer property was 4874.0 feet. As such, the elevation determined
by this modeling for post- mining conditions is well above the previously established FIS
100 -year flood plain elevation.
At the request of Weld County, model simulations were also conducted for lower flows
that include the 5 and 10 -year events. The 5 -year event (7,900 cfs) approximates flow at
the Bachofer property, which has been currently flooding the lower level of his house.
The flooding has been occurring since 2004 when L.G. Everist lined their pit with a
slurry wall. As can be seen on Table 1, the 5 -year event (4873.91 feet) for current mining
conditions is very close to the 100 -year permitted flood plain elevation of 4874 feet. The
model results show the 100 -year pre -mine surface to be 4873.95.
-7-
GANSER LUJAN & ASSOCIATES
Hydrogeological and Environmental Consultants
Model results for the pre -mining topographic surface (Table 1) for the 5 -year event show
a value (4872,43) well below flood elevations. Thus, the modeling results indicate that
the Bachofer property should not be flooding with pre -mining topography at 5 and 10
year flows. The model results are very similar (within 0.05 feet) at 4873.95 feet to the
established FIS 100 year flood elevation, which indicate that the model is well -calibrated
and that it is simulating real -world hydrologic conditions.
Discharge from the 10- year event (10,000 cfs) was also modelled to closely simulate
peak flows that occurred during flood events in September 2013 (10,300 cfs) and in May
2015 (10,500 cfs). Flooding occurred at the Bachofer house during both of these events
(Bachofer, 2015 Oral Comm.). As can be seen on Table 1, the model results validate
these events, At 10,000 cfs, the modelled water level elevation is 4874. 26, which is
measurably above the FIS 100 year flood plain elevation. Based on the 2008
configuration of the land surface, surface water levels at this flow now cause flooding.
Simulations using pre -mining topography show that the water surface for 10,000 cfs is
4872,66, which is well below the established flood plain level at the Bachofer property.
Figure 6 below shows output from the HEC-RAS model for existing mining conditions.
These cross -sections illustrate that the surface of the flood plain is highly irregular. The
high areas shown below may be interpreted as berms constructed around the reservoir
pits to prevent flooding. Cross-section 3193.73 represents the land surface that transects
the Bachofer property. As can be seen on this cross-section, there are five high areas that
are obstructing the flood plain and precluding dissipation of the flood waters across the
flood plain.
Figure 6
HEC-RAS Results
Existing Mining Conditions
2009MInIna Bachofer 6-12-2015
Rs. 47uQ7e
C PF3
NB PP
CGPF7
416 PF2
EGAN
'NS PF i
gaultl
Bard, Ste
Mut 3700 4000
S,,I 11n}
2008MinIng Bachofer B-12-2015
RS= 423311
EGPF3
416 PF3
GLPF3
06 'F 7
EOPF3
416 PP1
Govw
Barat$6
-8-
GANSER LUJAN Az ASSOCIATES
Hydrogeological and Environmental Consultants
Figure 6 (Continued)
200BMinino Bachorer8-12-2015
Rs. 235437
5
S
ww
2006Mining Bachafer8-12.2015
Rs. 318373
ea3rr
4885
WO)
EOPF3
yryg PIF3
858F2
Wa PF 2
EOPP1
17186 Pr !
riuund
e.WSf.
adman
3000
200BMIning Baohofere-t2-2015
RS= 514315
200BMIn' no Bachofer8.12.2015
R&= 188.89
_g_
GANSER LUJAN & ASSOCIATES
Hydrogeological and Environmental Consultants
Figure 7 below shows HEC RAS output for pre -mining conditions. Note the markedly dissimilar
topographic surface on the cross-section 3193.73 as compared to the existing mining surface as
shown in Figure 6. The uniform and relatively flat surface allows flood waters to spread across
the western floodplain thereby diminishing the probability of flooding on the east side of the river
channel.
Figure 7
HEC-RAS Results
Pre -Mining Conditions
Pre -Mining Bachafer6-12.2015
RS- 4=78
Pr -Mining Bachofar0-12-2015
RS= 318373
u
d
EMIT
1H6 PP]
EOPF2
vF` PA 7
EGPF1
y PF1
Ciewf
8.084
EGPF3
INS PF 3
EGPF]
vF PPS
EOPF1
1414 W1
rlculd
Bait ®e
841m (111
0 1800
r
463:-
4740
Pre -Mining Bac{Iofer6•12-2015
RS= 238437
44
2 21604)-.
3 r--- 5006
841m1e}
0 04
E2PF3
h PF 3
EGPF2
Wit PF2
EGPF I
WS PF1
Om Ind
Sink Eis
,10_
GANSER LUJAN & ASSOCIATES
Hydrogeological and Environmental Consultants
Figure 7 (Continued)
Pre -Mining Bachorora-12-2D15
RS= new
Pre -Mining Bachofer8-12-2D15
RS. 634.95
ESPF3
N6 PF3
EGPF2
AS PF2
EGPFI
NS PF
GeM4
Bak SIB
IMO
Pre -Mining Bachofer8-12-2015
RS= 19845
031+- 04 -
21110
EOM
04
a
X00
Contributions of Alluvial Flow
In addition to mining activities altering topographic relief and floodplain characteristics,
including the floodway and storage configurations, slurry wall construction around these
open -pit aggregate mines has created subsurface flow constrictions, which cause alluvial
groundwater to back up or mound behind these walls on the up -gradient side.
Groundwater levels tend to decline on the down -gradient side. As a result, mounding has
caused higher water levels than historic levels in the river alluvium upstream and
adjacent to the Bachofer property and, as a secondary result, has induced greater
groundwater flow into the river through this constriction. This phenomenon has been
noted in modeling studies by the USGS (Arnold et al., 2010) and show a mounding effect
where groundwater levels rise 2 to 4 feet on the south and east sides of lined pits across
the river from the Bachofer property.
-11-
GANSER LUJAN & ASSOCIATES
Hydrogeological and Environmental Consultants
Another modeling simulation shows groundwater levels rising 4 to 6 feet in the same area
around the pits. Although the mounding occurs on the west side of the river, flows are
diverted around the constrictions and raises water levels in the channel and on the east
side of the river with corresponding impact to the landowners located on the east side of
the river. As stated in the U. S.G.S. report (Arnold, 2010), the South Platte acts a sink to
groundwater when water levels are elevated in the alluvium. Hence, mounding can cause
increased groundwater discharge to the river, thus contributing to higher base flows in the
river. As such, a lined pit slurry wall will not only impact groundwater levels, but will
also elevate surface water flows in the river for any given storm flow.
Although the contribution of alluvial groundwater or "bank flow" in this study has not
been quantitatively determined by numerical modeling, it may be concluded that
additional alluvial discharge to the surface water system is occurring adjacent to the
Bachofer property, thereby contributing to the increase in river level. Due to the distance
to the gage downstream at the Fort Lupton gage, this localized discharge, which is part of
the combined bank flow and open -channel flow, would not affect the gage reading.
Hence, the results of the surface water model should be considered conservatively low,
since they don't reflect the total flow from local discharge to the river from groundwater.
Rebuttal to D&A Model Results
Pre Mining Topography
The pre -mining topographic map used by Deere and Ault in their 2008 modeling report
does not represent pre -mining conditions at the mine site. This topographic surface was
created from the more recent 2008 aerial survey data by altering natural and man-made
features from the surface. Hence, these data represent conditions after the mining began
and the land surface was significantly altered. To recreate a pre -mining surface from
present day data is mostly guess work and highly subjective. It cannot reflect a true pre -
mining picture of the land surface in this area along the South Platte River,
Model Not Calibrated to Site Conditions
The results of the D&A model for both 2008 topographic data and pre -mining data are
not indicative of or reflect present flows and stage levels in the river or established
baseline floodplain elevations. Model results show only negligible changes between
present and pre -mine simulations near Bachofer property under 100 -year flow event of
29,000 cfs, Yet the Bachofer property has been flooding above the 100 -year elevation for
the past 10 years or more with flows that are significantly lower than 29,000 cfs. Water
surface elevations from the model results should be tied to actual measured elevations at
the mine site that correspond to the 100 -year event. These results suggest that the model
is not calibrated to any real-time hydrologic data and as such is unreliable.
Manning's Coefcient
The D&A model used the same Manning's number (0.035) for both the channel and
overbank values. While this value is reasonable for the channel roughness and is similar
to the value that was determined by the U.S. Army Corps of Engineers and used in the
FIS for Fort Lupton, the FIS used a value that ranged from 0.045 to 0.120 for the
-12-
GANSER LUJAN & ASSOCIATES
Hydrogeological and Environmental Consultants
overbank flows. Given the gravelly nature of the overbank material and abundant
vegetation along the banks of the river channel that includes large cottonwoods, an "n"
value of 0.040 appears more appropriate than 0,035 that was used in the D&A model.
Water Level Surface Error
Water level elevations should decrease from upstream stations to downstream stations.
Between Station 60 and Station 50, the water elevation erroneously increases 0.34 feet.
Conclusions
• Results of the model simulations support the historical and current observations
that construction activities at the Everist mine have not only altered the
hydrologic regime of the South Platte River but have also changed the
morphology and topography of the original floodplain on the west side of the
river causing a flow constriction. Storm flows, which in the past could spread and
dissipate across the pre -mining flood plain and have no effect on the Bachofer
property on the east side, now flood his land.
▪ Results by D&A showed negligible impact of mining operations on the water
level surface from the 100 year event when compared to pre -mining conditions.
Modeling results from GL&A indicate a significant difference (an increase of
approximately 2 feet) in surface water elevations in the vicinity of the Bachofer
property between pre -mining and existing mining surface topography.
• Model simulations using the 5 -year event (7,900 cfs) indicate flood levels are now
close to the 100year event previously determined by the Flood Insurance Study
(FIS) for the Town of Fort Lupton (USDH and UD, 1978).
• The GL&A model was used to conduct a flood plain analysis for the 5-, 10-, and
100 -year design events, which corresponds to 7,900, 10,000, and 29,000 cfs, The
100 -year flood event (29,000 cfs) was used as the baseline flood event as
previously modeled by Deere and Ault in their Flood Hazard Development Permit
#355 submitted to Weld County December 9, 2008. Evaluation of only the 100 -
year event did not identify adverse impacts for the lessor and much more common
flood events, such as the 5- and 10 -year flow events.
• Pre -mining topographic data for this model were obtained from the 7.5 -minute
digital elevation model (DEM) data files from the USGS national elevation
dataset (NED). These data are digital representations of cartographic information
in a raster form. The 3-D raster data file is from the same period of time that the
1950 7.5 minute Fort Lupton quadrangle map was generated and was used to
create a 2-D contour map with 2 -foot contours by interpolation algorithms using
Autocad&11 Civil 3-D.
- 13 -
GANSER LUJAN & ASSOCIATES
Hydrogeologlcal and Environmental Consultants
• Modeling results indicate that existing mining conditions (based on the 2008
aerial survey conducted by L.G. Everist) result in a maximum change of
approximately 2 feet in the water surface elevation compared to pre -mining
conditions for the 100 -year flow event. At cross-section 3193.73, which transects
the Bachofer property, there is 1.86 -foot change in water surface between these
topographic surfaces for the 100 -year flood event.
• The modeled surface water elevation for the 100 -year event using the 2008
mining surface is 4875.81 and is well above the previously established 100 -year
flood plain elevation of 4874 determined by Weld County,
• Model simulations were also conducted for lower flows that include the 5 and 10 -
year events. The 5 -year event (7,900 cfs) approximates flow at the Bachofer
property, which has been recently flooding the lower level of his house. The 5 -
year event water surface (4873.91 feet) for current mining conditions is very close
to the 100 -year permitted flood plain elevation of 4874 feet, The model results
show the 100 -year pre -mine water surface to be 4873.95.
• The 10- year event (10,000 cfs) simulates peak flows that occurred during flood
events in September 2013 (10,300 cfs) and in the spring of May 2015 (10,500
cfs). Flooding occurred in the Bachofer house (Bachofer, Oral Comm, 2015)
during both of these events and model results validate these events with water
surface elevations above 4874 feet. Simulations using pre -mining topography
show that the water surface at 4872.66, which is well below the established flood
plain level at the Bachofer property. This indicates flooding of the residence is a
result of alterations of the original floodplain morphology and topography caused
by mining and pit -lining activities.
• Groundwater mounding has resulted from slurry wall construction, constriction or
restriction of subsurface flow, and has caused higher water levels than previous
and historic levels in the river alluvium upgradient of the slurry walls. As a result,
it has induced greater groundwater flow into the river. The results of the surface
water model should be considered conservative since they don't reflect the total
local discharge to the river from groundwater.
- 14 -
GANSER LUJAN c& ASSOCIATES
Hydrogeological and Environmental Consultants
References
Arnold L.R. et al, 2010, Land -Use Analysis and Simulated Effects of Land -Use Change
and Aggregate Mining on Groundwater Flow in the South Platte River Valley, Brighton
to Fort Lupton, Colorado, U.S.G.S. Open File Report 2010-5019
Bachofer, Ross, & Michael, 2015, Oral Communication and site visit to inspect house damage
from flooding
Deere and Ault, September 22, 2010 Letters from Mr. Ross Bachofer Regarding Flooding
of Property East of South Platte River near the Everist Fort Lupton Mine;
Deere and Ault Consultants, Inc., 2008, Supporting Documentation for Fort Lupton Sand
and Gravel Mine FHDP;Weld County Public Works Department
Ganser Lugan and Associates (GL&A), 2013, Evaluation of the Cause(s) of Water Level
Increases in the South Platte Alluvium at the Ross Bachofer Property; 7525 Highway 85, Fort
Lupton, Colorado
U.S. Department of Housing and Urban Development Federal Insurance Administration,
1978, Flood Insurance Study Town of Fort Lupton, Colorado Weld County
U.S. Army Corps of Engineers, 2010, HEC-RAS River Analysis System, Hydrologic
Engineering Center
U.S.G.S. 2013, USGS surface water data for Colorado, Curve Rating Data For Fort
Lupton Gage Water Information System Database, accessed September 2013.
-15-
Table 2
2008 Mining Model Output
Reach
River Station
Profile �
a Total '
Min Channel
Water Surface
Critical
Energy Grade
Energy Grade
Velocity '
Velocity
Velocity
now Area '
Top Width
Elevation
Elevation
Water Surface
Elevation
Slope
Left
Channel
Right
(cfs)
(ft)
(ft)
(ft}
(ft)
(ft/ftj
(ft/s}
(ft/s}
(ft/s)
(sq ft)
(ft)
South Platte River
4700.78
PF 1
7900
4870
4874.25
4874.27
0.000311
0.95
2.15
0.69
7498.21.
4746.06
South Platte River
4700.78
PF 2
10000
4870
4874,58.
4874.61.
0.00029
1.02
2.19
0.77
9111.82
4910.43
South Platte River
4700.78
PF 3
29000
4870
4876.21
4876.26
0.000363
1.56
3.02
1.38
17772.49
5673.36
South Platte River
4233.11
PF 1
7900
4869.98
4874.14.
4874.16
0.000178
0.69
1.58
0.66
10188.58
6115.46
South Platte River
4233.11
PF 2
10000
4869.98
4874.49
4874.5
0.000164
0.74
1.6
0.69
12303.681
6243.78
South Platte River
4233.11
PF 3
29000
4869.98
4876.1.
4876.13
0.000207
1.22
2.24
1.19
22529.4
6424.79
South Platte River
3193.73:
PF 1
7900
4868
4873.91
4873.94
0.000231.
0.83
2.31.
0.91
7867.69
4164.82
South Platte River
3193.73
PF 2
10000
4868
4874.26
4874.29
0.000233.
0.87
2.41
0.94
9748.55
5825.98
South Platte River
3193.73
PF 3
29000
4868
4875.81
4875.86
0.000314
1.41.
3.26
1.22
19064.01
6207.23
South Platte River
2354.37
PF 1
7900
4867.99
4873.6
4873.65
0.00055E,
1.08
2.56
_
5819.79
3847.91
South Platte River
2354.37
PF 2
10000
4867.99
4873.97
4874.01
0.000475
1.14
2.53
7276.74
4000.84
South Platte River
235437
PF 3
29000
4867,99
4875.42
4875.49
0.00058
1.73
3.45
0.84
15109.31
5693.67'
South Platte River
1388.01
PF 1
7900
4866
4871.61
4871.32
4872.41
0.003922
3.22
7.92
1.87
1388.74
712.04
South Platte River
1388.01
PF 2
10000.
4866
4872.17
_ 4871.76
4872.94
0.003509
2.71
8.12
1.94
2046.48
2256.27
South Platte River
1388.01
PF 3
29000.
4866
4874.09
4874.48
0.002076
2.66
7.77
2.94
8303.02
4220.32
South Platte River
634.85
PF 1
7900
4863.85
4870.57
4870.91
0.001036•
0.78
4.78
0.44
1889.88
1059.21
South Platte River
634.85
PF 2
10000
4863.85
4871.3
4871.62
0.000888.
0.9
4.81
0.82
3201.35
2549.19
South Platte River
634.85
PF 3
29000
4863.85
4873.08.
4873.39
0.001012.
1.59
6.08
1.79
11297.45
6184.24
South Platte River
188.69
PF 1
7900
4863
4868.02
4868.02
4869,74
0.009277
0.17
10.51.
0.26
752.27
252,85
South Platte River
188.69
PF 2
10000
4863
4868.72
4868.72
4870.57
0.008131
2.03
10.96
2.21
943.43
295.08
South Platte River
188.69
PF 3
29000
4863
4871.88
4871.88
4872.64
0.002826
2.35
9.3
2.94
6995.31
3999.99
Table 3
Pre Mining Model Output
Reach
River Station
Profile
QTotal
Min Channel
Water Surface
Critical
Energy Grade
Energy Grade
Velocity
Velocity
Velocity
Flow Area
Tap Width
Elevation
Elevation
Water Surface
Slope
Left
Channel
Right
(cfs)
(ft)
(ft}
(ft)
(ft}
(ft/ft}
(ft/s)
(ft/s)
(ft/s)
(sq ft)
(ft)
South Platte River
4700.78
PF 1
79001
4870
4873.27
4873.34
0.000769
0.88.
2.77
1.16
5388.12
5051.06
South Platte River
4700.78
PF 2
10000
4870
4873.48
4873.56
0.000771
1.02
2.91
1.23•
6510,61
5193.83
South Platte River
4700.78
PF 3
29000
4870
4874.85
4874.94
0.00075
1.76
3.62
1.75
13955.79
5605.99
South Platte River
4233.11
PF 1
7900
4870
4873
4873.04
0.000498
0.91
2.2
0.92
7121.97
5613.04
South Platte River
4233.11
PF 2
10000
4870
4873.22
4873.25
0.000501
1.02
2.32
0.98
8344.63
5692.2
South Platte River
4233.11
PF 3
29000
4870
4874.57
4874.63
0.000549
1.66
3.07
1.41
16352.26
6161.81
South Platte River
3193.73
PF 1
7900
4868
4872.43
4872.46
0.000588
0.98
2.84
1.24
6936.71
5690.08
South Platte River
3193.73
PF 2
10000
4868:
4872.66
_
4872.69
0.000556
1.08
2.87
1.28
8257.94
5727.01
South Platte River
3193.73
PF 3
29000
4868.
4873.95
4874.01
0.00062
1.75
3.64.
1.73
15761.48.
5921.48
South Platte River
2354.37
PF 1
7900
4868
4871.88
4871.92
0.000682
1.08
3.08
1.32
6570.1
5282.95
South Platte River
2354.37
PF 2
10000
4868.
4872.1
4872.14
0.000734
1.17
3.32.
1.46
7804.23
5969.37
South Platte River
2354.37
PF 3
29O00
4868,
4873.38•
4873.44
0.000699
1.8
3,88
1.87
15507.04
6120.19
I
South Platte River
1388.01
PF 1
7900
4866
4871.01
4871.11
0.000974
1,
3.95
1.65
5301.09
5401.73
South Platte River
1388.01
PF 2
10000
4866
I.
4871.35.
4871.42
0.000717
1.07
3,56
1.54
7160.61
5583.68
South Platte River
1388.01
PF 3
29000
4866
4872.54
4872.63.
0.000915
1.82
4.69
1.81
14508.15
6772.1
South Platte River
634.85
PF 1
7900
4864
4859.84
4867.83
4870.2
0.001389
0.94
5.17
1.28
2364.07
1986.8
South Platte River
634.85
PF 2
10000
4864
4870.11
4868.41
4870.57
0,001734
0.88
5.98
1.08
3204.88
5203.58
South Platte River
634.85
PF 3
29000
4864
4871.63
4871.03
4871.85
0.001146
1.82
5.77
1.66
12144.77
6374.85
South Platte River
188.69
PF 1
7900
4864
4868.84
4868.84
4869.32
0.002952
1.5
6.49
1.12
2382.78
2677,84
South Platte River
188.69
PF 2
10000
4864
4869.04
4869.04
4869.54
0.003206
1.81
6.99
1.35
2934.49
2883.04,
South Platte River
188.69
PF 3
29000
4864
4869.91
4869.91
4870.79
0.006039'
-- 3.7
10.88
2.37
5862.71
3911.98
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LEGEND
PRE -MINING INDEX CONTOUR (10')
PRE -MINING INTERMEDIATE CONTOUR (2')
-- +— — MINING INDEX CONTOUR (1D')
MINING INTERMEDIATE CONTOUR (2)
DACHOFERPROPERTY BOUNDARY
— — . RESERVOIR PROPERTY BOUNDARY
, — EDGE OF WATER BODY 1 RIVER
PAVED ROAD
UNPAVED ROAD
RIVER BANK
NOTE:
PRE -MINING CONTOURS GENERATED FROM USGS FORT LUPTON,
COLORADO 7.5' QUADRANGLE DIGITAL ELEVATION MODEL (DEM). MINING
CONTOURS TAKEN FROM FIGURE 5, EXISTING TOPO AS MODELED, BY
DEERE & AULT, DATED 11125f08.
GRAPHIC SCALE
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LEGEND
PRE -MINING INDEX CONTOUR (15')
PRE -MINING INTERMEDIATE CONTOUR (2')
— — — MINING --_ MINING INDEX CONTOUR 10')
MINING INTERMEDIATE CONTOUR (2')
BACHOFER PROPERTY BOUNDARY
RESERVOIR PROPERTY BOUNDARY
--- • , • — --- EDGE OF WAFER BODY r RIVER
PAVED ROAD
----- — ---
UNPAVED ROAD
RIVER BANK
NOTE:
FRE-MINING CONTOURS GENERATED FROM USGS FORT LUPTON,
COLORADO 7,5' QUADRANGLE DIGITAL ELEVATION MODEL (DEM). MINING
CONTOURS TAKEN FROM FIGURE 5, EXISTING TOPO AS MODELED, BY
DEERE & AULT, DATED 11!25106.
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GANSER LUJAN & ASSOCIATES, LLC SECTIONS 10-40
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informationagenserlujan.com fiELD COUNI7 COLORADOJl
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2008 AERIAL SURVEY GRADE EOUIF•I PLATTE RIVER 4
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4850 1956 USGS 7.5' GRADES 1 4850
4830 4830
0+00 2+00 4+00 6+00 8+00 10+00 12+00 14+00 16+00 18+00 20+00 22+00 24+00 26+00 28+00 30+00 32+00 34+00 36+00 36+00 40+00 42+00 44+03 46+00 46+00 50+00 52+00 54+00 58+00 56+00 60+00 62+00 64+00 66+00 68+00 70+00 72+00 74+00 76+00 77+55
SECTION 188.69
HORIZONTAL SCALE: 1" = 600'
VERTICAL SCALE: 1" = 100'
4925 4#25
4983 '`y 4000
2008 AERIAL SURVEY GRADE
SOUTH RIVER
4675 / 4675
4850 1950 UI000 ?.5' GRADE S 4050
4832 48.32
0+00 2+00 4+00 6+00 8+00 10+00 12+00 14+00 16+00 18+00 20+00 22+00 24+00 26+00 28+00 30+00 32+00 34+00 36+00 38+00 40+00 42+00 44+00 46+00 48+00 50+00 52+00 54+00 56+00 58+00 60+00 62+00 64+00 66+00 68+00 70+00 72+00 74+00 76+00 77+,03
SECTION 634.85
HORIZONTAL SCALE: 1" = 600'
VERTICAL SCALE: 1" = 100'
4300 4820
2006 AERIAL SURVEY GRADE
5CILtif I PLATTE RIVER 4
�^-
4875 - - 4875
.
4850 1950 USGS 7.5 GRADES 48550
4830 4930
0+00 2+00 4+00 6+00 6+00 10+00 12+00 14+00 16+00 18+00 20+00 22+00 24+00 28+00 28+00 30+00 32+00 34+00 38+00 38+00 40+00 42+00 44+00 48+00 48+00 50+00 52+00 54+00 58+00 58+00 60+00 82+00 64+00 66+00 88+00 70+00 72+00 74+'06176
SECTION 1386.01
HORIZONTAL SCALE: 1" = 600'
VERTICAL SCALE: 1" = 100'
GRAPHIC SCALE
a 50 1I0}0
il
I
4000 2506 AERIAL SURVEY GRADE 41
( IN FEET)
W'
g•
SOUTH PLATTE RIV1-_R 4
•187Si... 4875
1 inch=100ft.
``
'
GRAPHIC SCALE
4850 1550 USG 37.5•GRADES J ...-. --- 46.50
01i�
10/5/15
�" '
M r
^
4830 48,90
a 3a0 600
PLC *El
awatlan■
ass 4
Au I SHOIMVI
0+00 2+00 4+00 8+00 8+00 10+00 12+00 14+00 16+00 18+00 20+00 22+00 24+00 26400 28+0D 30+00 32+00 34+00 36+00 38+00 40+00 42+00 44+00 46+00 48+00 50+00 52+00 54+00 56+00 5B+00 60+00 62+00 64+00 66+00 68+00 70+00 (IN FEET)
1 inch =600ft.
SECTION 2354.37
HORIZONTAL SCALE: 1" = 600'
VERTICAL SCALE: 1" = 100'
"'..'.
1507004
w"°
3 .r 4
FIGURE:::
4 J
4900
O
4900
4875
46:34
2008 AERIAL SURVEY GRADE
1950 USGS l,5'GRADES
0+00 2+00 4+00 6+00 8+00 10+00 12+00 14+00 16+00 16+00 20+00 22+00 24+00 26+00 28+00 30+00 32+00 34+00 36+00 38+00 40+00 42+00 44+00 46+00 48+00 50+00
4000
an
4850
4830
SECTION 3193,73
HORIZONTAL SCALE: 1" = 600'
VERTICAL SCALE: 1" = 100'
2008 AERIAL SURVEY GRACE
1950 USGS '75 GRACES
SOUTH PLATTE RIVER 4.
4675
4850
4830
62+00 54+00 56+00 56+00 60+00 62+00 64+00 66+00 68+00 09+54
SOUTH PLATTE RIVER 4_
4900
4875
0+00 2+00 4+00 6+00 8+00 10+00 12+00 14+00 16+00 18+00 20+00 22+00 24+00 26+00 28+00 30+00 32+00 34+00 36+00 38+00 40+00 42+00 44+00 46+00 48+00 50+00 52+00 54+00 56+00 58+00 60+00 62+00 64+00 66+00 68+00 69+V
SECTION 4233.11
HORIZONTAL SCALE: 1" = 600'
VERTICAL SCALE; 1" = 100'
4900 2008 AERIAL SURVEY GRADE
4675
4850 1950 USGS 7.5' GRAQE5
4835
_/
.0UTH PLATTE RIVER
4850
4830
0+00 2+00 4+00 6+00 6+00 10+00 12+00 14+00 18+00 18+00 20+00 22+00 24+00 26+00 28+00 30+00 32+00 34+00 36+00 38+00 40+00 42+00 44+00 46+00 46+00 50+00 52+00 54+00 56+00 56+00 60+00 62+00 6490I'50
SECTION 4700.78
HORIZONTAL SCALE: 1" = 600'
VERTICAL SCALE: 1" = 100'
GRAPHIC SCALE
-0 300 100
I I
(IN FEET)
1 inch .80Oft
GRAPHIC SCALE
0 4 100
( IN FEET)
1 Inch .100ft,
4900
4075
4850
4635
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5
GANSER LUJAN & ASSOCIATES
JJ i irfro cPolo ie' l and En vlronmenta f Consmltanis
Prepared by:
Joel M, Sobol P.G.
Senior Hydrogeologist
1,51 sommali
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Reviewed by:
Sergius Hanson P.S., P.O.
President E-21 Engineering
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GI A GANSER LUJAN & ASSOCIATES
Hydrogeological and Environmental Consultants
October 4, 2013
Ross Bachofer
P.O. Box 652
LaSalle, Colorado 80645
RE: Evaluation of the Cause(s) of Water Level Increases in the South Platte Alluvium at the
Ross Bachofer Property; 7525 Highway 85, Fort Lupton, Colorado
Ganser Lujan and Associates (GL&A) have conducted a desktop study and literature search to
determine the impacts of aggregate mining adjacent to and in the vicinity of the Bachofer
property at 7525 Highway 85 in Fort Lupton on the hydrogeologic regime in the South Platte
Alluvium. This study was performed in general accordance with our proposal dated June 25,
2013 as accepted and approved on June 27, 2013.
Summary
The results of this study indicate that aggregate mining adjacent to and upstream of the subject
property has resulted in higher river flood stage levels, higher ground water levels and water
seepage into the lower level of the residence at this location. The impact of aggregate mining
to this property is based on the fact that slurry wall construction around open pit aggregate
mines has caused alluvial groundwater to back up or mound behind these walls on the up -
gradient side of the walls and decline on the down -gradient side. Mounding has caused higher
water levels than previous and historic levels in the river alluvium and as result has induced
greater groundwater flow into the river. The impermeable slurry walls from the Suburban Pit
and the Everist Pits are located in close proximity to the west bank of the South Platte River
and as such have also significantly reduced the bank storage capabilities of the river alluvium.
In addition, the slurry walls and berms have adversely altered the floodplain characteristics of
the river by obstructing the land adjacent to the river that was naturally reserved for base
flooding without increasing the water surface elevation. Historically, high flows in the past
have not caused flooding on the Bachofer property. Since 2004, flows that are considerably
lower than previous discharge are now flooding the subject's property.
Extensive mining operations with lined pits upstream of the Bachofer property have also
altered the river dynamics and regime causing what was normal river swells in the past to
exceed the 100 year floodplain elevation. These mining activities coupled with the L.G.
Everist pits located primarily along the west side of the South Platte River have altered the
river morphology contributing to an increase in the number and size of sand bars. These bars
have reduced the cross-sectional area of the river channel thus causing higher stage levels and
the potential for more frequent flooding.
As discussed above, all of these factors together have increased the likelihood of flooding
with lower river flows than what previously caused flooding on the Bachofer property.
12610 W Bayaud Ave, Unit 11, Lakewood, CO 80228
303-888-9078
GANSE1R LUJAN & ASSOCIATES
Hydrogenlogical and Environmental Consultants
As part of this study, several tasks were performed to support the premise that slurry wall
construction associated with aggregate mining has increased river and groundwater levels:
1. Performed site visit and visual inspection of the property and surrounding mining
operations;
2. Evaluated U.S.G.S. stream gauge records and monitoring well hydrographs in the
South Platte River in the vicinity of the Bachofer property and Fort Lupton area;
3. Reviewed pertinent Wright water Engineers and Deere and Ault reports;
4. Performed a document and records search at the Colorado State Engineers Office,
Colorado Division of Water Resources and the Colorado Division of Reclamation,
Mining and Safety to obtain pertinent and relevant information on water level data
and studies conducted at nearby mines;
5. Performed a review of U.S.G.S. Water Resources Investigation Report 02-4267,
Analytical and Numerical Simulation of the Steady -State Hydrologic Effects of
Mining Aggregate in Hypothetical Sand and Gravel and Fractured Crystalline Rock
Aquifers; and
6. Performed a review of U.S.G.S. Open File Report 2010-5019, Land -use Analysis and
Simulated effects of Land -Use Change and Aggregate Mining on Groundwater Flow
in the South Platte River Valley, Brighton to Fort Lupton, Colorado.
Site Visit to Bachofer Property
A site visit to the Bachofer property was conducted on July 9, 2013 to visually observe the
physical layout of the property and its relation to the river and surrounding landscape. The
visit also included an assessment of the impact and the damage due to flooding of the
property and the house. The Bachofer property is located on Lot 6 in the SW 1/4 of the NE
1/4 Section 30, T2N, and R66W adjacent to and along the east side of the South Platte
River. The property is situated between the river and Highway 85 and 1/2 mile south of
Weld County Road 18. The property is triangularly shaped and wedged between the river
on the west and the Spear Canal irrigation ditch to the east. The house sits on a flat area on
the eastern edge of the trees as shown in Figure 1 below. This flat area forms the post -
Piney Creek alluvial terrace deposit and is comprised of interbedded sand and gravel.
2
GANSI4at LUJAN N ASSOCIATES
Hydrogeolog kal and Environmental Consultants
Figure 1
The area is heavily vegetated with cottonwood, willow, and occasional elm trees, tall
bushes, and grass that extend from the house to the bank of the river.
According to Mr. Bachofer, the lower level of the house has repeatedly flooded since
August 2004, when the flows at the Fort Lupton Gauge reach 6,400 cubic feet per second
(cfs), whereas in the past the flows reached over 10,000 cfs and no flooding occurred. The
elevation of the door on the bottom level floor has been surveyed by a licensed surveyor and
is at 4875 feet above mean sea level elevation amsi). The 100 year flood plain level
determined by Weld County for obtaining a Flood Hazard Permit on 5/19/1987 was 4874.0
feet and as such Mr. Bachofer was not required to have a permit because his land was above
the 100 year floodplain level. There was also no history of flooding on this property prior to
this time.
During the visit, a sand- bagged berm was built along the west side of the concrete floor
leading to the basement entry to prevent flooding of the basement.
3
GANSER. LUJAN &, ASSOCIATES
Hydrogeologieal and Environmental Consultants
Evaluation of U.S.G. S. and CDWR Stream Gauge Records on the South Platte River
in the Vicinity of the Bachofer Property and Fort Lupton Area
Discharge data from the Fort Lupton Gauging Station on the South Platter River were
obtained online from the USGS National Water Information System. The station is located
on the South Platte River on CR 18 %Z mile north of the Bachofer property. Data have been
collected intermittently since 1930 to the present. Early data from 1930 to 1957 were
collected from the original station that was located approximately 3 miles upstream from the
present location on CR 18. No data were collected from 1958 to 2002 and the station was
moved to its present location in 2002 (oral comm. Rick Crowfood, USGS), where stream
data have been collected since 2003. Annual peak flows at Fort Lupton Station are shown
below on Figure 2.
Figure 2
Peak Flows at Fort Lupton
South Platte River
Cubic Feet per second (cis)
35,000
30,000
25,000
20,000
15,000
10,000
5,000
0
1.
...i.'
.. $
r ,
1
" , Prlorto slurry wall
Ino flooding occurs
o 1 with flows over
IDoaa cfs,
k �
it 1
a 1 �`A,k f�,' ',
C1,IV\ , 11,� , 1 1 Y 4 , y
`4;1 ;;Ii I 4' } 1 i"f ,p
I Ii
Co
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r O QI
a, IC)
Date
N
a,
cio
O/
rn
f0
Slurry wall constructed
around PAS
Properly floods
with flows over
5,440 cfs post
N
0
O
N
N
U,
N
O
N
N
O
—4,— Fort
Lupton
--4•-• Predicted
Flow data from the missing years, 1958 to 2002 were calculated by using a linear regression
analysis of flow data (2002 to 2012) from the Henderson station located approximately 14
miles upstream from the Fort Lupton station. Peak flows from this station are presented on
Figure 3 below. Data from this station are collected by the Colorado Division of Water
Resources (CDRW) State Engineer's Office and was obtained online.
The results of the regression analysis with the regression equation are shown on Figure 4
and were used to estimate discharge at the Fort Lupton station. Figure 2 shows these
estimated predicted data with a dashed line and hollow symbol marker. In the years
preceding the existence of the present Fort Lupton gauge from 1987 to 1995, Mr. Bachofer
reported that peak flows at the Henderson station exceeding 11,000 cfs on Figure 3, did not
4
GANSER LUJAN ASSOCIATES
Hydrogeologieal and Environmental Consultants
flood his property. These flows would correspond to peak flows of approximately 10,000
cfs at the Fort Lupton station if the station existed at the time.
Figure 3
Peak Flows at Henderson
South Platte River
Cubic Feet per second (cfs)
35,000
30,000
25,000
20,000
15,000
10,000
5,000
a I-
01 oiN QS
r r
qti Ze
1
Date
P rlor to slurry wall
no Rooding occurs
with flows over
11,000 cfs.
[At�4
0)
07
a
0)
OS
CQ
Slurry wan constructed
around pits
Property floods
with flows over
7,000 cfs post
slurry wall
Henderson
Flow
Slurry Wall
The L.G. Everist pit was lined in April 2004, and as of August 2004, the Bachofer property
started flooding and has continued to flood with flows of 6,440 cfs at the Fort Lupton gauge.
9,000
8,000
7,000
6,000
5,000
. 4,000
e.)
p 3,000
2,000
1,000
0
0
Figure 4
Fort Lupton versus Henderson
Peak Flows 2003-2012
y = 0.9042x - 70.194
R'=0.7397
2,000 4,000 6,000 8,000 10,000
Discharge (c(s)
Fart Lupton vs
Henderson
Linear (Fort
Lupton vs
Henderson)
5
GANSER I.UJAN & ASSOCIATES
Hydrogeologlcal and Environmental Consultants
Wright Water Enineerini Report , Response to Comments by DMG in Adequacy
Review of Ft. Lupton Sand and Gravel Mine Ap licalion (M14-1999 January .3 2005
The Wright Water Engineering, Inc. (WWE) report responds to the Division of Minerals
and Geology (DMG)) comments related to the potential impacts of constructing a slurry
wall around aggregate pits operated by L.G. Everist (LGE). DMG was concerned that
emplacement of a slurry wall would affect surface water, groundwater, and nearby wetlands
along the South Platte River near Fort Lupton Colorado.
Response to WWE Comment #7
LGE constructed a slurry wall around their aggregate mining pits in April of 2004 and
initiated a groundwater monitoring program to evaluate the impact to groundwater levels
surrounding the pits. Because this report was published so soon after (January 2005) the
slurry wall was completed, GL&A believes that the monitoring program had not been
conducted over a sufficient length of time to adequately determine the true impact to the
hydrologic system. The report erroneously concludes that mining operations would have
little or no negative effect on surface water flows and groundwater levels in the South Platte
River Alluvium.
More recent modeling studies from the USGS (Arnold et al., 2010) show a mounding effect
where groundwater levels rise 2 to 4 feet on the north and west sides of lined pits across the
river from the Bachofer property. Another modeling simulation shows groundwater levels
rising 4 to 6 feet in the same area around the pits. Although the mounding occurs on the
west side of the river the impact to the landowners located on the east side of the river
cannot be ruled out. As stated in the U.S.G.S. report (Arnold, 2010), the South Platte acts a
sink to groundwater when water levels are elevated in the alluvium. Hence, mounding can
cause increased groundwater discharge to the river, thus contributing to higher base flows in
the river. As such, a lined slurry wall will not only impact groundwater levels but will also,
affect surface water flows in the river.
Earlier modeling studies were also conducted by the USGS (Arnold et al, 2003) to
determine the hydrologic effects of aggregate mining in hypothetical sand and gravel and
fractured crystalline rock aquifers. Analytical solutions and numerical models were used
predict the extent of steady-state drawdown and mounding due to aggregate mining below
the water table. The results of the model simulations are very similar to the results
determined by the USGS Report in 2010. That is, mounding occurs on the up -gradient side
of lined pits and drawdown occurs on the down -gradient side.
Response to WWE Comment #8
WWE conducted a cumulative assessment of a lined slurry wall by using a numerical
groundwater model (USGS MODFLOW). The model was developed to predict
groundwater changes subsequent to the installation of the slurry walls around LGE's pit
cells. While the modeling simulations showed an increase of approximately 3 feet in
6
GANSER LU.JAN & ASSOCIATES
Hydrogeological and Environmental Consultants
groundwater levels due to LGE's pit expansion, there was no prediction of changes to
surface flows in the South Platte River. MODFLOW has the capability of simulating
surface flows by using the stream routing package. However, using this package was not
considered in the modeling scenarios and as a result the impact to surface flows was not
estimated.
In addition to failing to estimate stream flows, WWE wrongly concludes that their modeling
results are conservative because recharge to groundwater from Little Dry Creek and
irrigation ditches was not included in the model. While this statement is true for predicting
groundwater declines, it is an incorrect assumption for water level mounding. On the
contrary, the opposite is true in that the modeling results will underestimate the mounding
effect because there will be more water added to the system if recharge is included.
Response to WWE Comment #9
WWE report shows a rise in groundwater levels of 3 feet on the west side of mining
operation due to the slurry wall construction. Although this value is similar to the one in
Arnold's report (2 to 4 feet), WWE appears to ignore the impact of these results on the
South Platte River system. In the report WWE says that the mounding effect will be
mitigated by Little Dry Creek, which will act like a drain allowing higher groundwater
levels to discharge into the creek. Thus, lining of the aggregate pits will indirectly allow for
greater discharge into the South Platte River because Little Dry Creek confluences with the
South Platte River north of CR 18. Furthermore, a more direct impact will occur to the east
of the LGE Pits. Groundwater mounding on the south side of these pits also causes flow
toward the northeast which discharges into the South Platte River adjacent to the Bachofer
property. This discharge will increase surface flows in the channel.
Deere and Ault, Letters from Mr. Ross Bachofer Regarding Flooding of Property East
of South Platte River near the Everist Fort Lupton Mine; September 22, 2010
This report is a response by Deere and Ault to letters by Mr. Bachofer to the Colorado
Division of Reclamation, Mining, and Safety (DRMS). The report states unequivocally that
flooding on the Bachofer property is not related to the L.G.E.'s mining operation and the
construction of slurry walls.
Response to Deere and Ault Comment #1
There is ample evidence to show that mining activities have altered the floodplain and
channel morphology of the South Platte River. As stated previously, the 100 year flood
plain level was determined by Weld County for obtaining a Flood Hazard Permit to be
4874.0 feet amsl at Mr. Bachofer's location. Because the level of his basement floor was
survey to 4874.6 feet amsl, Mr. Bachofer was not required to have a permit because his land
was above the 100 year floodplain level. The fact that Deere and Ault prepared a Letter of
Map Revision (LOMR) on behalf of Weld County and Aurora is a strong indication that the
original 100 -year flood plain level has changed and no longer represents current flow
conditions in the river. The reason that the 100 -year level is no longer accurate is because
7
GANSER LUJAN & ASSOCIATES
Hydrogeolaglcal and Environmental Consultants
the morphology and geometry of the South Platte river and flood plain has changed due to
the construction of lined pits on the west side of the river. The statement by Deere & Ault
that the mine operator, L.G. Everist has not altered the South Platte River floodplain is
unsubstantiated and negated by the very application of the LOMR letter to establish a new
100 -year flood level.
Based on the Deere & Ault letter of notification dated August 19, 2011, the LOMR
application is for a considerable change in flood levels. The original 100 -year flood plain
elevation on the finished floor elevation at the Bachofer house was 4874 feet amsl. The new
proposed elevation according to the Deere & Ault map attached with the LOMR letter is at
4876.5 feet or 2.5 feet above the original flood level. This level was estimated using HEC-
RAS surface water modeling and was based on a steady-state discharge of 29,000 cfs
representative of a 100 -year event. The results of this modeling appear to be unreliable and
unrealistic when compared to actual stream flow data measured at the nearby Fort Lupton
gauging station. Recent peak flows measured 9,200 cfs at this station on 9/13/2013, while
the water level was at 4877.6 feet, 3 feet above the floor elevation of 4874.6 at the Bachofer
house. The disparity between the model and measured results suggest that the model is
poorly calibrated and does not represent actual hydrologic conditions. As such, the model
results should not be used as a basis to re-establish a new 100 -year flood plain map. The
model significantly underestimates the stage level in the entire reach of the model area and
specifically in the vicinity of the Bachofer property.
Response to Deere and Ault Comment #2
The comment states that the South Platte River lies between the L.G. Everist property and
the Bachofer property and that hydrologically forms a no -flow boundary and would be
modeled as such. This statement is categorically and unequivocally incorrect and indicates
that the author is inexperienced in the construction of numerical groundwater models and
does not understand boundary conditions in the hydrological system. The river is modeled
as a sink or a source depending on the stage in the river and head in the adjoining model
cells. A river is usually modeled with the river module or package which allows water to
either flow into or out of the river cells through a conductive layer. It is not a no -flow
boundary.
With respect to hydrogeologic conditions in the area near the Bachofer property, the
groundwater gradient is toward the river from both the west and east sides. Hence,
groundwater is flowing toward the northeast on the west side and toward the northwest on
the east side. While this condition appears to preclude an apparent hydrologic effect to the
Bachofer property, the opposite is actually true. Groundwater mounding or rise that occurs
on the west side due to the impermeable slurry wall around L.G. Everist pits has created
higher heads in this area and as a consequence, the alluvial aquifer is discharging and
contributing more water to surface flows in the river than what was occurring prior to the
slurry wall construction.
8
GANSER I.UJAN 6 ASSOCIATES
Hydrogeological and Environmental Consultants
Response to Deere and Ault Comment #3
Deere & Ault have misconstrued the results of the USGS Report 2010-5019. The USGS
report shows on Figure 29 report the direction of the hydraulic gradient in the alluvial
aquifer under steady-state conditions for the 2000 irrigation and non -irrigation seasons
without lined or unlined pits. Based on the orientation of the equipotential lines drawn on
this figure, less than half of the total alluvial flow would discharge into the river by the
Bachofer property while the remaining proportion of flow would discharge further north.
With the presence of lined pits, the configuration of the groundwater surface would be
significantly different. The outline of the lined pits as shown on Figure 34 of the USGS
report is located directly across the river from the Bachofer property and extends
approximately 4,000 feet westward across the flood plain to nearly the edge of the alluvial
valley. The lined pits physically extend across approximately 80 percent of the total width
of the alluvial floodplain. Although the report does not include a figure with model
simulations showing a hydraulic head configuration due to the lined pits, the report does
illustrate the change in groundwater levels. The presence of mounding 2 to 4 feet above
normal levels means that in addition to the actual length of the slurry wall, a hydraulic
barrier to flow has formed beyond the edges of the slurry wall that will block nearly all
alluvial flow.
The length of this barrier can be seen on Figure 34, which is denoted by the color blue and
delineates the total extent of the mounding, The blue shading covers the entire width of the
river alluvium from the southwest bank of the river to the western edge of the valley. This
means that essentially no alluvial groundwater can flow northward to discharge areas as in
the past. As a result, nearly all of this flow will be routed into the river channel as either
alluvial or surface flow ultimately increasing the base -flow and stage level in the river in
this area.
Given the above scenario, it may also be possible that groundwater levels on the east side of
the river by the Bachofer property have increased due to mounding on the west side of the
river. Frequent flooding has resulted from higher groundwater levels on the west side of the
river may have caused higher water levels on the east with recharge to the alluvial aquifer
occurring from the flood waters. As illuminated in the USGS report, mounding has and will
occur due to lining mine pits with low permeability barriers such as slurry walls.
Another factor that is contributing to flooding of the Bachofer property is the elevation of
the top of L.G. Everist's Golden cell located directly across the river from the Bachofer
property. The elevation, as shown by a topographic survey performed by King Surveyors,
Inc in 2009, shows that the elevation of the rim of the pit is 4877 feet. This elevation is 3
feet above the 100 year elevation at 4874 that was determined by Weld County for obtaining
a Flood Hazard Permit on 5/19/1987 on the Bachofer property. The surface elevation of this
pit precludes the use of this portion of the flood plain to mitigate a flood swell thus, causing
higher flood levels in the river and on adjacent areas such as the Bachofer property.
9
GANSER LUJAN & ASSOCIATES
Hydrogeological and Environmental Consultanis
Prior to construction of the Golden cell, the land was below the 100 year flood plain
elevation of 4874 and thus could mitigate a flood swell or higher stage in the river that was
above this value. Based on a pre -mining topographic map developed by Deere and Ault
(11/25/2008) and a survey of the Golden Cell by King Surveyors (5/19/2009),
approximately 60 percent of this area that was once part of the flood fringe that could
mitigate flood stage levels has been removed. The Golden Cell with L.G. Everist's pits to
the west has removed approximately 90 percent of the area that was historically at or below
the 100 year level. Removal of this area has significantly impacted the ability of the river to
dissipate stage levels and prevent flooding on the east side of the river.
Colorado Division of Reclamation, Mining and Safety (DBMS), L.G, Everist Fort
Lupton Sand and Gravel , Permit No. M-1999-120, Bachofer Property Flooding as
Result of Reclamation Slurry Walls, October 22, 2010
Conclusions
DRMS acknowledges several possible causes of flooding on the Bachofer property that
includes greater aquifer discharge due to higher groundwater levels as a consequence of
mounding from the slurry walls. Other reasons include mounding on the east bank, which
have caused active seeps and/or artesian groundwater conditions. However, DRMS
discounts these explanations as possible causes of flooding because they are not supported
by the results of the USGS Report (Arnold et al., 2010). The USGS report explicitly
concludes that mounding would occur on the upstream side of lined pits such as the L.G.
Everist pits located directly west of the Bachofer property. Furthermore, the report does
state on page 68 "Groundwater -level changes near the South Platte River generally are less
than 2 feet because the river lessens the hydrologic effects of the pits by contributing or
receiving water as water levels change". Although USGS does not explicitly state that a
higher river stage and possible flooding could result, the fact that the river is attenuating the
mounding effect close to the river substantiates the hydraulic connection and relationship
between river and aquifer. Higher groundwater levels on the west side coupled with the
hydraulic barrier created by the mounding suggests that most of the alluvial groundwater
flow will be redirected into the river.
The possibility that ground water is mounded on the east side of the river and manifested by
active seeps is highly unlikely. There are no active seeps on the Bachofer property or on
nearby properties. The presence of mounding on this area would also not create artesian
conditions. For artesian conditions to occur a much higher recharge source outside the area
would need to be available and the alluvial aquifer would have to be overlain by
impermeable clay to create a pressure head. The hydraulic head in the alluvial aquifer
system is solely due to elevation head.
There is some evidence to suggest that higher groundwater levels exist on the east of the
river at or about the time that the pits were lined. Data obtained from the Colorado Division
of Water Resources well listing report shows two wells in the vicinity of the Bachofer
property with a continuous monitoring record that dates back to 1989 and 1994,
respectively. Well SB02N6631 ACD, is located approximately 1 mile south of the study
10
GANSEU LUJAN & ASSOCIATES
Hydrogeologlca! and Environmental Consultants
area in NE 'A of the SW 1/4 of Section 31, T2N, R66W. The well is located just east of the
Spear Canal Irrigation Ditch and is an alluvial well completed to a depth of 50 feet. The
period from 2002 to 2012 is enclosed in window and appears to display a fairly constant
groundwater level as compared to past years. The average groundwater is level is shown
form two periods from 1989 to 2003 and from 2004 until 2012. The average elevation for
each of these periods is shown by the horizontal line on Figure 5 and shows that the average
groundwater level is higher for the later period.
To rule out the possibility that these higher levels were due to higher than normal
precipitation and recharge, annual precipitation data were obtained from the Western
Regional Climate Center for the Brighton Station. These data clearly show that the higher
than average groundwater levels for these years is unrelated to precipitation.
Figure 5
Water Level Elevations in Well
02N6631ACD
Elevation in ft amsl
4878
4877
4876 �r
4875 II
4874
4873
4872
4871
Average elevation
from 1989 to 2003
Average elevation
from 2004 to 2012
O1 r1 m In N. O1 r- m In n O)i .-i m
00 O1 O1o, Oh O1O O O O r1 r -I
O1 Q1 O1 O1 O1 O1 O O O O O O O
' r1 .-4 r1 r`I N N N N N NI N N
▪ 1
.-▪ I ▪ .-1 rI ,-I ▪ .-4 - r▪ a • .-I r4 r▪ i r▪ l ri r▪ 4 ra
N r_I ._-I N .1 ,`-I r_ -I .-4 N r_i r_ -I ._-I r_ -I
Date
21
18
15
12
3
0
Annual Preciptation in Inches
—4—Water
Level
Elevations
Prc ipitati
on
The other well, SB02N6620CD is located about 1 mile to the northeast of the study area in
the SW 1/4 of the SE 1/4 of Section 20 T2NR66W. Based on topographic contours of the area,
the well appears to be situated on the edge of the South Platte Alluvial valley. Water level
elevations on Figure 6 show a similar pattern to the water levels in SB02N6631ACD.
Average elevations from 1995 to 2004 and from 2005 to 2010 also show there is slightly
higher average elevation for the later period. Greater seasonal fluctuations also appear to be
noticeable primarily from recharge to the aquifer from irrigated and non -irrigated periods
from 2005 to 2010. Although this well is located further away from the river than well
SB02N6620CD, it appears to be affected by mounding on the east side. The impact from
mounding may be affecting water levels on the east side during certain periods of river flow.
It can be assumed that the river is primarily gaining except during flood stages when water
11
GANSER LUJAN &6 ASSOCIATES
Hydrageologlcal and Environmental Consultants
may flow back to recharge bank storage in the adjacent areas. Higher stage levels in the
river may reduce flow from aquifer on the east side because of a lower hydraulic gradient
towards the river. Under this scenario, less discharge from the aquifer would mean higher
water levels in the aquifer on the east side of the river. This same phenomenon may be the
cause of higher average water levels in the well SB02N6631ACD.
Figure 6
Water Level Elevations in
Well SB02N6620CD
4874
4873
4872
4871
4870
4869
0
P. 4868
Le • 4867
4866
4865
4864
Average elevation
from 1995 to 2004
ID
O_1
Additional Considerations
0
N
0
0
N N
Date i
0
N
0
N
25
Precipitation in Linches
f Water Level
Elevations
Precipitation
(Inches)
GL&A does agree with DRMS assessment of changing river conditions and the formation
of sand bars. Historic aerial photos from Google Earth from 2002 to 2012 are presented
below and show that the river morphology has changed. A noticeable increase in the
formation of sand bars has occurred in a reach of the river from approximately '/2 mile
upstream of the Bachofer property to the Fort Lupton gauging station % mile downstream.
While the earliest photos go back to 1993, only those photos with similar flow conditions in
the river were selected. Dates on the photos were matched with average daily discharge
data from the same date from the Colorado Division of Water Resources website for the
Henderson gauge on the South Platte River. Discharge values for these selected dates ranged
from 110 cfs to 215 cfs and depending on the year tend to represent low flow or average
base -flow in the river.
12
GANSER LUJAN &. ASSOCIATES
Hydrogeological and Environmental Consultants
The earliest photo from 12/11/2002 (Figure 7) was taken several years before the pits were
lined and shows there were fewer and less developed sand bars present than in later years.
When compared to 10/7/2012 (Figure 10), there are fewer less developed bars near the
Bachofer property. A long bar is visible directly across the river from the property on the
2002, but is wider and longer on the 2012 photo. North of the Bachofer property, there are
several other bars that are present on the 2012 photo but do not appear on the 2002 photo.
As noted on the series of Figures 7 to 10 from 2002 until 2012, there appears to be a steady
increase in the quantity of sand being deposited near the Bachofer property. As mentioned
in the DRMS's letter the presence of these bars reduces the cross-sectional area of the
stream channel and raises the stage level in the river upstream of the bars.
What has caused this change in the river over the past 10 years? GL&A believes that bars
may have formed due to alterations in the channel geometry upstream of the Bachofer
property. These alterations may be attributable to the presence of lined pits that are located
upstream of the Bachofer property as shown on Figure 34 of the USGS Report. In order for
the bars to form downstream, higher water velocities upstream have occurred because of the
change in energy grade due to the lined pits. Higher velocities upstream have increase the
bed load of the river and have carried more sediment downstream. Deposition of this bed
load has occurred in the formation of point bars where stream velocities have decreased. As
in the case of the river near the Bachofer property, the bars have formed near the inside
banks of the river, where stream velocities tend to be lower. The formation of these sand
bars coupled with groundwater mounding adjacent to sand bars has created a significant
change in the surface water levels and stage in this area of the South Platte River. As a
result of these two factors, more frequent flooding has been occurring on the Bachofer
property.
Close-up pictures of these bars are shown below in a sequence of site -visit photos that show
the South Platte River adjacent to the Bachofer property.
13
c;A1-'+i1 SE LUJA T ASSOCIATES
Hydrogeological and Environmental consultants
Figure 7 11/1212002 Mean daily #low@1?3 cfs
Figure 8 4/13/2003 Mean daily flow@215 cis
14
GANSER LUJAN & ASSOCIATES
Hydrogco!og kaI and Environtnental Consultants
e
Figure 9 4/5/2006 Mean daily flow @110 cfs
Figure 10 10/7/2012 Mean daily flow *180 cfs
15
GA t ElR LUJAN ASSOCIATES
Ftpdrogcolo8 ica! and Environmental Consultants
Photo #1 South Platte River from east side of the river and west bank of the property'
looking southwest.
Photo #2 South Platte River from east side of the river and west bank of the property
looking west toward aggregate mine. Note sand bar in middle of channel.
GANSER ER LUJAN & ASSOCIATES
Hy ►rogeologi al and Environmental Consultants
Photo #3 South Platte River from east side of the river and west bank of the property
looking southwest. Note several bars in the middle of the channel.
Photo #4 South Platte River from east side of the river and west bank of the property
looking north. Note bars on east and west sides of the river channel.
GANSEI_ LUJAN & ASSOCIATES
Hydrogeologlcal and Environmental Consultants
Conclusions
• Peak flows in the South Platte River corresponding to approximately 10,000 cfs at
the Fort Lupton station did not flood the Bachofer property prior to August 2004.
The L.G. Everist pit was lined in April 2004, and as of August 2004, the Bachofer
property started flooding and has continued to flood with flows of 6,440 cfs at the
Fort Lupton gauge.
• The 100 year flood plain elevation at the house that Mr. Bachofer currently owns at
7525 Highway 85was originally determined in 1987 to be at 4874 feet. The house
was surveyed to be at 4875 feet or 1 foot above the 100 year flood elevation. The
fact Weld County and Aurora Water would like to revise the flood plain elevation,
suggests that this level has risen with time.
• WWE monitoring program has not been conducted over a sufficient length of time to
adequately determine the true impact to the hydrologic system. The report
erroneously concludes that mining operations would have little or no negative effect
on surface water flows and groundwater levels in the South Platte River Alluvium.
• Modeling studies from the USGS (Arnold et al., 2010) show a mounding effect
where groundwater levels rise 2 to 4 feet on the north and west sides of lined pits
across the river from the Bachofer property. Arnold states in report, the South Platte
River acts a sink to groundwater when water levels are elevated in the alluvium.
Hence, mounding can cause increased groundwater discharge to the river, thus
contributing to higher base flows in the river. As such, a lined slurry wall will not
only impact groundwater levels but will also, affect surface water flows in the river.
• WWE's numerical model had the capability using the stream routing package in
MODFLOW to simulate changes to surface flow in the South Platte River.
However, using this package was not considered in the modeling scenarios and as a
result the impact to surface flows was not estimated.
• WWE wrongly concludes that their modeling results are conservative because
recharge to groundwater from Little Dry Creek and irrigation ditches was not
included in the model. While this statement is true for predicting groundwater
declines, it is an incorrect assumption for water level mounding. On the contrary,
the opposite is true in that the modeling results will underestimate the mounding
effect because there will be more water added to the system if recharge is included.
• WWE's modeling study shows a rise in groundwater levels of 3 feet on the west side
of mining operation due to the slurry wall construction. Although this value is
similar to the one in Arnold's report (2 to 4 feet), WWE appears to ignore the impact
of these results on the South Platte River system. WWE says that the mounding
effect will be mitigated by Little Dry Creek, which will act like a drain allowing
higher groundwater levels to discharge into the creek. Similar to the Little Dry
Creek, the South Platte will also act like a drain as mounded groundwater will flow
18
GANSER LUJAN & ASSOCIATES
Hydrogeologrcal and Environmental Consultants
toward the northeast and discharge into the South Platte River adjacent to the
Bachofer property. This discharge will increase surface flows in the channel.
• The results of Deere and Ault's HEC-RAS surface model and appear to be unreliable
and unrealistic when compared to actual stream flow data measured at the nearby
Fort Lupton gauging station. Peak flows during recent flooding measured 9,200 cfs
at this station on 9/13/2013, while the water level was at 4877.6 feet. The model
results produced a 100 year level at 4876.5 feet with flows of 29,000 cfs. The model
results should not be used as a basis to re-establish a new 100 -year flood plain map.
• Groundwater mounding that occurs on the west side due to the impermeable slurry
wall around L.G. Everist pits has created a higher heads in this area and as a
consequence , the alluvial aquifer is discharging and contributing more water to
surface flows in the river than what was occurring prior to the slurry wall
construction.
• Deere & Ault have misconstrued the results of the USGS Report 2010-5019. The
presence of mounding 2 to 4 feet above normal levels means that in addition to the
actual length of the shiny wall, a hydraulic barrier to flow has formed beyond the
edges of the slurry wall that will block nearly all alluvial flow. This means that
essentially no alluvial groundwater can flow northward to discharge areas as in the
past. As a result, nearly all of this flow will be routed into the river channel as either
alluvial or surface flow ultimately increasing the base -flow and stage level in the
river in this area.
• There is some evidence to suggest that higher groundwater levels exist on the east of
the river at or about the time that the pits were lined. Data obtained from the
Colorado Division of Water Resources well listing report shows two wells in the
vicinity of the Bachofer property with a continuous monitoring record that dates
back to 1989 and 1994.
• Historic aerial photos from Google Earth from 2002 to 2012 show that the river
morphology has changed. A noticeable increase in the formation of sand bars has
occurred in a reach of the river from approximately 1/2 mile upstream of the Bachofer
property to the Fort Lupton gauging station %2 mile downstream meaning that the
river stage has increased and caused flooding of the subject property.
• There is ample evidence that indicates construction of the sand and gravel pits
located on the west of the South Platte River from the Bachofer property has
disturbed the natural topographic surface of the area. Prior to construction of the
Golden cell, the land was below the 100 year flood plain elevation of 4874 and thus
could mitigate a flood swell or higher stage in the river that was above this value.
Based on a pre -mining topographic map developed by Deere and Ault (11/25/2008)
and a survey of the Golden Cell by King Surveyors (5/19/2009), approximately 60
percent of this area that was once part of the flood fringe that could mitigate flood
stage levels has been removed. More importantly, the Golden Cell together with
19
GANSER LUJAN Si ASSOCIATES
Hydrogeologkal and Environmental Consultants
L.G. Everist's pits to the west has removed approximately 90 percent of the area that
was historically at or below the 100 year level. Removal of this area has
significantly impacted the ability of the river to dissipate stage levels and prevent
flooding on the east side of the river.
Prepared by:
cue ,f14,()
Joel M. Sobol
Reviewed by:
Donald R. Ganser, P.G.
Senior Hydrogeologist Principal Hydrogeologist
20
GANSER LUJAN Si, ASSOCIATES
Hydrogeolagical and Environmental Consultants
References
Arnold L.R. et al, 2010, Land -Use Analysis and Simulated Effects of Land -Use Change and
Aggregate Mining on Groundwater Flow in the South Platte River Valley, Brighton to Fort
Lupton, Colorado, U.S.G.S. Open File Report 2010-5019
Arnold L.R.et al., 2003, Analytical and Numerical Simulation of the Steady -State
Hydrologic Effects of Mining Aggregate in Hypothetical Sand -and -Gravel and Fractured
Crystalline -Rock Aquifers, U.S.G.S. Open File Report 02-4267
Colorado Division of Reclamation, Mining and Safety (DRMS), October 22, 2010, L.G.
Everist Fort Lupton Sand and Gravel, Permit No. M-1999-120, Bachofer Property Flooding
as Result of Reclamation Slurry Walls
Colorado Division of Water Resources, Groundwater levels in the South Platte River Basin,
Accessed September 2013
Deere and Ault, September 22, 2010 Letters from Mr. Ross Bachofer Regarding Flooding of
Property East of South Platte River near the Everist Fort Lupton Mine;
U.S.G.S. 2013, USGS surface water data for Colorado, Peak flows stream flow statistics
form National Water Information System Database, accessed September 2013.
Wright Water Engineering Report, January 3, 2005 Response to Comments by DMG in
Adequacy Review of Ft. Lupton Sand and Gravel Mine Application (#M-1999),
21
Prepared in cooperation with the City of Fort Lupton and the City of Brighton
Land -Use Analysis and Simulated Effects of Land -Use
Change and Aggregate Mining on Groundwater Flow in the
South Platte River Valley, Brighton to Fart Lupton, Colorado
. .; ;if.;Vn►.
1r
kte
1 •:_'
Scientific Investigations Report 2010-5019
U.S. Department of the interior
U.S. Geological Survey
Land -Use Analysis and Simulated Effects
of Land -Use Change and Aggregate Mining
on Groundwater Flow in the South Platte
River Valley, Brighton to Fort Lupton,
Colorado
By L.R. Arnold, C.S. Mladinich, W.H. Langer, and J.S. Daniels
Prepared in cooperation with the City of Fort Lupton and the City of Brighton
Scientific investigations Report 2010-5019
U.S. Department of the Interior
U.S. Geological Survey
U.S. Department of the Interior
KEN SALAZAR, Secretary
U.S. Geological Survey
Marcia K. McNutt, Director
U.S. Geological Survey, Reston, Virginia: 2010
Far mare information on the USGS—the Federal source far science about the Earth, its natural and living resources,
natural hazards, and the environment, visit http://www usgs.gov or call 1-888-ASK-USGS
For an overview of USGS information products, including maps, imagery, and publications,
visit http://www usgs.gov/pubprod
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Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the
U.S Government
Although this report is in the public domain, permission must be secured from the individual copyright owners to
reproduce any copyrighted materials contained within this report
Suggested citation:
Arnold, L R,, Mladinich, C S., Langer, W.H., and Daniels, J.S., 2010, Land -use analysis and simulated effects of land -
use change and aggregate mining on groundwater flow in the South Platte River valley, Brighton to Fort Lupton, Colo
rado: US Geological Survey Scientific Investigations Report 2010-5019, 117 p
Contents
Abstract 1
Introduction 2
Purpose and Scope 2
Study Area Description 4
Physiography and Climate 4
Streams and Ditches 4
Aggregate Mining 7
Wetlands 9
Groundwater Hydrology 9
Geologic Setting 9
Aquifer Characteristics 12
Groundwater Levels 12
Hydraulic Properties 14
Aquifer Inflows 16
Recharge 16
Subsurface Inflow 24
AquiferOutflows............................................................................ 24
Flow to the South Platte River 24
Well Withdrawals 24
Phreatophyte Evapotranspiration 27
Mine Dewatering and Subsurface Outflow 27
Land -Use Analysis 27
Socioeconomic Trends 27
Land -Use Trends 27
Predictions of Land -Use Change 29
Wetland Mapping 33
Simulation of Groundwater Flow 39
Mathematical Methods 40
Model Design 40
Spatial Discretization ...40
Boundary Conditions and Hydrologic Stresses 40
Hydraulic Conductivity 43
Parameterization 44
Model Calibration 44
Observations and Prior Information 46
Calibration Assessment .............. .................................. 47
Statistical Measures of Overall Model Fit 47
Randomness, Independence, and Normality of Residuals 55
Simulated Steady -State Groundwater Flow in the Alluvia! Aquifer58
Sensitivity Analysis. 58
ModelNonlinearity. ....... .............. •..... ,.......... ........... ................................... 62
Simulated Effects of Land -Use Change and Aggregate Mining on Groundwater Flow 62
iv
Simulated Hydrologic Effects of Land -Use Change 62
Simulation 1 —Land -Use Conditions in 2020 62
Simulation 2 —Land -Use Conditions in 2040 65
Summary of Land -Use Change Simulations 65
Simulated Hydrologic Effects of Aggregate Mining 65
Simulated Cumulative Hydrologic Effects of Reclaimed Aggregate Pits 65
Simulation Design 67
Considerations for Transient Simulations 67
Simulation 3 —Reclaimed Pits in 202068
Simulation 4 —Reclaimed Lined Pits in 2040 71
Simulation 5 —Reclaimed Unlined Pits in 2040 72
Summary of Reclaimed Pit Simulations75
Simulated Hydrologic Effects of Actively Dewatered Pits 75
Simulation 6 —One Actively Dewatered Pit 76
Simulation 7 Two Closely Spaced, Actively Dewatered Pits76
Simulation 8 —Two Widely Spaced, Actively Dewatered Pits 79
Simulation 9 —Three Closely Spaced, Actively Dewatered Pits .... 79
Summary of Actively Dewatered Pit Simulations.. 83
Simulated Effects of Pit Spacing and Configuration on Groundwater
Levels Near Pits 83
Simulated Effects of Three Aligned Pits 83
Simulated Effects of Three Offset Pits 85
Summary of Simulated Pit Spacing and Configuration Effects 90
Model Limitations and Transferability of Results 90
Summary and Conclusions 92
Acknowledgments 94
References Cited 94
Appendix —Color infrared aerial photographs used by this study to map wetlands
and surface water in the South Platte River valley, Brighton to Fort Lupton, Colorado 99
V
Figures
1. Location map of South Platte study area 3
2. Location map of weather stations and stream gages used by the study 5
3. Bar graph showing mean monthly streamflow of the South Platte River at
Fort Lupton, Colorado 6
4. Bar graph showing sum of mean monthly diversions from the South Platte River
by Brighton, McCanne, Lupton Bottom, and Platteville Ditches 6
5. Two methods of dry -mining aggregate below the water table 8
6. Bar graph showing distribution of pit sizes and methods used to reclaim pits,
Brighton to Fort Lupton, Colorado 8
7. Landforms and stratigraphic units of the South Platte River valley and
its tributaries 10
8. Geologic sections through alluvium of the South Platte River valley, Brighton to
Fort Lupton, Colorado 11
9. Map showing saturated thickness and generalized water -table conditions of the
South Platte alluvial aquifer, Brighton to Fort Lupton, Colorado 13
10. Graphs showing groundwater -level fluctuations in four wells completed in the
South Platte alluvial aquifer, Brighton to Fort Lupton, Colorado, 1954-2003 15
11. Graph showing relation of transmissivity determined by aquifer tests to
transmissivity determined from specific capacity for 27 wells in the South
Platte alluvial aquifer between Denver and Greeley, Colorado... ........... , ..... ... 16
12. Map showing hydraulic -conductivity distribution and location of wells used
to estimate hydraulic conductivity of the South Platte alluvial aquifer, Brighton
toFort Lupton, Colorado ....................19
13. Bar graph showing mean monthly gain or loss in flow of the South Platte River
between stream gages located near Henderson and Fort Lupton, Colorado 26
14. Bar graph showing sum of mean municipal -well withdrawals by Brighton and Fort
Lupton during the irrigation season IMay—October) and non -irrigation season
(November —April). ....... ................26
15. Bar graph showing populations of Brighton and Fort Lupton, Colorado,1910-2000 28
16. Bar graph showing farm and non -farm employment in Adams and Weld
Counties, Colorado, 1970-2005 28
17-20. Maps showing:
17 Generalized land use, Brighton to Fort Lupton, Colorado......, ..... 30
18. Predicted extent of urban land use, Brighton to Fort Lupton, Colorado 34
19. Predicted extent of aggregate mining, Brighton to Fort Lupton, Colorado 36
20. Location and extent of wetlands and surface water mapped by this study,
Brighton to Fort Lupton, Colorado 38
21. Model grid and boundary conditions of the simulated South
Platte alluvial aquifer, Brighton to Fort Lupton, Colorado 41
22. Layer thickness of the simulated South Platte alluvial aquifer, Brighton to
Fort Lupton, Colorado. 42
23. Hydraulic -conductivity zones of the simulated South Platte alluvial aquifer,
Brighton to Fort Lupton, Colorado, ....... ,..,„ 45
24. Location of hydraulic -head observations and stream gages used to estimate
streamflow gain -loss observations for the simulated South Platte alluvial aquifer,
Brighton to Fort Lupton, Colorado 48
vi
25-28. Graphs showing:
25. Relation of weighted residuals to weighted simulated values 56
26. Relation of unweighted residuals to unweighted hydraulic -head observations..,56
27. Normal probability of weighted residuals 57
28. Relation of weighted observations to weighted simulated values 57
29. Map showing simulated steady-state distributions of hydraulic head in the
South Platte alluvial aquifer, Brighton to Fort Lupton, Colorado .... 59
30. Map showing location of largest wetlands mapped as part of this study and
riparian herbaceous flora, Brighton to Fort Lupton, Colorado 61
31. Bar graph showing composite scaled sensitivities of parameters for the
calibrated steady-state model of the South Platte alluvial aquifer, Brighton
to Fort Lupton, Colorado................................................................................... ................ . 63
32-34. Maps showing:
32. Simulation 1 —Steady-state drawdown resulting from predicted
land -use conditions in 2020, Brighton to Fort Lupton, Colorado 64
33. Simulation 2 —Steady-state drawdown resulting from predicted land -use
conditions in 2040, Brighton to Fort Lupton, Colorado 6€
34. Simulation 3 —Groundwater -level changes in 2035 resulting from the potential
extent of reclaimed aggregate pits in 2020, Brighton to Fort Lupton, Colorado69
35. Graphs showing simulated transient groundwater -level changes, Brighton to
Fort Lupton, Colorado 70
36-41. Maps showing:
36. Simulation 4 —Groundwater -level changes in 2055 resulting from the
potential extent of reclaimed aggregate pits in 2040, Brighton to Fort Lupton,
Colorado 73
37. Simulation 5 —Groundwater -level changes in 2055 resulting from the
predicted extent of reclaimed aggregate pits in 2040, Brighton to
Fort Lupton, Colorado 74
38. Simulation 6 Drawdown resulting from a single actively dewatered
pit in the South Platte alluvial aquifer after 1 year and after 15 years of
dewatering 78
39. Simulation 7—Drawdown resulting from two closely spaced, actively
dewatered pits in the South Platte alluvial aquifer after 1 year and after
15 years of dewatering 80
40. Simulation 8—drawdown resulting from two widely spaced, actively
dewatered pits in the South Platte alluvial aquifer after 1 year and
after 15 years of dewatering 81
41, Simulation 9—Drawdown resulting from three closely spaced,
actively dewatered pits in the South Platte alluvial aquifer after 1 year
and after 15 years of dewatering B2
42. Seven configurations used to simulate the effect of pit spacing and
configuration on groundwater levels near reclaimed lined pits... 84
43. Maps showing simulated groundwater -level changes resulting from three
1,400 -ft -wide lined pits 86
44. Graphs showing relation of groundwater -level changes to pit spacing 89
45. Graphs showing relation of groundwater -level changes to pit offset distance 91
VII
Tables
1. Aquifer transmissivity and hydraulic conductivity estimated from aquifer tests
and specific capacity of wells completed in the South Platte alluvial aquifer,
Brighton to Fort Lupton, Colorado 17
2. Seasonal groundwater -level rises and estimated recharge from applied
irrigation water, Brighton to Fort Lupton, Colorado ................................. 20
3. Mass -balance estimation of gain or loss of water along the South Platte
River between stream gages located near Henderson and Fort Lupton for
the periods 1954-60,1974-80, and 1997-2003.. .............. ........ ............... ......... 25
4. Area and percent change of urban, irrigated, and non -irrigated land uses,
1957-2000, and predicted urban land use in 2020 and 2040, Brighton to
Fort Lupton, Colorado 29
5. Summary statistics for wetlands and surface water mapped by this study,
Brighton to Fort Lupton, Colorado, July —August, 200439
6. Initial and final estimated parameter values for the calibrated steady-state
model of the South Platte alluvial aquifer, Brighton to Fort Lupton, Colorado 46
7. Hydraulic -head observations used to calibrate the steady-state model of the
South Platte alluvial aquifer, Brighton to Fort Lupton, Colorado 49
8. Flow observations used to calibrate the steady-state model of the South Platte
alluvial aquifer, Brighton to Fort Lupton, Colorado54
9. Statistics used to assess calibration of the steady-state model of the South
Platte alluvial aquifer, Brighton to Fort Lupton, Colorado 55
10. Groundwater budgets for the calibrated model and simulations of the
hydrologic effects of predicted land -use change and reclaimed pits in
2020 and 2040, Brighton to Fort Lupton, Colorado60
11. Groundwater budgets for simulations of the hydrologic effects of actively
dewatered pits in the South Platte alluvial aquifer 77
viii
Conversion Factors
Inch/Pound to SI
Multiply
By
To obtain
Length
inch (in,)
foot (ft)
mile (mil
2.54
0.3048
1.609
centimeter (cm)
meter (m)
kilometer (km)
Area
acre
square foot (112)
square mile (mil)
0.4047
0.09290
2.590
Volume
hectare (ha)
square meter (m2)
square kilometer (km2)
acre-foot (acre -fl)
1,233
cubic meter (m3)
Flow rate
cubic foot per day (ft3/d)
cubic foot per second (fi3/s)
gallon per minute (gal/min)
acre-foot per year (acre-il/yr)
gallon per minute per foot
[(gikl/minl/f111
foot per day (f/d)
0.02832
0, 02832
0.06309
1,233
Specific capacity
cubic meter per day (m3/d)
cubic meter per second (m3/s)
liter per second (L/s)
cubic meter per year (m3/yr)
0.207(1 liter per second per meter [(L/s)/m]
Hydraulic conduc Eivity
0.3048 meter per day (mid)
foot per mile (ft/mil
Hydraulic gradient
0.1894 meter per kilometer (m/km)
Transmissivity*
foot squared per day (112/d)
0.09290 meter squared per day (m2/dl
Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:
°C=1°F-32)11.8
Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929
{NGVD 291 unless otherwise noted.
Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 831.
Altitude, as used in this report, refers to distance above the vertical datum.
*Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot times
foot of aquifer thickness [(ft3/d)/ft9ft, In this report, the mathematically reduced form, foot
squared per day (ft2/d), is used for convenience.
Other abbreviations used in this report:
L Length
T Time
L/T Length per time
Land -Use Analysis and Simulated Effects of Land -Use
Change and Aggregate Mining on Groundwater Flow in
the South Platte River Valley, Brighton to Fort Lupton,
Colorado
By L.R. Arnold, C.S. Mladinich, W.H. Langer, and J.S. Daniels
Abstract
Land use in the South Platte River valley between the
cities of Brighton and Fort Lupton, Colo,, is undergoing
change as urban areas expand, and the extent of aggregate
mining in the Brighton —Fort Lupton area is increasing as the
demand for aggregate grows in response to urban develop-
ment. To improve understanding of land -use change and the
potential effects of land -use change and aggregate mining on
groundwater flow, the U.S, Geological Survey, in cooperation
with the cities of Brighton and Fort Lupton, analyzed socio-
economic and land -use trends and constructed a numerical
groundwater flow model of the South Platte alluvial aquifer in
the Brighton —Fort Lupton area. The numerical groundwater
flow model was used to simulate (1) steady-state hydrologic
effects of predicted land -use conditions in 2020 and 2040, (2)
transient cumulative hydrologic effects of the potential extent
of reclaimed aggregate pits in 2020 and 2040, (3) transient
hydrologic effects of actively dewatered aggregate pits, and
(4) effects of different hypothetical pit spacings and configu-
rations on groundwater levels. The SLEUTH (Slope, Land
cover, Exclusion, Urbanization, Transportation, and Hillshade)
urban -growth modeling program was used to predict the extent
of urban area in 2020 and 2040. Wetlands in the Brighton —Fort
Lupton area were mapped as part of the study, and mapped
wetland locations and areas of riparian herbaceous vegeta-
tion previously mapped by the Colorado Division of Wildlife
were compared to simulation results to indicate areas where
wetlands or riparian herbaceous vegetation might be affected
by groundwater -level changes resulting from land -use change
or aggregate mining.
Analysis of land -use conditions in 1957, 1977, and
2000 indicated that the general distribution of irrigated land
and non -irrigated land remained similar from 1957 to 2000,
but both land uses decreased as urban area increased. Urban
area increased about 165 percent from 1957 to 1977 and
about 56 percent from 1977 to 2000 with most urban growth
occurring east of Brighton and Fort Lupton and along major
transportation corridors. Land -use conditions in 2020 and
2040 predicted by the SLEUTH modeling program indicated
urban growth will continue to develop primarily east of
Brighton and Fort Lupton and along major transportation
routes, but substantial urban growth also is predicted south
and west of Brighton.
Steady-state simulations of the hydrologic effects of
predicted land -use conditions in 2020 and 2040 indicated
groundwater levels declined less than 2 feet relative to simu-
lated groundwater levels in 2000, Groundwater levels declined
most where irrigated land was converted to urban area and
least where non -irrigated land was converted to urban area.
Simulated groundwater -level declines resulting from land -use
conditions in 2020 and 2040 are not predicted to substantially
affect wetlands or riparian herbaceous vegetation in the study
area because the declines are small and wetlands and riparian
herbaceous vegetation generally are not located where simu-
lated declines occur.
Transient simulations of the cumulative hydrologic
effects of multiple reclaimed pits in 2020 and 2040 indi-
cated that lined and fines-backfilled pits caused groundwater
levels to decline downgradient from pits and to rise upgradi-
ent from pits, whereas unlined pits had the opposite effect,
The maximum decline resulting from the cumulative effects
of reclaimed pits in 2020 and 2040 ranged from about 9 to
1 feet, and the maximum rise was about 5 to 9 feet. Ground-
water levels changed most during the first year, and ground-
water levels ceased to change substantially in most areas of
the simulated aquifer within about 10 years. Some wetlands
or areas of riparian herbaceous vegetation are located where
simulated groundwater levels resulting from the effects of
reclaimed pits in 2020 and 2040 changed more than 2 feet,
indicating that groundwater -supported wetlands or riparian
herbaceous vegetation at these locations might be affected by
the changed groundwater levels. Some areas where ground-
water -level rises resulting from the cumulative effects of
reclaimed pits in 2020'and 2040 occurred where the simu-
lated depth to water is less than 5 feet, potentially creating
2 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
conditions favorable to the formation of new wetlands at these
locations.
Transient simulations of hypothetical actively dewatered
pits were used to predict the magnitude and extent of draw -
down resulting from a single pit, two closely spaced pits, two
widely spaced pits, and three closely spaced pits. The maxi-
mum extent of drawdown of 2 or more feet ranged from about
12,400 feet for a single dewatered pit to about 13,100 feet for
three closely spaced dewatered pits. Most drawdown occurred
during the first year, and drawdown extent after 1 year was
almost as great as after 15 years. Because dewatering typically
occurs for multiple years and most drawdown occurs rapidly
during the first year, groundwater -supported wetlands in areas
of 2 or more feet of drawdown might be affected by lower
grotutdwater levels resulting from pit dewatering.
Transient simulations of three hypothetical lined pits
were used to evaluate the effect that pit spacing and configura-
tion have on groundwater -level changes resulting from lined
pits, Simulations indicated that groundwater -level changes
resulting from lined pits became larger as pit size increased
and became smaller as pit spacing increased. Groundwater -
level changes resulting from lined pits decreased by suc-
cessively smaller amounts as the distance between pits was
increased. Offsetting the center pit upgradient or downgradi-
ent from other pits decreased the hydrologic effects of lined
pits to a greater extent than increasing the distance between
aligned pits. Simulations indicated that offset pits with a spac-
ing of 200 to 400 feet provide a configuration that reduces the
hydrologic effects of lined pits by the greatest amount while
minimizing the distance between pits.
Introduction
The South Platte River valley between the cities of
Brighton in Adams County and Fort Lupton in Weld County,
Colo. (fig. I), is underlain by an extensive unconfined allu-
vial aquifer and is undergoing land -use change as urban
areas expand and the extent of aggregate mining increases
in response to urban development. Changes in land use and
land cover can have substantial influence on economic and
environmental conditions at multiple scales and can affect
the distribution and quantity of aquifer recharge (Bauer and
Vaccaro, I 990; Harbor, 1994; Scanlon and others, 2005),
which can alter groundwater levels and flow directions in
shallow alluvial aquifers underlying areas of land -use change.
The presence of large aggregate -mining pits excavated below
the water table also can affect groundwater levels and flow
directions in shallow alluvial aquifers. When a pit is mined
in a dry condition (dry mining), groundwater is pumped or
otherwise removed from the pit, and drawdown occurs in the
surrounding aquifer as the pit is deepened (Knepper, 2002).
When aggregate mining is completed, pits typically are either
backfilled, allowed to refill with water, or lined with a low -
permeability barrier to create reservoirs for water storage. Pits
backfilled with fine sediments can alter local aquifer hydraulic
conductivity and also can affect the direction of groundwater
flow. Pits allowed to refill with water create areas of evapora-
tion, causing groundwater losses to the atmosphere, Pits lined
with low -permeability material create barriers to groundwater
.flow that can cause groundwater levels to rise upgradient from
pits and decline downgradient from pits (Arnold and others,
2003), Groundwater levels near aggregate pits commonly are
monitored during the life of mining operations, but isolating
the hydrologic effects of pits, whether active or reclaimed,
can be difficult because multiple pits can affect groundwater
levels and flow directions in a complex manner along an entire
river reach. In addition, other hydrologic stresses, such as
well pumping, can affect groundwater levels near pits. Where
the water table is near land surface, wetlands might depend
on groundwater to support vegetation, and large changes in
groundwater levels resulting from land -use change or aggre-
gate mining could adversely affect wetlands, Groundwater -
level declines could cause wetlands to dry up, and ground-
water -level rises could flood wetlands or create conditions
favorable for the formation of wetlands at new locations,
in 2004, the U.S. Geological Survey (USGS), as part of
the Central Region Integrated Science Partnership program,
initiated a study of land -use change and the cumulative effects
of land -use change and aggregate mining on groundwater flow
in the alluvial aquifer of the South Platte River valley between
the cities of Brighton and Fort Lupton. As part of the study, a
numerical groundwater flow model of the South Platte alluvial
aquifer was developed to simulate the steady-state hydrologic
effects of land -use change and multiple reclaimed pits, and
wetlands were mapped to indicate areas where wetlands might
be affected by changes in groundwater levels. In 2005, the
study was expanded in cooperation with the City of Fort Lup-
ton and the City of Brighton to include simulation of short-
term transient hydrologic effects of both active and reclaimed
pits and the effects of different hypothetical pit spacings and
configurations on groundwater flow.
Purpose and Scope
The purpose of this report is to present results of the
land -use analysis and the simulated effects of land -use change
and aggregate mining on groundwater flow in the South Platte
River valley between the cities of Brighton and Fort Lup-
ton, Colo, Results also provide an indication of areas where
wetlands and areas of riparian herbaceous vegetation might be
affected by groundwater -level changes.
Socioeconomic and land -use data were compared and
analyzed to improve understanding of the kinds and locations
of changes that are taking place and the rate at which changes
are occurring, Historical land -use data were used as input to
the urban -growth modeling program SLEUTH (Slope, Land
cover, Exclusion, Urbanization, Transportation, and Hill -
shade) (U.S. Geological Survey and University of California
at Santa Barbara, 2001) to predict the potential extent of urban
Introduction 3
,
COLORADO
LARIMER
0
WELD
Study
�Icn I,l>rl, area
ADAMS
~DENVER
I�l)l t:l,.lti
EXPLANATION
Lake or pond
Stream or river
Canal or ditch
I 2 MILES
I
i I
2 KILOMETERS
40°04'N -
40°02'N
40°N
39°59'N
104°52'W
{
104°50'W
finagery modified from U S Department of Agriculture -farm Solylcc Agency National Agflcrllture Imagery PIoglant,
140,000, 2005
Streams, lakes, and ditches modified from US Geological Survey National Hydrogrephy ➢atoset, 1:190,000
Ronda modified from Col undo Department of Transportation
North American Datum of 1983
1
Figure 1. Location of South Platte study area. Brighton to Fort Lupton, Colorado.
104°48W
104°461A/
4 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo,
development in the study area in 2020 and 2040. Land -use
conditions in 1957, 1977, and 2000 were selected for use in
analyzing historical land -use change because spatial datasets
(U.S. Geological Survey, 1999, 2001b, c) were available for
these time periods. Wetland locations were mapped by using
false -color infrared aerial photographs (appendix) to identify
probable wetland areas, and those areas were subsequently
verified by field inspection,
The USGS modular groundwater modeling program
MODFLOW-2000 (Harbaugh and others, 2000) was used to
simulate the potential effects of land -use change and aggre-
gate mining on groundwater flow. The model was calibrated
to land -use and hydrologic conditions representative of 1957,
1977, and 2000 to facilitate simulations concerning the effects
of land -use change. Hydrologic data for 3 years before and
after each calibration year were used as input to the model
to better represent average hydrologic conditions in each
time period. Simulations include (I) steady-state hydrologic
effects of predicted land -use conditions in 2020 and 2040, (2)
transient cumulative hydrologic effects of the potential extent
of reclaimed aggregate pits (lined, unlined, and backfilled with
fine sediments) in 2020 and 2040, (3) transient hydrologic
effects of actively dewatered aggregate pits, and (4) effects
of different pit spacings and configurations on groundwater
levels.
Areas of simulated groundwater -level decline and rise
were compared to wetland locations mapped by this study
and to areas of riparian herbaceous vegetation mapped by the
Colorado Division of Wildlife (2007a, b) to indicate areas
where groundwater -supported wetlands or riparian herbaceous
vegetation might be adversely affected by groundwater -level
changes or conditions might be favorable for the formation of
new wetlands. The South Platte River valley between the cities
of Brighton and Fort Lupton was selected for study because
it is experiencing substantial land -use changes as agricultural
land is converted to urban and suburban areas and is undergo-
ing rapid development of new aggregate mines.
Study Area Description
Physiography and Climate
The study area (fig. 1) is an 88 -mil region along an I l-mi
reach of the South Platte River in northeastern Colorado,
approximately 20 mi northeast of the city of Denver. The study
area consists of river valley and alluvial terrace landforms
with relief generally less than about 250 ft between the valley
floor and adjacent upland areas (Robson, 1996; Robson and
others, 2000). Land -surface altitude along the river valley
ranges from about 4,870 ft at the northern boundary of the
study area to about 4,980 ft at the southern boundary. The
historically agricultural region also is part of the Wattenberg
field, Colorado's second-largest and the Nation's eighth -
largest gas field (Energy Information Administration, 2006)
and part of the last major source of gravel aggregate in the
Denver metropolitan area (Lindsey and others, 1998). The
South Platte River, its tributaries, and irrigation ditches dissect
the agricultural landscape. There are two main municipalities
in the study area. In the south is the city of Brighton, which
straddles Adams and Weld Counties, and in the north is the
city of Fort Lupton, in Weld County. Both Brighton and Fort
Lupton are situated primarily along the east side of the South
Platte River and along U.S. Highway 85. Between Brighton
and Fort Lupton, on the west side of the South Platte River,
is the small town of Wattenberg. Many farmsteads dot the
landscape surrounding these municipalities. Both dry -land
farming and irrigated agriculture are practiced in the study
area. Irrigated agricultural land is supported by surface -water
diversions from the South Platte River and by groundwater
pumped from the alluvial aquifer. Flood irrigation is the
most common method of irrigation used in the study area,
but irrigation using center -pivot sprinklers also is common
(Colorado Decision Support Systems, 2004), For the purposes
of this report, the irrigation season is considered to be the
period May through October. The region typifies a rural
agricultural landscape transitioning to become a suburban
extension of the greater Denver metropolitan area.
Temperature and precipitation data are available for two
National Weather Service stations (Fort Lupton 2 SE and
Brighton; fig. 2) in or near the study area (Western Regional
Climate Center, 2007). Data from the Fort Lupton 2 SE
station are available for the period 1948 to 1976. Data from
the Brighton station are available for the period 1973 to the
present (2007). Based on the period of record for the two
stations, the mean July maximum temperature in the study
area is 89,6°F, and the mean July minimum temperature
is 56.3°F (Western Regional Climate Center, 2007). The
mean January maximum temperature is 42.7°F, and the
mean January minimum temperature is 13.2°F. Mean annual
precipitation in the study area is 13.3 in. with about 67 percent
of precipitation occurring during the irrigation season from
May through October (Western Regional Climate Center,
2007).
Pan -evaporation data are available in the vicinity of
the study area from two weather stations (Fort Collins and
Wiggins 7 SW; fig. 2) (Western Regional Climate Center,
2007). Mean annual pan evaporation in the study area esti-
mated on the basis of data from these two stations and the
U.S, Department of Commerce (1968) is about 48 in., which
greatly exceeds mean annual precipitation. About 82 percent
of the pan evaporation occurs during the irrigation season
from May through October.
Streams and Ditches
The primary surface -water feature in the study area is
the South Platte River. Streamflow data collected by the U.S,
Geological Survey (2006a) and the Colorado Division of
Water Resources (2006a) indicate streamflow in the South
Platte River varies throughout the year with highest flows
generally in May and June resulting from snowmelt from the
Introduction 5
40°30'N
40°15'N
40°N
105"W
104°45'W
104°30'W
Streams nwdified from U S Geological Survey National Hydrogrnphy Dutaset, 1: LOOM()
North American Datum cf 1983
EXPLANATION
104°15'W
• City or town
Fort Lupton 2 SE r' Weather station and station name —Precipitation and temperature
Wiggins 7 SW Weather station and station name —Precipitation, temperature, and evaporation
06721000 A Stream gage and station number
Figure 2. Location of weather stations and stream gages used by the study.
134°W
6 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
nearby Rocky Mountains. Mean monthly streamflow in the
South Platte River at Fort Lupton (station number 06721000)
(fig. 3) ranges from about 63 to 106 million ft3/d (728-
1,230 ft3fs) during May —June for the time periods (1954-1960,
1974-1980, and 1997-2003) analyzed by this study, After
peak runoff, mean monthly streamflow generally decreases
throughout the summer to a value ranging from about 11 to
33 million ft3/d (128-387 ft3/s), Mean monthly streamflow
generally was greatest for the period 1997-2003 and least for
the period 1954-1960. River stage based on mean monthly
streamflow at Fort Lupton varies by less than 2 ft throughout
the year for the time periods compared; however, river stage
temporarily can increase as much as about 4 ft during periods
of high runoff, Flow in the South Platte River through the
cry 120
100
0
w 80
a
W 60
LL.
40
Lu
cc
0 20
W
2
0
study area is regulated by upstream diversions and releases
and is not representative of natural conditions.
Water is diverted from the South Platte River to irrigate
crops in the study area primarily from about mid -April
to mid -October (Colorado Division of Water Resources,
2006b; fig. 4). Brighton Ditch, Lupton Bottom Ditch, and
Platteville Ditch divert water from the South Platte River in
the study area; although no longer in use, McCanne Ditch
(now occupied by Third Creek) historically diverted water
from the South Platte River in the study area. Brantner Ditch
and Fulton Ditch divert water from the South Platte River
upstream from the study area but supply water to fields in
the study area. During the periods 1954-1960, 1974-1980,
and 1997-2003, the mean of total diversions by the Brighton,
McCanne, Lupton Bottom, and Platteville Ditches was about
June
Jan. Feb. Mar. Apr, May
July
MONTH
Aug.
1
Sept. Oct. Nov. Dec.
I
IN 1954-1960 .
❑ 1974—1980
n 1997-2003
Figure 3. Mean monthly streamflow of the South Platte River at Fort Lupton, Colorado (station
06721000), for the periods 1954-1960,1974-1980, and 1997-2003.
z pC
— w
O w
LL L±L
O L_]
LIJ
cc CO
W
0
Z rn
uJ
IJ.J
J
2 2
18 _
16I-
14
12
10
8
6
4
rt
0
Jan,
Feb, Mar, Apr.
May
June July Aug. Sept.
MONTH
In 1954-1960
❑ 1974-1980 1
El 1997-2003
I
Y�LJJ I J `t
Oct. Nov. Dec.
Figure 4. Sum of mean monthly diversions from the South Platte River by Brighton, McCanne, Lupton
Bottom, and Platteville Ditches for the periods 1954-1960, 1974-1980, and 1997-2003.
Introduction 7
2.1 billion ft3(48,600 acre -fl). These diversions were used to
irrigate an average of about 16,500 acres (Colorado Division
of Water Resources, 2006b), resulting in average water
application of about 3 ft during the irrigation season, Ditches
in the study area generally are unlined (Bob Stahl, Division
1 Water Commissioner, oral comnum,, 2004) and above the
water table.
Tributaries to the South Platte River in the study area
include Big Dry Creek, Little Dry Creek, and Third Creek
(fig. I). Big Dry Creek is the largest tributary in the study area
with mean monthly flow ranging from about 2 to 5 million
ft'Id (23-60 fe/s) (U.S. Geological Survey, 2006a) near
the creek's confluence with the South Platte River (station
number 06720990). Little Dry Creek and Third Creek flow
intermittently. Flow in Little thy Creek is intercepted by
Lupton Bottom Ditch before it reaches the South Platte River.
Aggregate Mining
Second only to crushed stone, more sand and gravel is
produced in the United States than any other nonfuel mineral
commodity in terms of volume and value. During 2005, about
1.4 billion tons of sand and gravel, with a value of $7.46
billion were produced by about 3,800 companies from more
than 6,000 operations in 50 States (Bolen, 2005). In Colorado,
about 46,6 million tons of sand and gravel (about 10 tons per
capita) worth about $256 million were produced during 2005
(Bolen, 2005).
Most sand and gravel is used as construction aggregate
and can be prepared from deposits containing a wide range of
particle sizes, To be an economically valuable resource, the
deposits need to be minable and accessible to nearby markets.
The mining site needs to qualify for all necessary land -use
and environmental permits, and the operation needs to meet or
exceed all costs including acquisition, operation, compliance
with regulations, and reclamation in order to be profitable.
Permit applications and reclamation plans need to consider
many factors including hydrology, geology, land use and
zoning, air quality, cultural and scenic features, vegetation,
and wildlife habitat (Knepper, 2002). Several years may be
required to consider all factors before a permit is issued.
Site preparation for aggregate mining includes removing
vegetation and stripping sufficient overburden to access the
resource, Topsoil commonly is separated from the overburden
and stockpiled for reclamation activities. Site preparation also
includes construction of access roads, fences, berms, haul
roads, drainage ditches, culverts, settlement ponds, process-
ing and maintenance facilities, and other plant infrastructure
(Langer and others, 2004). The life of an aggregate operation
is variable. Mining plans on file with the Colorado Division of
Mining Reclamation and Safety (CDMRS) indicate aggregate
mining operations in the study area plan to operate from 6 to
29 years with an average life of about 14 years. During the life
of the operation, aggregate within the permitted boundaries
typically will be mined in phases lasting an average of about
3 years each,
Sand and gravel can be mined wet by dredging from
water -filled pits, or groundwater can be removed from the
pit so that the materials can be mined dry using conventional
earth moving equipment. Some pits are dewatered by collect-
ing groundwater in drains in the floor of the pit and pumping
the water out of the pit (Langer and others, 2004; fig. 54).
This dewatering technique generally continues for the life of
the pit and may be completed in phases for large pits as min-
ing progresses. As the pit is dewatered, the water table in the
vicinity of the pit is towered, and drawdown resulting from
the dewatering can affect nearby wetlands, wells, and streams.
In some instances, slurry walls are used to isolate the pit from
groundwater during dewatering (Knepper, 2002; fig. 5B). Once
the slurry wall is in place, the pit can be dewatered without
substantially affecting the surrounding aquifer. However, the
slurry wall creates a barrier to groundwater flow that causes
groundwater levels to rise on the upgradient side of the pit and
decline on the downgradient side of the pit.
Following excavation, sand and gravel is processed to
remove unwanted material, Ifthe gravel has a diameter larger
than about 1.5 in., it commonly is crushed and screened to
create properly sized particles. Depending on the product,
the gravel may be washed to remove fine particles, and waste
fines typically are sent to a settling pond. Conveyors move
the sorted sand and gravel to separate stockpiles awaiting sale
(Langer and others, 2004).
Excavation area varies depending on distribution of the
gravel, the amount of pennitted land, and existing infrastruc-
ture, such as roads, buildings, and oil and gas transmission
lines, Pits generally range in size from about 3 acres to more
than 300 acres, Because of the start-up costs of permitting
and operating sand and gravel operations, pits generally are
larger than 10 acres. Based on mining plans for 58 existing or
proposed pits on file with the CDMRS for the study area as of
October 2006, 81 percent of pits are larger than 10 acres and
50 percent are larger than 25 acres (fig. 6). Mining phases for
pits range from about 6 to 81 acres with an average phase size
of about 34 acres. Pits in the study area typically are excavated
down to the bedrock surface.
Following the excavation of available sand and gravel,
pits in the study area may be left unlined and allowed to refill
with water, backfilled with sediments, or lined or surrounded
with a low -permeability barrier, such as a slurry wall or clay
liner, to isolate the pit from the surrounding aquifer (fig. 6).
Pits left open and allowed to refill with water can alter ground-
water flow by creating large areas where sediments comprising
the aquifer have been removed, and groundwater is exposed to
evaporation. Pits backfilled with overburden sediments hav-
ing hydraulic conductivity similar to the surrounding aquifer
typically have little long-term effect on groundwater flow.
Pits backfilled with fine sediments from aggregate processing
can alter groundwater flow because of decreased hydraulic
conductivity within the area of the backfilled pit. Pits lined or
surrounded with a low -permeability barrier at the conclusion
of mining can have the same effect on groundwater levels and
flow as pits surrounded with slurry walls prior to dewatering
8 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
A
Stream channel
-----A Gravel pit
B
Low -permeability bedrock
Water table - sand and gravel
Groundwater collected in
drains and pumped from pit
Slurry walls
Gravel pit
Law -permeability bedrock
Water table
Stream channel
Sand and gravel
Slurry wall trench keyed
into low -permeability bedrock
Figure 5. Two methods of dry -mining aggregate below the water table: (A) Pit dewatered without a slurry wall;
(B) Pit dewatered after installing a slurry wall.
NUMBER OF PITS
20
18
18
14
12
10
8
rr
A
0
Less than 5
❑ Backfilled with overburden orunknown
❑ Backfilled with fine sediments
ri Unlined
ra Clay liner
❑ Slurry wall
5-10 10-25 25—50
PIT SIZE, IN ACRES
50-100
Aft
More than 100
Figure S. Distribution of pit sizes and methods used to reclaim pits, Brighton to Fort Lupton, Colorado.
Groundwater Hydrology 9
at the start of mining. After reclamation, former aggregate pits
can be used for a variety of applications including residential
or commercial property, recreation, natural areas such as wet-
lands, or water storage (Knepper, 2002). Reclamation plans on
file with the CDMRS indicate that about 50 percent of back -
filled pits in the Brighton -Fort Lupton area will be reclaimed
as wetlands,
Wetlands
The term wetlands collectively includes all ecosystems
that contain transitional zones between land and water, encom-
passing many different ecosystem types throughout the world
(Verhoeven, 2003; Mitsch and Gosselink, 2000). A number
of classification systems for wetlands have been developed
for different purposes. One widely used classification system
was prepared by the U.S. Fish and Wildlife Service (USFWS)
and described in a report by Cowardin and others (1979),
Five general wetland systems were identified and divided
into 10 subsystems, 55 classes, and 121 subclasses, all of
which are characterized by examples of dominant types of
plants or animals. Another widely used classification system
was developed to legally define wetlands in the United States
in order to delineate areas that are under the jurisdiction of
Section 404 of the Clean Water Act. The term jur•isdictiortctl
wetland comes from that classification system. Various formal
definitions of wetlands have been developed for scientific and
management purposes over the years (Mitsch and Gosselink,
2000), hut generally they all include the presence of standing
water for some period during the growing season, hydric soils
(with evidence of reducing conditions), and vegetation adapted
to, or tolerant of, saturated conditions. Within the study area,
small wetlands are common along the South Platte River,
irrigation ditches, and around surface -water bodies. Larger
wetlands occur in topographic depressions away from the
river, particularly in abandoned oxbows formed by the South
Platte River. Cottonwood trees and other riparian vegetation
also commonly grow along the South Platte River and within
abandoned oxbows.
En general, wetlands are among the most productive
natural ecosystems on Earth (Hammer, 1991). Wetlands are
valuable for the commercial products they provide, such as
food, fiber, lumber, and energy resources, and for recreational
endeavors, such as boating, hiking, hunting, fishing, and
bird -watching. However, wetlands also provide food, shelter,
nesting areas, breeding grounds, and stopovers for many
types of resident and migratory forms of life, and they support
a large diversity of plants, birds, microbes, invertebrates,
amphibians, reptiles, fish, and mammals, many of which
could not live anywhere else. In addition, wetlands are
valuable because they are capable of flood mitigation, storm
abatement, erosion control, aquifer recharge, and water -quality
improvement (Louisiana Coastal Wetlands Conservation
and Restoration Task Force, 2007), At an even larger and
more complex scale, wetlands influence the global cycles
of nitrogen, sulfur, methane, and carbon dioxide (Louisiana
Coastal Wetlands Conservation and Restoration Taslc Force,
2007; Mitsch and Gosselink, 2000).
Specific vegetation thrives in wetlands because it
has special adaptations to survive the anaerobic rooting
environment and fluctuating groundwater levels that can flood
emergent plant parts. Wetland plants have the unique ability to
enhance oxygen transport below ground through aerenchyma
(an airy root tissue), grow taller in deeper water through
development of elongate leaf stalks, and even stimulate
shallow and above -ground rooting in deep water so the roots
can he in contact with oxygen -containing water higher in the
water column (Crook and Fennessy, 2001).
Although wetland plants have adapted to surviving in
saturated soils, they are similar to terrestrial plants in that they
cannot survive long without water. Length of survival time
depends on the species, its general health, and on numerous
environmental factors, such as temperature, humidity, soil
type, and available nutrients. Plant species that can survive in
saturated conditions for lengthy periods but can also tolerate
drier soil conditions for part of their growing season are called
facultative wetland plants (Reed, 1988).
Wetlands can be supported by surface water,
groundwater, or both, if groundwater levels decline only
slightly beneath groundwater -supported wetlands, effects such
as plant death or a change in plant species composition might
not occur for some time, depending on the water -holding
capacity of the soil, the plant species present, and the weather.
Even periodic rain events can substantially delay or mitigate
the effects of slightly lower groundwater levels on upland
facultative wetland plants. However, if groundwater levels
decline substantially, such as below root depth, the effects
can occur much faster. A large decline in groundwater levels
for an extended period of time could lead to changes in plant
species community composition, wildlife and fish use, and
functions performed by the original wetland communities.
Once the original plants show substantial signs of water stress,
the wetland and all its associated functions likely will change
permanently. On the other hand, if groundwater levels rise
to near land surface, hydrologic conditions favorable to the
formation of new wetlands could be created or the hydrology
of existing wetlands could be altered. The ability of a wetland
to adapt to changes in hydrologic conditions is affected by
the timing and rate of the changes. Many wetland species can
migrate with changing hydrologic conditions if the rate of
change is slow.
Groundwater Hydrology
Geologic Setting
Three categories of alluvial landforms occur along
the South Platte River based on age, relative elevation, and
degree of dissection (fig. 7). From oldest (highest) to youngest
(lowest), they are (1) dissected alluvial fans and terraces
10 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
- /r
\ �- z /may -• f 1 .
- v x
f�1,�IJ,-
..
ti
Gneiss and
AGE
Holocene and
Plcfstocene
1IE❑tcnc
5G1ltllr;mdt,oly
I'Irisu ',vaic.
\\\
G ti
'mow
ft"
. f. - r f.
-
�If.Li:
i f u` y s- r
-,SefIIn1CI11aIy -'' ~ - �'
-'-� he Beek : ; : ; : ; Unconsolidated .....-....1._:::,v.:.,.,,,--.
EXPLANATION
LANUFDRM
Dune fields
C-I�l
Flood plain and
low terraces
Alluvial terraces
Dissected alluvial
fans and tell noes
STRATIGRAPHIC UNIT
Eolian sand and silt
PosI-Piney Creek and
Piney Creek Alluviums
Broadway and
i-fulviets Alluviums
Slocum, Verdes,
Rocky Flats, and pre -
Rocky Flats Alluviums
Figure 7, Landforms and stratigraphic units of the South Platte River valley and its tributaries.
located near the mountain front and on divides between major
streams, (2) alluvial terraces located along the margins of
major stream valleys, and (3) flood plains and low terraces
that compose the modern South Platte River valley and the
valleys of its tributaries (Lindsey and others, 2005). In the
Brighton -Fort Lupton area, dissected alluvial fans and terraces
commonly are composed of sediments of inferior quality for
construction purposes and seldom are mined for aggregate.
Furthermore, they generally are small, topographically high
relative to modern streams, well -drained, and isolated from the
South Platte alluvial aquifer and South Platte River. Therefore,
these landfornzs are not included in the aquifer extent
simulated in this study.
Sediments of the alluvial terrace (Broadway terrace) and
low ten -ace associated with the modern flood plain (post -Piney
Creek terrace) in the study area (fig. 8) generally constitute the
South Platte alluvial aquifer and are included in simulations of
groundwater flow by this study. The Broadway terrace forms
a broad alluvial plain that extends along the eastern side of the
South Platte River from Denver to downstream from Greeley
(Lindsey and others, 2005). In the Brighton —Fort Lupton
area, the Broadway terrace is about 20 ft above the flood
plain of the South Platte River (fig. 8). Alluvial sediments of
the Broadway terrace consist of laterally extensive layers of
gravel, sand, Si It, and clay, but clay layers thicker than about 2
Modified fi ran S I Cmsisy. 1978
F .
ft generally are discontinuous. Gravel of the Broadway terrace
is as much as 50 ft thick near Brighton (fig, 8, section L —L').
The flood plain and post -Piney Creek terrace occupy a
narrow, modern valley, which is less than one-half the width of
the. valley defined by the Broadway terrace, Three gravel units
have been identified within the post -Piney Creek terrace in the
study area (Lindsey and others, 2005). The gravel units are,
from bottom to top, a basal, yellowish -brown coarse gravel; a
middle, yellowish -brown sandy gravel; and an upper, grayish -
brown gravel. The basal gravel rests on the valley floor incised
through the Broadway terrace and into the underlying bedrock.
The upper gravel contains interbedded sand in places and
locally contains cottonwood and willow logs. The three gravel
units can be traced in borehole logs and gravel pits as far north
as Fort Lupton, where all three are sandy (Lindsey and others,
1998).
Total thickness of unconsolidated sediments in the South
Platte River valley between Brighton and Fort Lupton ranges
from 0 ft in areas where bedrock crops out along valley
margins to about 70 ft where the valley of Little Dry Creek
joins the main valley of the South Platte River (Robson, 1996;
Robson and others, 2000, sheet 1). Sediment thickness beneath
the South Platte River generally is 20-40 ft, whereas sediment
thickness between the South Platte River and the valley mar-
gins commonly is 40-60 ft (Robson, 1996; Robson and others,
2000, sheet I).
Groundwater Hydrology 11
104°52'W
o 1 2 MILES
L I
I i I II
0 1 2 KILOMETERS
104°50'W
104°48'W
EXPLANATION
Approximate extent of alluvial aquifer
L L" Trace of section
104°45'W
Figure 8. Geologic sections through alluvium of the South Platte River valley, Brighton to Fort Lupton,
Colorado.
12 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
K
Bedrock
Bedrock
Vel lieu)
exaggel Olion
40:1
Post -Piney Cltek
Upper gravel
Middle giavcl
Basal grovel
Saudi Maw River
Letran;
South Moe h ee+
Past -Trine Qsek Lei ace
Modified fiom Lihdeey and others (2005)
Broadway ten ace
L
V el I ieal exxaggel or: on 40:1
AI nad tvay terrace
J 4,000 8,000 FEET
0 1,000 2000 METHS
510
505
550
495
490
FEET ABOVE SEA LEVEL
K'
-- 5050
5000)
4950
4900
4K50
FEET ABOVE SEA LEVEL
EXPLANATION
EA
Sail, celluvium, overhank
sill., and artificial tilt
Gravel > sand
Sand > gravel
Sand
Silt and clay
[lase nF Holocene (eatimaled)
Borehole cattier
Figure B. Geologic sections through alluvium of the South Platte River valley, Brighton to Fort Lupton, Colorado, —Continued
Nearly all alluvium in the study area overlies bedrock of
the Arapahoe Formation of Cretaceous age (Robson, 1983,
sheet 1). The Arapahoe Formation consists of 400 to 700 ft
of interbedded conglomerate, sandstone, siltstone, and shale.
Lithologic logs of wells and test holes in the study area
indicate that bedrock directly beneath alluvium of the South
Platte River valley in the Brighton -Fort Lupton area generally
is composed of dark blue -gray to black shale that provides
a sharp contact between the bedrock and alluvium. Eolian
(wind-blown) sand and silt, generally less than 3 ft thick,
covers the alluvium in some places and much of the bedrock
outside main stream valleys (Colton, 1978; Trimble and
Machette, 1979).
Aquifer Characteristics
The South Platte alluvial aquifer ranges from about 2 mi
wide near the northern end of the study area to about 4 mi
wide near its southern end (Robson, 1996; Robson and others,
2000). The aquifer boundary is well-defined in most places by
bedrock that crops out to form upland areas along both sides
of the valley, Saturated thickness generally is 20-40 ft but
commonly is 10-20 ft along valley margins and locally might
be as much as 60 ft, particularly along a paleochannel on the
west side of the valley between Big and Little Dry Creeks
(fig. 9). Depth to water generally is less than 20 ft in the study
area and typically is less than 10 ft near the South Platte River;
however, depth to water commonly is 20-40 ft along the
eastern side of the valley (Robson, 1996; Robson and others,
2000, sheet 5). The South Platte alluvial aquifer generally is
unconfined, but small areas of semi -confined conditions can
exist where clay and sand layers are well stratified. Ground-
water Clow in the aquifer predominantly is down valley and
toward the South Platte River and its tributaries (Robson,
1996; Robson and others, 2000, sheet 3) (fig. 9). The water -
table gradient generally is 0.002-0,003 (10-15 ft/mi) down the
South Platte River valley and averages about 0,005 (25 ft/mi)
from the valley margins toward the river,
Groundwater Levels
The water table in the study area generally is highest in
summer and early fall, likely as a result of increased aqui-
fer recharge from infiltration of irrigation water applied to
fields, seepage from unlined ditches, and greater precipitation.
Groundwater levels generally decline throughout the non -
irrigation season (November to April) and reach their lowest
levels in the spring prior to the irrigation season. However, in
Groundwater Hydrology 13
144°52'W
4D°06'N
40°04'N
4D°02'N -
40°N —
39°50'N
Saturated thickness and water -level contours modified fi om Robson (1996); Robson and others (2000)
Sheams modified from U S Geological Survey National Hydrograpby Dataset; 1:100,000
North American Datum of 1983
Saturated thickness, in feet
0-20
20-40
40-60
104°5o'W
104°48'W
In
15
co
C
Fort ' 8
Lupton m
o m
n.
Sa001066D613
d
O
S1)001066304 1066304 DA
3
��a� ♦+
5
i-J
EXPLANATION
104°40'N/
O 1
f l i t l i I
O 1 2 KILOMETERS
2 MILES
—4900- Water -table contour —Shows approximate altitude of watei table
Contour interval is 20 feet Datum is National Geodetic Vertical
Datum of 1929
t
SB00106630ADA O
Generalized direction of groundwater flow
Location of well with hydrograph—Number is U S.
Geological Survey local well number
Figure 9. Saturated thickness and generalized water -table conditions of the South Platte alluvial
aquifer, Brighton to Fort Lupton, Colorado.
14 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
localized areas of heavy well pumping, groundwater levels
could be lower during the irrigation season than at other times
of the year. The magnitude of seasonal groundwater -level
fluctuations (fig. 10) varies by location and from year to year.
Groundwater -level data (U.S. Geological Survey, 2006b;
Michael Schaubs, Colorado Division of Water Resources,
written commun., 2006) indicate groundwater -level fluctua-
tions between the irrigation season and the non -irrigation
season during 1954-2003 generally ranged from about 0 to 7
ft. Groundwater -level fluctuations near the South Platte River
generally were less than near valley margins because of the
stabilizing influence of the river. The magnitude of annual
(year to year) groundwater -level fluctuations commonly was
about one-half the magnitude of the groundwater -level fluc-
tuation between the irrigation season and the non -irrigation
season. Annual groundwater levels generally were stable or
slightly declining during the periods of measurement at each
location.
Hydraulic Properties
Transmissivity of the alluvial aquifer was estimated from
aquifer -test results presented in published reports (Wilson,
1965; Smith and others, 1964) and from specific -capacity data
reported by McConaghy and others (1964), Schneider (1962),
and well -construction records on file with USGS and the
Colorado Division of Water Resources. The method of Theis
and others (1963, p. 331-341) was used to estimate aquifer
transmissivity from specific -capacity data using a modified
form ofTheis's equation I as presented by Prudic (1991). The
modified form of the equation is given as:
T = 15.32(Q/s)(-0.577—] n[r2S/4Tt]) (1)
where
T is aquifer transmissivity, in feet squared per day,
O r is specific capacity of the pumped well, in gallons
per minute per foot,
r is effective radius of the pumped well, in feet,
S is storage coefficient of the aquifer (dimensionless),
and
1 is elapsed pumping time, in days.
Given input values of OGs•, r, S and 1, aquifer transmissiv-
ity (7) can be determined by providing an initial estimate of
Tin the right side of the equation and iteratively substituting
calculated Tfrom the left side of the equation back into the
right side until the values for Ton both sides of the equation
are essentially the same. Because the South Platte alluvial
aquifer generally is unconfined, specific yield was substituted
for .S' in equation 1. The value of specific yield in equation
1 was estimated to be 0.25 from data provided by Johnson
(1967), who reported specific yield values ranging from 0.12
to 0.35 with a mean value of about 0.25 for materials ranging
from medium sand to coarse gravel. A diameter of 18 in. was
assumed for wells without casing information, based on the
most common well diameter reported on well -construction
records. Because storage coefficient (5) and well radius (f') are
within the natural -log Leon of equation 1, calculated transmis-
sivity is relatively insensitive to errors in estimates of S and P.
Transmissivities calculated by using specific capacity of wells
with small pumping rates typically were found by this study to
be an order of magnitude less than those with large pumping
rates. Therefore, only wells with pumping rates greater than
100 gal/min (similar to aquifer -test conditions) were used to
estimate transmissivity from specific capacity.
Comparison of transmissivities calculated using equa-
tion 1 to transmissivities determined by aquifer tests indicates
that transmissivities based on specific capacity generally are
less than those from aquifer tests. To improve the accuracy of
transmissivity estimates based on specific capacity, transmis-
sivity values from aquifer -test results provided by Wilson
(1 965) and Smith and others (1964) for 27 wells in the South
Platte alluvial aquifer between Denver and Greeley were
regressed (fig. 11) against transmissivity estimates based on
specific capacity, and the resulting linear regression equation
was used to adjust the transmissivity estimates. The regression
has a correlation coefficient of 0.89 and a coefficient of deter-
mination (R2) of 0.80. The equation used to adjust transmis-
sivities calculated from specific capacity data is:
TJ=1.2T4,+7,755
(2)
where
T' is the final, adjusted transmissivity value, in feet
ad
squared per day, and
T. is transmissivity estimated from specific capacity, in
feet squared per day.
Because the regression line of the equation does not
intercept the graph origin, the regression equation is not valid
for transmissivity values substantially less than the small-
est transmissivity value determined from specific capacity.
Only wells with construction similar to those used to develop
the regression equation were used in the estimation of final,
adjusted transmissivity values (table 1).
Hydraulic conductivity was estimated by dividing cal-
culated transmissivity by the length of the perforated interval
at each well location (table 1). The hydraulic -conductivity
distribution and location of wells used to estimate hydraulic
conductivity are shown in figure 12. Estimated hydraulic con-
ductivity ranges from 390 ft/d at one location near the margin
of the aquifer to 2,100 ft/d in a small area near the center
of the aquifer south of Brighton. Most estimated hydraulic -
conductivity values range from about 600 to 1,400 ft/d with
the greatest values generally along the central part of the val-
ley. Aquifer tests conducted in the Arapahoe bedrock aquifer
underlying the study area indicate hydraulic conductivity
values ranging from 0.3 to 1.5 ft/d (Robson, 1983), which is 2
to 3 orders of magnitude less than the hydraulic conductivity
of the alluvial aquifer.
Groundwater Hydrology 15
DEPTH TO WATER, IN FEET BELOW LAND SURFACE
10
15
20
25
30
11950 1955 1960 1955 1970 1975 1980 1985 1990 1995 2000 2D05
S B 00206631 AC D
10
15
20
25
-r--L . •l
30 '
1950 1955 1960 1965 1970 1975 1980 1985 199D 1995 2000 2005
SB00106608BCD
10
15
20
25
. T. —r ter,tee, r•e r , I , •
S B 0010663 DAD A
30 E I I L
1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005
10
15
20
25
• -rrI—.
SC00106607CCB
30 I t . �. J_. .� • • I . . 1 • I.
1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005
YEAR
Figure 10. Groundwater -level fluctuations in four wells completed in the South Platte alluvial aquifer, Brighton
to Fort Lupton, Colorado, 1954-2003. (The 13 -digit well identifier is the U.S. Geological Survey site name or the
Colorado Division of Water Resources location number.)
16 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
w 80,000
w
u_
z
Lu
w
w 60,000
U-
a o 50,000
m cc
e,
w d
?w
Ma
w�
w
0
20,000
2 10,000 -
z
70,000
40,000
30,000
0-,
I
♦
♦
1 1 1 1 1
10,000 20,0o0
I
T9I1 =12 T„#7,755
R2=0.80
1 I I I E I I I I I
30,000 40,000
I I _
♦
50,000 60,000
TRANSMISSIVITY DETERMINED FRDM SPECIFIC CAPACITY,
IN FEET SQUARED PER DAY
Figure 11. Relation of transmissivity determined by aquifer tests to transmissivity determined from
specific capacity for 27 wells in the South Platte alluvial aquifer between Denver and Greeley, Colorado.
Aquifer Inflows
Inflows to the South Platte alluvial aquifer in the study
area are primarily from infiltration of precipitation and water
applied to irrigated agricultural fields, seepage from irrigation
ditches, and subsurface inflow from the upgradient end of
the aquifer and tributary valleys. Depending on the hydraulic
gradient between the alluvial aquifer and the underlying
Arapahoe bedrock aquifer, the South Platte alluvial aquifer
also could receive groundwater inflow from the Arapahoe
aquifer, but because the hydraulic conductivity of the
Arapahoe aquifer is much less than that of the alluvial aquifer,
inflow from the Arapahoe aquifer likely is a small component
of the total water budget for the alluvial aquifer.
Recharge
Recharge from infiltration at the land surface varies spa-
tially and temporally and depends on many factors, such as the
amount, rate, and timing of precipitation and applied water;
evapotranspiration; surface cover (including vegetation); soil
type; geology; and slope of the land surface. For the purposes
of this study, recharge in the Brighton —Fort Lupton area was
distributed by land use (native and non -irrigated land, irrigated
agricultural land, and urban areas) because land use generally
considers differences in applied water, evapotranspiration,
and surface cover. Precipitation, soils, geology, and slope
are considered to be substantially uniform with respect to
recharge within the extent of the South Platte alluvial aquifer
in the study area and are not used as discriminating factors for
recharge.
Recharge beneath native and non -irrigated land.
Recharge from infiltration of precipitation beneath native
grasslands and shrublands in arid and semiarid regions, where
evaporation exceeds precipitation most days of the year, typi-
cally is small (Scanlon and others, 2005). A synthesis of 26
recharge studies by Scanlon and others (2006) indicates mean
annual recharge beneath predominantly natural ecosystems in
semiarid and arid regions ranges from about 0.008 to 1.4 in.,
which represents 0.1-5 percent of precipitation. When native
vegetation is converted to dry -land (non -irrigated) agriculture,
recharge can increase because tilled and fallow fields may
allow more infiltration, or it can decrease because crops with
deeper roots might more efficiently capture water infiltrating
down through the soil. Recharge estimates for native grassland
converted to dry -land agriculture computed using a deep -per-
colation model indicated little change for semi -arid conditions
similar to those in the study area (Bauer and Vaccaro, 1990).
Based on those results, recharge for native land and non -
irrigated land was considered to be equal for the purpose of
this study.
Because topography in the study area generally is gently
sloping and sediments of the South Platte alluvial aquifer are
Groundwater Hydrology 17
Table 1. Aquifer transmissivity and hydraulic conductivity estimated from aquifer tests and specific capacity of wells completed in
the South Platte alluvial aquifer, Brighton to Fort Lupton, Colorado.
[CDWR, Coloiado Division of Water Resources; USGS, U S Geological Survey; (gal/min)/11, gallons per minute per foot; Ft, feet; ft/d, feet per day; ft2/d, feet
squared per day]
Well
identifier'
Transmissivity
Specific value estimated
capacity from specific
[(gal/min)/ft] capacity
(ft'/d )
Adjusted
Transmissiv- transmissivity
illy value from value from
aquifer test linear
(ft2/d) regression
(ft2/d)
Saturated
thickness'
(ft)
Estimated
hydraulic
conductivity
(ft/d[
Source
15273RF 32 4,100
SC00106701CCA1 17 2,200
20026RF 53 7,200
B1-66-8bcd 37
SB00206736ACC1 31 3,300
131-66-I8ddc 70
B1-66-31ada 49 3.200
C1-66-6abb 58
13409F 15 2,200
B1-66-19bbc 57
131-67-12acc 76 8,500
962RF 33 5,000
B1-66-30dad 49
24152F 37 5,700
B1-67-13bdd 69
11749RF 45 5,200
6818R 52 8,200
C1-66-18bbd 116 10,200
SC00106605BBB1 53 5,000
13413E 28 3,200
71]0RF 45 7,000
l]082F 53 6,100
B1-66-18ddd 49
11383k 56 8,600
131-67-25acc 63 4,500
1108SRF 41 4,700
C1-67-13dcc 58 4,600
6588R 39 6,600
B1-66-8bdc
19805F 38 5,800
BI-67-36cdd2 60 5.700
SB00106607ACD1 60 8.300
SC00106617CBD2 25 4,300
12,600 33 390 CDWR
10,400 22 470 USGS
16,400 34 480 CDWR
10,000 IS 560 Wilson (1965)
11,700 12 600 USGS
20,100 33 610 Wilson (1965)
11,600 19 610 Schneider (1962)
9,400 IS 630 Wilson (1965)
10,400 16 650 CDWR
10,700 16 670 Wilson (1965)
17,900 27 680 Schneider (1962)
13,800 20 690 CDWR
15.400 22 700 Wilson (1965)
]4,500 20 730 CDWR
22,700 30 760 Wilson (1965)
14,000 17 820 CDWR
17,500 20 880 CDWR
19,900 22 910 Schneider (1962)
13,800 15 920 USGS
11,600 12 970 CDWR
16,100 16 1,010 CDWR
15,100 IS 1,010 CDWR
24,100 23 1,050 Smith and others
(1964)
18,100 17 1,060 CDWR
13,100 I2 1,100 Schneider (1962)
13,300 12 1,110 CDWR
13,300 12 1,110 Schneider (1962)
15,700 14 1,120 CDWR
20,100 17 1,180 Wilson (1965)
14,700 12 1,220 CDWR
14,600 12 1,220 Schneider ([962)
17,700 14 1,270 USGS
12,800 10 1,280 USGS
18 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
Table 1. Aquifertransmissivity and hydraulic conductivity estimated from aquifer tests and specific capacity of wells completed in
the South Platte alluvial aquifer, Brighton to Fort Lupton, Colorado. Continued
[CDWR, Coloiado Division of Water Resources; USGS, U S _ Geological Survey, (gal/mini/ft, gallons per minute per font; R, Feet; ft/d, feet per clay; fWd, feet
squared per day]
Well
identifier'
Specific
capacity
[(gallminlift]
Transmissivity
value estimated
from specific
capacity
(ft'/d)
Adjusted
Transmissiv- transmissivity
ity value from value from
aquifer test linear
(112/dl regression
(fialdl
Saturated
thickness'
(fit
Estimated
hydraulic
conductivity
(ftldl
Source
C1-66-7dacd 42
CI-67-12ccdh
20138R
C1-66-7dhb
C'1-67-13aac
92
89
294
55
8,000
10,900
14,300
4.100
66,800
17,400
20,800
24,900
12,600
13 1,290
15 1,390
13 1,910
34 2,010
6 2,100
McConaghy and
others (1964)
McConaghy end
others (1964)
CDWR
Smith and others
(1964)
Schneider (1962)
'identifier for USGS wells and Smith and others (1964) is local well number Identifier for CDWR wells is well permit number
and others ( I 964), Schneider (1962), and Wilson (1965) is location number
'Saturated thickness estimated as saturated thickness within perforated interval efwell
moderately to highly permeable, mean annual recharge from
infiltration of precipitation likely is near the upper end of
the range indicated by Scanlon and others (2006). Based on
long-term mean annual precipitation of 13.3 in., mean annual
recharge beneath native and non -irrigated areas in the study
area is estimated to be 0,3-0,7 in, (about 2-5 percent of pre-
cipitation), or about 0.5 in.
Recharge beneath irrigated agricultural land.
Recharge beneath irrigated agricultural land in the study
area includes infiltration of precipitation, infiltration of water
applied to irrigated fields, and seepage losses from irrigation
ditches. Net recharge beneath irrigated agricultural land was
estimated by using the water -table -fluctuation method (WTF)
described by Healy and Cook (2002). The WTI' method is
based on the premise that rises in groundwater levels in uncon-
fined aquifers are caused by recharge water arriving at the
water table and that no other sources or sinks affect groundwa-
ter levels during the recharge event. Recharge is calculated as:
R = 5 (dhldt) (3)
where
R is aquifer recharge, in length per time,
S' is specific yield (dimensionless), and
c/h/dt is the change in water -table hydraulic head
over time, in length per time.
'typically, the WTF method is applied over short time
periods to estimate recharge that occurs from individual
recharge events. However, the method also can be used to
provide an estimate of seasonal or annual net recharge to
an aquifer. Seasonal groundwater -level changes in 18 wells
Identifier for McConaghy
located on irrigated agricultural land were used to estimate
net recharge during the irrigation season in the study area
(table 2). Groundwater -level rises during the irrigation season
measured during various years from 1955 through 1991
ranged from 0 to about 9 ft with a mean of about 3 ft. Because
recharge from infiltration of precipitation is estimated to be
small, groundwater -level rises beneath irrigated land during
the irrigation season are assumed to be primarily caused by
infiltration of irrigation water applied to agricultural fields
and seepage from irrigation ditches, However, seasonal net
recharge estimated by using the WTF method also includes
any other sources of recharge active in areas of irrigated
agriculture during the irrigation season, such as from farm
septic systems. Similarly, seasonal net recharge estimated by
using the WTF method includes losses to the aquifer, such as
from well pumping, Because the South Platte alluvial aquifer
in the study area has high hydraulic conductivity, the water
table recovers rapidly from the local effects of well pump-
ing, and groundwater -level measurements made before and
after the irrigation season (when pumping is inactive) likely
do not reflect the localized effects of drawdown resulting
from pumping. Therefore, groundwater -level rises observed
in the study area are assumed to be representative of seasonal
water -table fluctuations in the aquifer and not representative of
localized groundwater -level recovery after seasonal pumping
ceases. Using an assumed specific -yield value of 0,25 (based
on the mean value for medium sand to coarse gravel sedi-
ments reported by Johnson, 1967), mean net recharge beneath
irrigated land in the study area during the irrigation season
is estimated to be 8.0 in. with a standard deviation of 5,1 in.
(table 2). Compared to an average surface -water application
Groundwater Hydrology 19
40°06'N
40°04'N
40°02'1
40°N
26°50'N
104°52'W
104°50'W
Car, ,
I04°48'W
104°46'W
L.7.4
• &20
39L'
•
120 • Fort
• Lupton
•dft0
Lde
•
1 270 •V 180
060' 73G
• .•
X10 1050
6g0
3•
;1C-
35Cti
1.h0 7rro
i •
110 920
•
O1O
• 9/0
•
Brig#rtdn
x`010
2,tOy 1.2D3
f' •r •
t- _ 914
1,280
1.110 • f
Streams modified from U.S, Geological Survey National Hydrography Dataset; 1:100,000
North American Datum of 1983
0 1
Study Area Boundary
EXPLANATION
Hydraulic conductivity,
in feet per day
0-1,000
1,000-2,000
2,000-3,000
Approximate extent of
alluvial aquifer
700 G
2 MILES
4 I.
DI 1 2 KILOMETERS
Location of aquifer test —Number is estimated
hydraulic conductivity, in feet per day
1,100. Locution of specific -capacity test —Number is
estimated hydraulic conductivity, in feet per day
Figure 12. Hydraulic -conductivity distribution and location of wells used to estimate hydraulic
conductivity of the South Platte alluvial aquifer, Brighton to Fort Lupton, Colorado.
20 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Cole.
Table 2. Seasonal groundwater -level rises and estimated recharge from applied irrigation water,
Brighton to Fort Lupton, Colorado.
Date of first Date of second Elapsed Groundwater- Estimated
Well identifier' water -level water -level time level rise2 recharge'
measurement measurement (days) (feet) (inches)
SB00106608BAD2 03/20/67 11/01/67 226 4 56 13.7
SB00106608BAD2 03/18/68 11/04/68 231 7.18 21.5
SB00106608BAD2 03/11/69 11/03/69 237 8.81 26.4
SB00106608BAD2 03/16/70 10/23/70 221 4.69 14.1
SB00106608BAD2 03/23/71 11/02/71 224 5.05 15.2
SB00106608BAD2 03/23/72 11/24/72 246 5.29 15.9
SB00106608BCD 04/04/55 11/08/55 218 3.54 10.6
SB0010660813CD 04/17/56 11/13/56 210 4.58 13.7
SB00106608BCD 04/15/58 11/13/58 212 2.56 7.7
SB00106608BCD 04/01/59 11/11/59 224 2.72 8,2
SB00106608BCD 04/05/60 11/02/60 211 5.67 17.0
SB00106608BCD 03/31/61 11/08/61 222 4.47 13.4
SB00106608BCD 03/26/62 11/05/62 224 4.87 14.6
SB00106608BCD 04/12/63 11/05/63 207 5.03 15,1
SB00106608BCD 03/18/64 11/10/64 237 3.43 10,3
SB00106608BCD 04/12/65 11/26/65 228 4.50 I3,5
SB00106608BCD 03/16/66 11/17/66 246 0 0
SB00106608BCD 03/18/68 11/04/68 231 6.47 19.4
SB00106608BCD 03/11/69 11/03/69 237 6.65 20.0
SB00106608BCD 03/16/70 10/23/70 221 3.82 11.5
SB00106608BCD 03/23/71 11/02/71 224 3.83 11.5
SB00106608BCD 03/23/72 11/24/72 246 0 0
SB00106608CCD 03/20/67 11/01/67 226 4.31 12.9
SB00106608CCD 03/18/68 11/04/68 231 5.32 16,0
SB00106608CCD 03/11/69 11/03/69 237 3.35 10,1
SB00106608CCD 03/16/70 10/23/70 221 3.35 10,1
SB00106608CCD 03/23/71 11/02/71 224 2,15 6.5
SB00106608CCD 03/23/72 11/24/72 246 3.08 9.2
SB00106617CCD2 03/20/67 11/01/67 226 2.92 8,8
SB00106617CCD2 03/18/68 11/04/68 231 4.26 12.8
SB00106617CCD2 03/11/69 11/03/69 237 2.94 8.8
SB00106617CCD2 03/16/70 10/23/70 221 3.47 10,4
SB00106617CCD2 03/23/71 11/02/71 224 1 .19 3,6
SB00106620CBD 03/20/67 11/01/67 226 4.24 12.7
SB00106620CBD 03/18/68 11/05/68 232 3.96 11.9
SB00106620CBD 03/11/69 11/03/69 237 416 12.5
SB00106620CBD 03/16/70 10/23/70 221 2.87 8,6
SB00106620CBD 03/23/71 11/09/71 231 2 53 7.6
SB00106620CBD 03/23/72 11/24/72 246 2.99 9.0
SB00106630ACA 03/27/89 10/30/89 217 0.85 2,5
SB0DI06630ACA 03/20/90 11/12/90 237 4.70 14.1
Groundwater Hydrology 21
Table 2, Seasonal groundwater -level rises and estimated recharge from applied irrigation water,
Brighton to Fort Lupton, Colorado. —Continued
Date of first Date of second Elapsed Groundwater- Estimated
Well identifier1 water -level water -level time level rises recharge'
measurement measurement (days) (feet) (inches)
SB00106630ACA 03/13/91 12/17/91 279 2.25 6.8
SB00106630ADA 04/05/55 11/08/55 217 2.48 7.4
SB00106630ADA 04/17/56 11/13/56 210 2.62 7.9
SB00106630ADA 05/07/57 11/12/57 189 4.25 12.8
S1300]06630ADA 04/07/58 11/13/58 220 3.24 9.7
SB00106630ADA 04/01/59 11/11/59 224 3.12 9.4
SB00106630ADA 04/12/60 11/02/60 204 3.98 11.9
SB00106630ADA 03/31/61 11/08/61 222 3.28 9.8
SB00106630ADA 03/26/62 11/05/62 224 5.38 16.1
SB00106630ADA 03/18/64 11/10/64 237 3.00 9.0
SB00106630ADA 04/12/65 11/26/65 228 2.73 8.2
SB00106630ADA 03/16/66 11/18/66 247 2.49 7.5
SB00106630ADA 03/22/67 11/01/67 224 0.77 2.3
SB00106630ADA 03/18/68 11/05/68 232 3.61 10.8
SB00106630ADA 03/11/69 11/03/69 237 3,92 11.8
SB00106630ADA 03/16/70 10/23/70 221 2,76 8.3
SB00106630ADA 03/23/71 11/02/71 224 2.75 8.3
SB00106630ADA 03/23/72 11/24/72 246 2.08 6.2
SB00106632CDC 03/20/67 10/31/67 225 3.69 11.1
SB00106632CDC 03/18/68 11/04/68 231 4.08 12.2
SB00106632CDC 03/11/69 11/03/69 237 5.71 17.1
SB00106632CDC 03/16/70 10/23/70 221 4.89 14.7
SB00106632CDC 03/22/71 11/02/71 225 2,52 7.6
SB00106632CDC 03/23/72 11/24/72 246 2.39 7.2
SB00106701BDC 04/15/68 10/14/68 182 0.39 1.2
SB00106701BDC 03/10/69 11/04/69 239 2.01 6.0
SB00106701BDC 03/17/70 10/23/70 220 0.88 2,6
SB00106701BDC 03/22/71 11/04/71 227 1.08 3.2
SB00106701BDC 03/23/72 11/19/72 241 1.32 4,0
SB00106712ACC 03/20/67 10/31/67 225 1,92 5.8
SB00106712ACC 03/18/68 11/04/68 231 1.98 5.9
SB00106712ACC 03/10/69 11/04/69 239 2.66 8.0
SB00106712ACC 03/17/70 10/23/70 220 1.58 4.7
SB00106712ACC 03/22/71 11/04/71 227 2.02 6,1
SB00106712ACC 03/23/72 11/19/72 241 1.16 3.5
SB00106713BDD 03/20/67 10/31/67 225 1.97 5.9
SB00106713BDD 03/18/68 11/04/68 231 2.07 6.2
SB00106713BDD 03/17/70 10/23/70 220 1,23 3.7
SB00106713BDD 03/22/71 11/04/71 227 1.38 4,1
SB00206629ABC2 03/20/67 11/01/67 226 3.45 10.4
SB00206629ABC2 03/19/68 11/05/68 231 3,19 9.6
SB00206629ABC2 03/11/69 11/04/69 238 5.13 15.4
22 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
Table 2. Seasonal groundwater -level rises and estimated recharge from applied irrigation water,
Brighton to Fort Lupton, Colorado. —Continued
Date of first Date of second Elapsed Groundwater- Estimated
Well identifier' water -level water -level time level rise' recharge'
measurement measurement (days) (feet) (inches)
SB00206629ABC2 03/17/70 10/23/70 220 2.79 8.4
SB00206629ABC2 03/23/71 11/06/71 228 2.12 6 4
SB00206629ABC2 03/21/72 11/21/72 245 1.02 3.1
SB00206629CCD 04/04/55 11/08/55 218 1.49 4.5
SB00206629CCD 04/17/56 11/13/56 210 0.94 2.8
SB00206629CCD 05/06/57 11/12/57 190 3.90 11.7
SB00206629CCD 04/02/58 11/12/58 224 2.24 6.7
SB00206629CCD 04/01/59 11/11/59 224 1.21 3 6
SB00206629CCD 04/05/60 11/02/60 211 2.14 6.4
SB00206629CCD 03/31/61 11/08/61 222 2.44 7.3
SB00206629CCD 03/26/62 11/05/62 224 1.96 5.9
SB00206629CCD 04/12/63 11/05/63 207 0.93 2 8
SB00206629CCD 03/18/64 11/10/64 237 1.67 5.0
SB00206629CCD 04/12/65 11/21/65 223 1.99 6 O
SB00206629CCD 03/16/66 11/18/66 247 0.04 0.1
SB00206629CCD 03/22/67 11/01/67 224 1.19 3.6
SB00206629CCD 03/19/68 11/05/68 231 0.80 2 4
SB00206629CCD 03/11/69 11/04/69 238 3.01 9 0
SB00206629CCD 03/17/70 10/23/70 220 1.96 5.9
SB00206629CCD 03/23/71 11/06/71 228 0,35 1.1
SB00206629CCD 03/23/72 11/21/72 243 0.56 1.7
SB00206630ADD 03/20/67 11/01/67 226 0.47 1.4
S1300206630ADD 03/19/68 11/05/68 231 0.70 2.1
SB00206630ADD 03/11/69 11/04/69 238 2.42 7.3
SB00206630ADD 03/17/70 10/23/70 220 1.10 3.3
SB00206630ADD 03/23/71 11/06/71 228 0 0
SB00206630ADD 03/23/72 11/11/72 233 4.35 13.1
SB00206631BDA 03/20/67 11/01/67 226 0 0
SB00206631BDA 03/19/68 11/05/68 231 0 0
SB00206631BDA 03/11/69 11/04/69 238 1.63 4.9
SB00206631BDA 03/17/70 10/23/70 220 0.38 1.1
SB00206631BDA 03/23/71 11/04/71 226 0,13 0.4
SB00206631BDA 03/23/72 11/19/72 241 0.16 0.5
SB00206725CDC 03/20/67 11/01/67 226 2.58 7.7
SB00206725CDC 03/19/68 11/05/68 231 3,18 9.5
SB00206725CDC 03/11/69 11/04/69 238 2.36 7.I
SB00206725CDC 03/17/70 10/23/70 220 1.72 5.2
SB00206725CDC 03/23/71 11/04/71 226 1.53 4.6
SB00206725CDC 03/23/72 11/19/72 241 0 0
SB00206736DBB 03/20/67 11/01/67 226 2.18 6.5
SB00206736DBB 03/19/68 11/05/68 231 1.44 4.3
SB00206736DBB 03/11/69 11/04/69 238 1.53 4,6
Groundwater Hydrology 23
Table 2. Seasonal groundwater -level rises and estimated recharge from applied irrigation water,
Brighton to Fort Lupton, Colorado. —Continued
Date of first
Well identifier' water -level
measurement
Date of second
water -level
measurement
SB00206736DBB
SB00206736DIIII
SC00106618CDC1
SC00106618CDC1
SC00106618CDC1
SC00106618CDC1
SC00106618CDC1
SC00106618CDC 1
Mean
Standard Deviation
03/17/70
03/23/71
03/20/67
03/18/68
03/10/69
03/16/70
03/22/71
04/04/72
10/23/70
11/09/71
10/31/67
11/04/68
11 /03/69
10/01/70
10/30/71
11/22/72
Elapsed
time
(days)
220
231
225
231
238
199
222
232
Graundwater-
level rise'
(feet)
0.59
0.25
2.18
3,41
3,95
3.85
0.69
3.64
Estimated
recharge'
(inches)
1 -8
0.8
65
10,2
11.9
11.6
21
10.9
2.67 80
1.69 5.1
' Well identifier is site name in the U S Geological Survey National Water Information System (http://waterdata usgs
gov/co/nwis/gw)
'Water -Level -rise values equal to 0 indicate altitude of the second water -level measurement is Less than or equal to the
altitude of the first water -level measurement
Estimated recharge represents net recharge during the irrigation season and is calculated using the water -table fluctua-
tion method described by Healy and Cook (2002) with an assumed specific -yield value of 0 25
of about 3 ft, mean recharge from irrigation represents about
22 percent of applied irrigation water and about 18 percent
of applied irrigation water (3 ft) and precipitation during the
irrigation season (67 percent of mean annual precipitation, or
about 9 in.) combined,
Recharge estimated beneath irrigated fields represents
average conditions and does not distinguish between recharge
beneath flood- and sprinkler -irrigated sites, which can have
substantially different recharge rates. Susong (1995) used
a water -budget method to estimate that about 49 percent of
applied irrigation water and precipitation became recharge
beneath a flood -irrigated field, whereas about 10 percent
became recharge beneath a sprinkler -irrigated field when
crop water requirements were exceeded. Similarly, Roark and
Healy (1998) estimated recharge beneath two flood -irrigated
alfalfa fields to he 14-43 percent of applied water and precipi-
tation, depending on soil permeability. Recharge beneath a
sprinkler -irrigated site about 7 mi north of the Brighton —Fort
Lupton study area was estimated to be about 12 percent of
applied water and precipitation for soils similar to those in the
study area (Gaggiani, 1995).
Seepage losses from ditches in the study area likely are
substantial. Average seepage loss for Lupton Bottom Ditch is
estimated to be about 30 percent of the diverted inflow based
on flow measurements along the ditch (Lupton Bottom Ditch
Company, oral commun,, 2004). A study of Fulton Ditch indi-
cated overall seepage loss is about 20 percent of the diverted
inflow (George McDonnell, Fulton Irrigating Ditch Company,
oral commun., 2004).
Recharge beneath urban areas. Recharge beneath urban
areas commonly is assumed to be less than recharge beneath
native and non -irrigated areas (Savini and Kammerer, 1961;
Harbor, 1994; Arnold and Friedel, 2000) because greater
impervious cover —such as roads, parking lots, rooftops, and
driveways —increases runoff (Leopold, 1968; Barfield and
others, 1981) and prevents infiltration of water at the land
surface. However, Lerner (2002) presents results of several
studies that indicate recharge beneath urban areas might be
greater than beneath native and non -irrigated land because
of leaky water mains and sewers and overirrigation of lawns,
trees, and gardens. In areas of commercial development,
impervious surfaces generally compose a large percentage of
the land cover, and irrigated landscapes occupy only small
areas. Conversely, residential areas generally have moderate
impervious cover, and irrigated lawns and landscaping occupy
substantial area. Therefore, recharge beneath commercial areas
is likely to be less than recharge beneath native and non -irri-
gated land, whereas recharge beneath residential areas might
be greater. The amount of impervious area in the study area
was estimated from 2006 images of the Brighton —Fort Lupton
area viewed by using Google Earth (accessed August 1, 2006,
at http.//earth.google corn) at scales ranging from about 1:750
to 1:2,000. Based on visual inspection of the images, commer-
cial areas in Brighton and Fort Lupton are estimated to have
about 50-95 percent impervious cover, and residential areas
are estimated to have about 20-50 percent impervious cover.
However, commercial and residential areas are combined in
the analysis of land -use change (see "Land -Use Analysis"),
and average combined recharge beneath urban areas might be
24 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
less than or greater than recharge beneath native and non -
irrigated land.
Subsurface Inflow
Subsurface inflow to the South Platte alluvial aquifer in
the study area can be estimated using a form of Darcy's Law
(Fetter, 1994):
O = —KA (dh/dl) (4)
where
O is subsurface inflow, in cubic feet per day,
K is aquifer hydraulic conductivity, in feet per day,
4 is aquifer cross-sectional area, in square
feet, and
dh/d1 is water -table hydraulic gradient
(dimensionless),
Using values representative of average hydraulic con-
ductivity (1,000 ft/d), cross-sectional area (550,000 ft2),
and hydraulic gradient (—0.003), subsurface inflow at the
upgradient end of the South Platte alluvial aquifer in the
study area is calculated using equation 4 to be 1,650,000 ft3/d
(19,1 Vs). Combined subsurface inflow from the tributary
valleys of Big and Little Dry Creeks (assuming K= 450 ft/d,
A = 150,000 ft2, d{t/dl = -0.005) calculated using equation 4
is 337,500 ft3/d (3.9 ftVs). Total estimated subsurface inflow
from the upgradient end of the South Platte alluvial aquifer
and tributary valleys is then 1,987,500 ft3/d (23.0 ft3/s). In
addition to subsurface inflow from the upgradient end of the
aquifer and tributary valleys, inflow also likely occurs along
the east aquifer boundary from return flow of irrigation water
applied upgradient and outside the eastern limit of the aquifer.
Assuming average hydraulic conductivity at the valley margin
is 650 ft/d, saturated cross-sectional area is 580,000 ft'-, and
hydraulic gradient is —0,005, average subsurface return flow
along the east side of the aquifer calculated using equation 4 is
about 1,885,000 ft3/d (21.8 ft3/s) during the irrigation season.
Inflow along the west aquifer boundary is considered negli-
gible because unconsolidated sediments on hilislopes west
of the aquifer commonly are thin or absent (fig. 8), limiting
subsurface return flow of irrigation water applied upgradient
and outside the western limit of the aquifer.
Aquifer Outflows
Outflows from the alluvial aquifer in the study area pri-
marily are discharge to the South Platte River, well withdraw-
als, phreatophyte evapotranspiration, aggregate -pit dewater-
ing, and subsurface outflow at the downgradient end of the
aquifer,
Flow to the South Platte River
The direction of flow between the South Platte River and
the alluvial aquifer within the hyporheic zone near the river
is highly variable (McMahon and others (1995), but, overall,
the general direction of flow is from groundwater to the river,
A monthly mass -balance analysis of strearnflow and diver-
sion data (table 3) (U.S. Geological Survey, 2006a; Colorado
Division of Water Resources, 2006a, b) for substantial inflows
(Big Dry Creek) and outflows (Brighton Ditch, McCanne
Ditch, Lupton Bottom Ditch, and Platteville Ditch) along the
South Platte River between stream gages located at Henderson
(station 06720500) (fig. 2) and Fort Lupton (station 06721000)
indicates that the river generally gains groundwater in all
months of the year (fig. 13). For the time periods analyzed
(1954-1960, 1974-1980, and 1997-2003), strearnflow gains
generally were greatest during the irrigation season from May
through October and least during the non -irrigation season
from November through April when groundwater levels
decline, reducing the aquifer hydraulic gradient toward the
river. During the irrigation season, mean monthly gains to the
South Platte River ranged from about 3.0 to 21.0 million ft'/d
(35-243 ft3/s) with a mean value of about 9.0 million ft3/d
(104 ft'/s), During the non -irrigation season, mean monthly
flow to and from the South Platte River ranged from a loss of
about 1.1 million ft'/d (13 ft'/s) to a gain of about 6.4 million
ft'/d (74 ft'/s). Strearnflow gains correlate with the magni-
tude and timing of diversions from the South Platte River for
irrigation (fig. 4), indicating that water diverted for irrigation
possibly infiltrates rapidly to the water table and increases
discharge to the river during the irrigation season. However,
strearnflow gains appear to begin declining sooner than diver-
sions during the irrigation season, possibly because groundwa-
ter withdrawals from irrigation wells generally are greatest in
mid to late summer (Hurr and others, 1975), and groundwater
withdrawals can capture recharge from seepage of diverted
surface water applied to fields,
Well Withdrawals
Irrigation -well withdrawals from an area of the South
Platte alluvial aquifer that encompasses the study area were
estimated by Smith and others (1964) to be about 17,900 ft3/d
(150 acre-ft/yr) per well in 1956 and about 7,160 fe/d (60
acre-ft/yr) per well in 1957. T-lurr and others (1975) estimated
mean irrigation withdrawals from the South Platte alluvial
aquifer between Henderson and the Colorado -Nebraska State
line to range from about 14,320 to 26,260 ft3/d (120 to 220
acre-ft/yr) per well for the period 1960-70. Withdrawals
from irrigation wells in the study area during the time periods
considered (1954-1960, 1974-1980, and 1997-2003) are
estimated to be within the range (7,160-26,260 ft'/d per well)
indicated by these studies for other wells in the South Platte
aquifer. However, because withdrawal rates and the number
and location of irrigation wells active during each irrigation
Groundwater Hydrology 25
Table 3. Mass -balance estimation of gain or loss of water along the South Platte River between stream gages located near
Henderson and Fort Lupton for the periods 1954-60,1974-80, and 1997-2003.
IA]] values arc mean monthly sircarrflow in cubic feet per day]
Month
South Platte South Platte
River at River at
Henderson Fort Lupton
(06720500)1 (06721000)a
Big Dry Brighton McCanne
Creek Ditch Ditch
(06720990)2 (810)' (868)'
Lupton
Bottom
Ditch
(812(3
Platteville Gain or loss
Ditch Henderson to
(813)3 Fort Lupton'
1954-60
January 5,947,751 9,011,904 1,986,728 0 0 0 0 1,077,425
hchruaiy 8,215,646 10,710,577 1,955,577 0 0 0 0 539,354
March 14,263,773 17,956,660 2,159,487 0 0 0 0 1,533,400
April 19,357,503 20,638,133 4,146,215 249,741 0 822,046 680,924 —1,112,874
May 78,915,009 80,877,552 3,368,800 1,397,152 0 3,275,284 2,686.403 5,952,583
June 66,702,596 62,868,025 3,196,041 2,499,465 0 5,924,246 4,799,796 6,192,895
July 39,363,937 35,146,201 3,023,282 2,307,750 0 5,283,616 4,794,275 5,144,623
August 28,099,123 25,288,893 2,936,902 1,972,895 0 4,582,453 4,114,611 4,922,826
September 11,855,487 11,071,293 3,800,697 1,547,817 0 3,745,289 3,283,659 3,991,874
October 8,532 224 10,231,678 2,936,902 762,481 0 2,146,494 1,334,243 3,005,770
November 7,219,237 9,847,745 2,418,625 0 0 0 0 209,882
December 6,387,721 9,587,254 1,986,728 0 0 0 0 1,212,806
1974-80
January 21,582,398 23,859,887 1,986,728 0 205.552 0 0 496.313
February 23,986,779 27,156,402 1,955,577 0 30,105 0 0 1,244,151
Mulch 27,859,031 3(1,754,071 2,159,487 0 0 71,669 0 807222
April 39,064,478 40,342,805 4.146,215 856,195 0 1,180,406 757,040 —74,248
May 82,410,476 90,310,819 3,368,800 1,942,635 33,446 3,270,905 2,952,773 12,731,302
June 91,101,899 97,414,541 3,196,041 2,814,624 66,241 5,008,805 5,196,008 16,202,278
July 58,133,714 51,473,651 3,023.282 3,285,239 86,401 6,494,429 6,369,405 6,552,129
August 40,468,041 36,581,179 2,936,902 2,602,788 36,233 5,282,422 5,258,930 6,356,608
September 23,527,475 23,487,566 3,800,697 1,795,090 0 3,203,018 4,039,053 5.196,555
Octobei 22,620,806 24,389,045 2,936,902 1,155,069 199,977 1,499,082 2,210,598 3,896,064
Novembci 19,449,335 22,579,1 18 2,418,625 15,635 316,805 19,338 134,539 1.197,474
December 17,485,306 20,943,745 1,986,728 0 316,804 0 0 1,788.515
1997-2003
January 31,140,687 33,630,870 2,417,603 0 0 0 0 72,581
February 28,005,362 30,013,766 2,110,272 0 0 0 0 —101,868
March 30,826,138 33,278,261 2,935,254 394,539 0 292,044 848,404 1,051,857
April 50,413,078 55,192,626 5,146,141 1,784,034 0 1,732,501 3,251,485 6,401,427
May 77,964,196 86,120,025 4,939,604 2,132,562 0 3,284,3(19 4,141.885 12,774,981
Junc 95,962,587 106,253,626 3,486,826 2,479,140 0 4,787,001 6,923.745 20,994,098
July 56,950,774 62,563,978 3,976,369 3,013,238 0 6,472,664 7,258.821 18,381,558
August 58,298,951 64,075,285 3,580,436 2,257,486 0 5,005,307 5,985,577 15,444.267
September 30,998,304 33,428,664 3,309,374 1,541,329 0 3,637,651 4,778,566 9,078,532
October 30,987,791 33,459,475 3,145,882 794,489 U 1,546,379 3,374,717 5,041,387
November 30,290,224 32,634,907 2,812,896 61,139 0 204,113 227.894 24,932
December 26,930,118 28,910,823 2,171,578 0 (1 319,438 0 128,564
'Number is station identifier Data from Colorado Division of Water Resources (2006a)
'Number is staiine number Data from U S Geological Survey (20061
'Number is structure identifier Data from Colorado Division of Watei Resources (2006b)
'Positive values indicate gain to river from groundwater Negative values indicate loss from river to groundwater
26 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
cn
Ja
Z CC
w
O -
▪ w 10
w
U -
CC U
°°
<
LO o
w
25 _
20
15
•a Iff
r
• 1954-1960
1974-1980
o 1997-2003
Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.
MONTH
Figure 13. Mean monthly gain or loss in flow of the South Platte River between stream gages located near
Henderson and Fort Lupton, Colo„ for the periods 1954-1960, 1974-1980, and 1997-2003. [Streamflow and
diversion data used to estimate gain or loss are from U.S. Geological Survey 12006) and Colorado Division of
Water Resources (2006a,b),]
season have large uncertainty, total irrigation -well withdrawals
are largely uncertain.
Municipal -well withdrawals are estimated based on
pumping data provided by the two main municipalities in the
study area and historical population estimates. Data pro-
vided by the city of Fort Lupton (Steve Nguyen, Clear Water
Rights, Inc,, written commun,, 2005) and the city of Brighton
(Dawn Hessheimer, city of Brighton, written comrnun., 2007)
indicate municipal -well withdrawals during the irrigation
season from May through October are 2 to 3 times greater
than those during the non -irrigation season from November
through April (fig, 14). The sum of mean withdrawals from all
municipal wells during May —October for the years considered
(1954-1960, 1974-1980, and 1997-2003) range from about
976,000 to 1,550,000 ft3/d (22.4-35,6 acre-ft/yr), whereas the
sum of mean withdrawals from all municipal wells during
November —April range from about 376,000 to 752,000 ft3/d
(8,6-17.3 acre-ft/yr). Municipal -withdrawal data prior to 2000
were not available for Fort Lupton; so mean withdrawals for
Fort Lupton during 1997-2003 reflect mean withdrawals from
only 2000 to 2003. Historical (1954-1960 and 1974-1980)
municipal withdrawals for Fort Lupton were estimated based
on per capita use during 2000-2003 multiplied by population
estimates (Colorado State Demography Office, 2007) provided
by State censuses taken in 1960 and 1980, Mean municipal
withdrawals for Brighton are based on withdrawal data pro-
vided for each year considered.
2.00
0
�J November —April
_ ❑ May —October
1954-1960
1974-1982
YEAR
1997-20t3
Figure 14. Sum of mean municipal -well withdrawals
by Brighton and Fort Lupton during the irrigation
season (May —October) and non -irrigation season
(November —April) for the periods 1854-1960, 1974-1980,
and 1997-2003. [Withdrawal data provided by Dawn
Hessheimer, city of Brighton, written commun. (2007) and
Steve Nguyen, Clear Water Rights, Inc., written commun.
(2005).]
Land -Use Analysis 27
Phreatophyte Evapotranspiration
Phreatophyte evapotranspiration occurs when plants
or trees have roots deep enough to penetrate the water table
and use water directly from the aquifer. Phreatophytes in
the South Platte study area consist primarily of cottonwood
trees (Colorado Division of Wildlife, 2007a, b), however,
Russian olive and willow trees also are present in the study
area. Annual evapotranspiration by phreatophytes consisting
primarily of cottonwood and cottonwood -willow trees has
been estimated to range from about 24 to 36 in. in the North
Platte basin of Wyoming (VanKlaveren and others, 1975) and
the South Platte River valley (Hurr and others, 1975). Most of
the evapotranspiration occurs concurrently with the irrigation
season from May through October. Phreatophyte evapotrans-
piration from the South Platte alluvial aquifer in the study
area is assumed to fall within this range on the basis of similar
climatic and hydrologic conditions.
Mine Dewatering and Subsurface Outflow
The rate of mine dewatering for individual aggregate
pits in the study area is highly variable depending on the size
and timeframe of the mining operation but typically is greater
than 192,500 f13/d (1,000 gal/min) and can be continuous
for several years. Subsurface outflow at the downgradient
end of the alluvial aquifer is estimated using equation 4 as
—1,200,000 ft3/cl (-13.9 ft3/s), assuming average hydraulic
conductivity is 1,000 ft/d, cross-sectional area is 400,000 ftz,
and hydraulic gradient is 0,003. The negative sign denotes that
the direction of groundwater flow is out of the aquifer.
Land -Use Analysis
Socioeconomic Trends
In the early 1990s population growth in northeastern
Colorado was largest in Douglas County to the south of
Denver (Douglas County, 2002). Since about 2000, substantial
growth has shifted to the agricultural region north of Denver
in Adams, Weld, Boulder, and Larimer Counties. Proximity
to Denver, access to major transportation corridors and an
international airport, moderate land values, and most impor-
tantly, the availability of water are the prime drivers for urban
growth in this region (Mladinich, 2006). Water supply, largely
in the form of reservoirs (from early irrigation development
and aggregate mining) and groundwater, already is in place
in the region, which is one of the main reasons for the shift-
ing growth from Douglas County (Wagner, 2002; Parton and
others, 2003).
The South Platte study area is in the northern growth
area, but the degree of growth in Brighton has been different
than that of Fort Lupton. Growth in Brighton (at the southern
end of the study area) has been larger and faster than in Fort
Lupton because Brighton is closer to Denver, has a railway
stop, and is the seat of Adams County (Wagner, 2002). Both
cities were incorporated between 1887 and 1890 and were
trading centers whose production and service industries grew
to support the surrounding agricultural region. The main eco-
nomic drivers in the region historically have been agriculture
and mineral extraction, and the region has followed the typical
cycles of economic booms and busts experienced throughout
Colorado (Kendall, 2002; Parton and others, 2003). These
cycles have resulted from national and international economic
trends and also have corresponded with drought cycles. Each
drought period has led to a downturn in agriculture and the
industries supporting them.
The population of Brighton and Fort Lupton increased
gradually through the early to mid -1900s and began to
increase more rapidly during the second half of the century
(Colorado State Demography Office, 2007) (fig. 15). Rapid
population growth began in Brighton around 1960, whereas
rapid population growth in Fort Lupton began around 1980.
Rapid population growth in both cities continued through
2000.
Starting in the mid -1960s through the mid -1980s, the
region reflected the shift in the State's economy from tradi-
tional resource -based industries, including agriculture and
mineral extraction, to high technology, energy, and service
industries (Kendall, 2002). Land values rose and farm incomes
fell during the 1970s and 1980s, resulting in a downturn for
farmers and ranchers in the region and throughout the State. In
the vicinity of the study area, the number of farm owners and
farm employment were declining during this time period (U.S.
Department of Commerce, 2007) (fig. 16), and by the 1970s,
all the sugar -beet mills in the region had closed (Kendall,
2002).
The State's economic rebound in the 1990s in the high-
technology and communications industries were reflected
by an economic upturn in the region. However, in the late
1990s, declining prices and cutbacks in Federal farm programs
reduced farm incomes. Since the late 1990s, the region has
attracted retirees and high-technology and service -industry
employees because of its small-town atmosphere, moderate
land values, and relatively low cost of living (Kendall, 2002).
Land -Use Trends
Land-use/land-cover data sets for 1957, 1977, and 1997
(U.S. Geological Survey, 1999, 2001b, c,) were analyzed and
compared to determine the type, extent, and rate of land -use
change in the study area. The land-use/land-cover data set
for 1997 was used to represent approximate land -use condi-
tions in 2000. Land use in the study area was classified into
three major types based on similarity of recharge conditions
for input to the numerical groundwater flow model. The
three major land -use classifications used in the study are (1)
urban areas, (2) irrigated agriculture, and (3) non -irrigated
land. Urban areas include all land uses related to urban
28 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
25,000
Fort Lupton
20,000El Brighton
NUMBER OF PERSONS IN NONFARM EMPLOYMENT
POPULATION
250,000
200,000
150,000
100,000
50,000
0
15,000 -
10,000 -
5,000
rmEl
r
L
IL II
I
1
1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
YEAR
Figure 15. Populations of Brighton and Fort Lupton, Colo., 1910-2000. [Data from Colorado State
Demography Office (2007]].
Li Adams County Nonfarm Employment
Weld County Nonfarm Employment
— 6— Adams County Farm Employment
- Weld County Farm Employment
1970
1975 1980
1985 1990
YEAR
1995
2000
2005
8,000
7,000
6,000
5,000
4,000
3,000
2,000
1,000
Figure 16. Farm and nonfarm employment in Adams and Weld Counties, Colo., 1970-2005, [Data from Colorado
State Demography Office (2007)],
NUMBER OF PERSONS IN FARM EMPLOYMENT
Land -Use Analysis 29
development, such as commercial, industrial, and residential
areas. Irrigated agricultural areas represent cultivated ]and
irrigated for crop production. Within the classification, no
distinction is made between areas irrigated by surface water
or groundwater. Similarly, no distinction is made between
irrigation methods used, such as sprinkler or flood irrigation,
Non -irrigated land includes all other land uses in the study
area, primarily dry -land agriculture and native land. Small
farm plots indicated as residential areas were included as part
of the dominant land use (irrigated agriculture or non -irrigated
land) surrounding the farm.
The general distribution of irrigated and non -irrigated
land in 1957, 1977, and 2000 (figs. 17A —C) is similar for
each year. Non -irrigated land generally is located near the
South Platte River and on upland areas outside the limits
of the alluvial aquifer, whereas irrigated agricultural land
generally is located on well -drained soils away from the
river and along tributaries to the South Platte River. Locally,
relatively small conversions between irrigated agricultural
land and non -irrigated land have occurred. In some places,
irrigated land in 1957 was converted to non -irrigated land
by 1977 and converted back again to irrigated land by 2000.
Similar conversions of non -irrigated land to irrigated land
also occurred between 1957 and 2000. Overall, the total area
of non -irrigated and irrigated land decreased slightly from
1957 to 2000 (table 4), and the reduction in irrigated land
was slightly larger than that for non -irrigated land. Irrigated
land use decreased 6-7 percent between each time period
compared, and non -irrigated land use decreased about 4
percent between each time period. By contrast, urban land
use increased about 165 percent between 1957 and 1977 and
about 56 percent between 1977 and 2000, Although urban
development increased greatly between 1957 and 2000, the
total land area covered by urban development in the study
area in 2000 was relatively small (about 13 percent) compared
to irrigated land (about 38 percent) and non -irrigated land
(about 50 percent). Most urban development in the study area
is associated with the cities of Brighton and Fort Lupton.
Urban growth of both cities between 1957 and 2000 primarily
has been to the cast (away from the South Platte River) and
along the corridor of U.S. Highway 85 and the Union Pacific
Railroad line,
Predictions of Land -Use Change
Predictions of future urban extent in the study area in
2020 and 2040 were made using the SLEUTH urban -growth
modeling program (U.S. Geological Survey and University
of California at Santa Barbara, 2001). SLEUTH is a cellular
automaton -based modeling program that derives its name from
input required to conduct simulations (Slope, Land cover,
Exclusions, Urbanization, Transportation, and Hillshade).
Slope and land cover provide information about the likeli-
hood of urban growth occurring in an area, whereas exclusions
define areas where urban growth will not likely occur because
N
N
r
U,
m in
C]
a
i0
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:110
a
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ro
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v
0 0 0 0
00 th D ao
6 W b
ni N �n
10 N
d
a fig
t °7 N
d
C ifl
s
a
e
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a
,+
C
0
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R
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5
I -
30 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
104°52'W
Streams modified from U S Geological Survey National Hydrography Ratner, 1:100,000
Land use modified from U S Geological Survey (2001 bi
Roads modified from Colorado Department of Transportalian
North American Do him of 1983
Land Use
104°50'W
EXPLANATION
Urban
In igaied ngiioullure
Non -irrigated
104°40'W
Approximale exlenl of
alluvial aquifer
0
I I
I l 1 I_
0 1 2 KILOMETERS
104°45'W
1 2 MILES
Figure 17A, Generalized land use in 1957, Brighton to Fort Lupton, Colorado.
Land -Use Analysis 31
104'52'W
Streams modified From U S Geological Survey National Hydiography Dnlnset, 1:100,000
Land use modified from U S Geological Survey (200th)
Ronda modified from Col mado Department of Transportation
North American Datum of 1983
Land Use
mitt
104°50'W
EXPLANATION
104°48'W
Urban — Approximate extent of
alluvial aquifer
Irrigated agiiculture
Non -ii rigated
104°46'W
0 I 2 MILES
—1_.Ll._ _I I.._I�1_-�
0 E 2 KILOMETERS
Figure 178. Generalized land use in 1977, Brighton to Fort Lupton, Colorado.
32 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
104°52•W
104°50'W 10n°ASw
So earls modified From US Geological Survey National }lydrography Dalasel, 1:100,000
Land Use modified from U S Geological Survey (1955)
(toads nmdifi ed Soon Colorado Department of Transportation
North Ameiiean Datum or 1983
EXPLANATION
Land Use
Urban
Irrigated agiicultuie
I Non -irrigated
- Approximate extenl of
alluvial aquifer
0 I 2 MILES
0 I 2 KILOMETERS
Figure 17C. Generalized land use in 2000, Brighton to Fort Lupton, Colorado.
Land -Use Analysis 33
of development restrictions or physical [imitations, such as
water bodies. Transportation is used to define the location of
roads along which urban development commonly proceeds.
Uillshade is used to provide spatial context to simulations.
SLEUTH predicts urban growth by comparing historical urban
extent and growth over time to determine a set of coefficients
that represent different factors contributing to urban growth.
The probability that existing urban areas will expand and the
probability that new urban areas will develop are considered
in the modeling program based on the coefficients determined
during calibration to historical urban development.
In this study, slope and hillshade were determined for
SLEUTH model input by using a USGS Digital Elevation
Model (DEM) with 30-m resolution (accessed June 10, 2005
at htfp,//rock}weh. CP: usgs.gov/elevation), Land cover was
defined as either urban or nonurban for the model on the basis
of land -use categories indicated by U.S. Geological Survey
(1999, 200la, b, e). Areas of irrigated agriculture and non -
irrigated land were not changed in the simulation except where
urbanization was predicted to occur. Exclusion areas were
defined on the basis of water bodies indicated by U.S. Geolog-
ical Survey (2001d). Land -use zoning maps were not used to
define exclusion areas because such maps were not available
in digital format suitable for SLEUTH model input. Trans-
portation corridors were defined on the basis of roads and
railroads indicated by U.S. Geological Survey (2001e, 0. The
SLEUTH model was calibrated to urban extent in 1937, 1957,
1977, and 2000 (U.S. Geological Survey, 1999, 2001a, b, c)
using a 50-m by 50-m cell size to determine characteristics
of urban growth in the study area. Coefficients determined by
calibration then were used to predict urban extent in 2020 and
2040. Because land -use zoning was not considered in model
input, predicted urban extent was compared to city and county
zoning maps and development plans (city of Brighton, 2003;
city of Fort Lupton, 2006; Weld County, 2006) at the conclu-
sion of the simulation, and predicted urban extent was modi-
fied to reflect areas most likely to be developed on the basis
of the maps and plans. The modified results of the SLEUTH
simulations (figs. l84 and 188) generally indicate areas where
the predicted probability of urban growth is 70 percent or
more. Similar to historical trends, urban growth is predicted to
occur predominantly to the east of Brighton and Fort Lupton
and along major transportation routes (U.S. Highway 85,
Interstate 76, and Union Pacific Railroad), However, substan-
tial growth also is predicted to the south and west of Brighton
as areas of low urban density are more fully developed. The
amount of urban area predicted in 2020 and 2040 is provided
in table 4. The modified results of the model simulations
represent one possible outcome of urban growth in the study
area based on historical trends, Actual urban growth could be
substantially different from that shown in figures 18/1 and 188
if factors affecting future urban growth are different than those
used by the simulation. Errors associated with predictions of
urban growth by the SLEUTH model simulations are difficult
to quantify and were not estimated as part of this study.
As of late 1999, five aggregate mining operations were
evident in the study area (Google Earth, accessed January 4,
2007, at http://earth google.com), To estimate the potential
extent of future aggregate mining in the study area, mining
and reclamation plans on file with the CDMRS were reviewed
in October 2006. At the time of the review, 58 aggregate pits
were either reclaimed, active, or planned for development by
2020. The potential extent of aggregate mining in 2020 and
planned methods of reclamation based on permit records is
shown in figure 194. A proposed lined water -storage facil-
ity unrelated to aggregate mining is included in figure 194
because its effect on groundwater flow would be the same as
that of a lined pit. Although actual mining extents and methods
of reclamation are subject to change based on market condi-
tions and the needs of mining companies, results shown in
figure 19.4 provide an indication of how mining might develop
in the Brighton -Fort Lupton area.
The potential extent of aggregate mining in 2040
(fig. 19B) is not based on existing mining plans but rather on
extrapolation of the size, spacing, and density of pit develop-
ment in 2020. Aggregate -in ining extent in 2040 represents
potential conditions when mining within the study area is
approximately fully developed. Pits generally were added
and shaped with consideration of existing roads, ditches,
houses, and oil wells shown on USGS 7.5 -minute topographic
quadrangle maps (1994) at a scale of 1:24,000 and with
consideration of wetlands mapped as part of this study (see
"Wetland Mapping") and areas of riparian herbaceous vegeta-
tion indicated by the Colorado Division of Wildlife (2007a,
b). Aggregate in the tributary valleys of Big and Little Dry
Creeks was assumed to not be of sufficient quality to mine;
therefore no pits were added at these locations. In addition,
pits were added only where SLEUTH model simulations did
not predict urban development in 2040. Some areas, particu-
larly near urban areas, were left open for future urban develop-
ment or aggregate mining beyond 2040. Because the potential
extent of aggregate mining in 2040 is not based on mining and
reclamation plans, the actual extent of aggregate mining in
2040 could be substantially different than that shown in figure
198, As with the SLEUTH model simulations, error associated
with the potential extent of aggregate mining in both 2020 and
2040 is difficult to quantify and was not estimated as part of
this study.
Wetland Mapping
Wetlands in the study area were mapped to verify areas
designated as wetlands on existing USGS land-use/land-cover
maps (U.S. Geological Survey, 1999, 2001a, b, c). Wetland
mapping was completed by the USGS and U.S. Bureau of
Reclamation during July —August, 2004. False -color infrared
aerial photographs were obtained at 18 locations (fig, 20) in
the central and western parts of the study area at a scale of
1:24,000. The aerial photographs are presented in the appen-
dix. The easternmost part of the study area was not covered by
34 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
104`52'W
19490' W
Streams modified from U S Geological Survey National Ely'drograplry Dataset, 1:100,005
Land use modified from U S Geological Survey (1999)
Roads modified from Colorado Deparunert (di) an:poi-Imo a
Nordl American Datum of l 983
EXPLANATION
Land Use
Urban
rI
Irrigated agi iuulture
Non -ii dgated
104'49'W
�— Approximaleextent of
alluvial aquifer
111,1°4S'W
0 1 2 MILES
T I
13 1 2 KILOMETERS
Figure 18A. Predicted extent of urban land use in 2020 with irrigated and non -irrigated land use
shown as unchanged from 2000 except where precluded by changes in urban land use, Brighton
to Fort Lupton, Colorado.
Streams modified from US Geological Survey National Hydrogmphy ➢ataset, 110D OUD
Lund use modified from U.S Geological Survey (1959)
Roads modified from Colorado ➢cpar0nonl of Transportation
North American Datum of 1983
Oft
Land -Use Analysis 35
1D4°52'W
Land Use
Urban
104°50'W
EXPLANATION
Irrigated agriculluie
i Non -in wmedagricullure
I and rrtuwt land
104°48'w
Approximate extent of
alluvial aquifer
104°46'W
C
l I
I I
0
1 2 MILES
I I I
I� I
2 KILOMETERS
Figure 188. Predicted extent of urban land use in 2040 with irrigated and non -irrigated land use
shown as unchanged from 2000 except where precluded by changes in urban land use, Brighton
to Fort Lupton, Colorado.
36 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
40`06'N
4o'01•N
4C"0214
3s°58'N
104°52'W
Water
storage
facility
Streams modified from U S Geological Survey Nnlionai Hy rography Milner, 1:I UU,004
Roads modified horn Colorado Dcpaitmenl ofTranspnnotien
Nerth American Datum of 1983
It
10-0°50'w
EXPLANATION
Pit Completion
Lined
Unlined
Backfilled with fines
Overburden backfill
or unknown
1D4 4D'w
i
For!
Lupton
Brighton
51
- Apprnximale exlent ai
alluvial aquifer
104°45'w
2 MILES
Tr
2 KILOMETERS
Figure 19A. Predicted extent of aggregate mining in 2020, Brighton to Fort Lupton, Colorado,
104°50'W
Land -Use Analysis 37
104"SM
Streams modified fl em Ll S Geological Survey National Hydragraphy Dalaset, 1:100,000
Rands modified rrom Colorado Dopartmanl of I'ransporta!ion
North American Datum of 1983
Water
storage
facility
EXPLANATION
Pit Completion
Lined
Unlined
13aekfilled with fines
Overburden backfill
ul unknown
10d"48'W
_LI Pit added after 2020
Approximate extent of
alluvial aquifer
104"46'W
0 1
I I
I I rI I
0 l 2 KILOMETERS
2 MILES
Figure 198. Predicted extent of aggregate mining in 2040, Brighton to Fort Lupton, Colorado.
38 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
40°05'N
40°04'N
40°02'N
40°N
39°58'N
104°52'W
104°50'W
104°48'W
104°46'W
Streams modified from U S Geological Survey National Hydrography Dataset; I:100;000
Roads modified flora Colorado Department of Transportation
North Amei ican Datum of 1953
EXPLANATION
Welland
Surface wafer
App.: oximale extent of alluvial aquifer
Location of aerial phologi aph Celli' oid
and photogi aph number (Photogiaphs in Appendix)
104°44' W
0 1 2 MILES
1 L,r ..11..1 L
0 1 2 KILOMETERS
Figure 20, Location and extent of wetlands and surface water mapped by this study, Brighton to
Fort Lupton, Colo., July —August, 2004,
Simulation of Groundwater Flow 39
the photography, but depth to water in this area generally pre-
cludes the development of groundwater -supported wetlands.
Preliminary photo interpretation was performed to identify
probable wetland areas, and those areas were subsequently
verified by field inspection. In addition to verifying locations
of probable wetland areas identified from aerial photographs,
specific evaluations were made at less -obvious wetland areas
to determine their status. Wetlands on public land or otherwise
accessible areas were evaluated by identifying the existence
of wetland plant species, the above -ground hydrology, and the
existence of reducing soils (necessary for evidence of hydric
soils). Several inaccessible sites on private land were visually
evaluated from the closest public road. Details of the wetlands
mapping effort are presented by Sales (2005).
Wetland types occurring in the study area can be
described using the USFWS wetland classification of Shaw
and Fredine (1956), as seasonally flooded areas (waterlogged
during variable periods, but well drained during much of the
growing season), fresh meadows (waterlogged to within about
an inch of the land surface), shallow fresh marshes (covered
with > 0.5 ft of water), deep fresh marshes (covered with
0.5-3.3 ft of water), and open fresh water (water deeper than
3.3 ft). For ease in mapping and quantifying the wetland areas
for the purpose of this study, areas were categorized simply
as wetlands or surface water. Areas identified as wetlands
included seasonally flooded areas, fresh meadows, shallow
marshes, and deep marshes that contained visible wetland veg-
etation, whereas surface water included all open water areas,
including sewage and stock ponds. Small wetlands associ-
ated with irrigation ditches in the study area were not mapped
because they were too small for the scale of the investigation.
Summary statistics for mapped wetlands and surface water
in the study area are presented in table 5, and the locations of
mapped wetlands and surface water are shown in figure 20.
Wetlands that were easily identified by the existence of
surface water (such as along creeks, canals, rivers, and ponds)
typically contained the following wetland plants: coyote
willow (Sal ix exigua), cottonwood (Poptilus l'remonlii),
reed canaiygrass (Photons arundihacea), cattail (Typha
spp ), hardstem bulrush (Schoenaplectus acuius), three -
square bulrush (S. pungens), algae and/or submersed aquatic
vegetation (not identified), and Russian olive (Eloeaghus
angustifbliu), Wetlands that were identified as seasonally
flooded or fresh meadows typically contained saltgrass
(Distichlis spieala), wire rush (Juncns ha/lieus), clustered
field sedge (Carex praegracilis), curly dock (Rumex crispus),
foxtail (.41opecurus spp), and cattail (Typha
Simulation of Groundwater Flow
The hydrologic effects of land -use change and aggre-
gate mining on groundwater flow and wetlands in the study
area were simulated using the USGS modular groundwater
modeling program MODFLOW-2000 (Harbaugh and others,
2000). Pre- and post -processing of MODFLOW-20001iles
Table 5. Summary statistics for wetlands and surface water
mapped by this study, Brighton to Fort Lupton, Colorado, July —
August, 2004.
[Source: Salas (2005)]
Total
Feature Number area
(acres)
Mean
size
(acres)
Minimum
size
(acres)
Maxi-
mum
size
(acres)
Wetland
Surface
water
28 262.5 9.37 0.04 132.6
75 302.7 4.04 0.05 48,6
primarily were completed using the MODFLOW Graphical
User Interface (Winston, 2000) for the Argus ONE geographic
information system (Argus Interware, 1997), The model was
calibrated to seasonal groundwater -level and flow conditions
of the South Platte alluvial aquifer during the irrigation and
non -irrigation seasons in 1957, 1977, and 2000 using the
Observation, Sensitivity, and Parameter -Estimation processes
(Hill and others, 2000) of MODFLOW-2000. The calibrated
model then was used to simulate (1) steady-state hydrologic
effects of predicted land -use conditions in 2020 and 2040, (2)
transient cumulative hydrologic effects of the potential extent
of reclaimed aggregate pits in 2020 and 2040, (3) transient
hydrologic effects of actively dewatered aggregate pits, and
(4) effects of different pit spacings and configurations on
groundwater levels.
Nine numerical simulations of the potential hydrologic
effects of land -use change, reclaimed pits, and actively
dewatered pits are presented as follows:
Simulation I —The hydrologic effects of reduced aquifer
recharge resulting from the conversion of
non -irrigated and irrigated land to impervi-
ous urban area are simulated for land -use
conditions predicted by the SLEUTH urban -
growth model for 2020.
Simulation 2 —The hydrologic effects of reduced aquifer
recharge resulting from the conversion
of non -irrigated and irrigated land to
impervious urban area are simulated
for land -use conditions predicted by the
SLEUTH urban -growth model for 2040.
Simulation 3 —The cumulative hydrologic effects of multiple
reclaimed pits are simulated for the potential
extent of aggregate mining in 2020. The
simulation includes a combination of lined
pits, unlined pits, and pits backfilled with
fine sediments.
Simulation 4 —The cumulative hydrologic effects of multiple
reclaimed pits are simulated for the potential
extent of aggregate mining in 2040. Pits
excavated after 2020 are simulated as lined,
40 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
Simulation 5 —The cumulative hydrologic effects of multiple
reclaimed pits are simulated for the poten-
tial extent of aggregate mining in 2040.
Pits excavated after 2020 are simulated as
unlined.
Simulation 6 The hydrologic effects of a single dewatered
pit added after 2020 are simulated.
Simulation 7 The hydrologic effects of two closely spaced,
dewatered pits added after 2020 are
simulated.
Simulation 8 —The hydrologic effects of two widely spaced,
dewatered pits added after 2020 are
simulated.
Simulation 9 —The hydrologic effects of three closely
spaced, dewatered pits added after 2020 are
simulated.
In addition, simulations of the hydrologic effects of three
hypothetical lined pits are used to assess the effect that pit
spacing and configuration (size and location relative to other
pits) have on groundwater levels near reclaimed lined pits.
Mathematical Methods
MODFLOW-2000 simulates three-dimensional move-
ment of groundwater of constant density through porous
earth material using the following partial differential equation
(McDonald and i-larbaugh, 1988);
dx K„ ax ) + dy V C,'}, dy) -F I Kz az ) + W = 55, �h
where
(5)
K ,K and Kzz are values of hydraulic conductivity along
the x, y, and z coordinate axes, which are assumed
parallel to the major axes of hydraulic conductivity in
the aquifer (L/T);
h is hydraulic head (L);
W is volumetric flux per unit volume from a hydrologic
source or sink as a function of location and time, with
W<0 for flow out of the aquifer and W>0 for flow into
the aquifer (T');
S,, is specific storage of the porous material (L1); and
t is time (T).
MODFLOW-2000 solves equation (5) using a finite -dif-
ference method in which the model domain is discretized into
a grid of cells, and hydraulic head is computed at the center
of each cell, Flows into or out of the aquifer from hydrologic
sources and sinks and head -dependent boundaries also are
computed for each simulation, Changes in aquifer storage are
computed for transient (time -varying) simulations.
Model Design
Spatial Discretization
The South Platte alluvial aquifer represented by the
model (fig. 21) is about 11 rni long, extending from the south
end of the study area to the north end, and about 3.6 mi wide
at its widest point at the south end of the study area. The simu-
lated area includes saturated alluvium in tributary valleys of
Big and Little Dry Creeks within 0.7-0.85 mi of the main stem
of the South Platte River valley. The model grid has 116 rows
and 38 columns with a uniform cell size of 500 ft by 500 ft.
The model simulates groundwater flow by using one layer
under unconfined conditions with rewetting capability active.
Layer thickness ranges from 12 to 65 ft (fig. 22) as determined
by the difference between land -surface altitude estimated from
a USGS DEM with 30 -in resolution (accessed June 10, 2005
at http://rockytveb.cr.usgsgov/elevation) and bedrock altitude
at the base of the alluvium indicated by Char and Arnold
(2002).
Boundary Conditions and Hydrologic Stresses
The base and most of the west side of the model are
simulated as no -flow boundaries (inactive cells) (fig. 21) to
represent relatively low -permeability bedrock in contact with
the alluvial aquifer. Although the east side of the model also
is underlain by relatively shallow bedrock, unconsolidated
sediments on hillslopes east of the aquifer generally are
thicker (fig. 8), and the east side of the model is simulated
as a specified -flow boundary by using the MODFLOW
Well Package to represent subsurface irrigation return flow
occurring through the unconsolidated sediments from ditch
seepage and infiltration of water beneath irrigated fields
located upgradient and outside the model domain to the east.
Simulated return flow during the non -irrigation season is
one-half of that during the irrigation season to represent less
inflow resulting from reduced recharge and lower groundwater
levels during the non -irrigation season. The upgradient and
downgradient ends of the aquifer, including the upgradient
ends of tributary valleys, are simulated as general -head
boundaries by using the MODFLOW General -Head Boundary
package to allow groundwater flow into and out of the
model, while also permitting hydraulic head to change at the
boundaries in response to land -use changes and aggregate
mining. General -head boundaries are defined using a saturated
thickness, hydraulic conductivity, and hydraulic gradient
representative of the aquifer at each boundary location.
Hydrologic stresses simulated by the model include distributed
recharge at the water table, streams and ditches, well
pumping, and phreatophyte evapotranspiration. Definition and
values of parameters used to represent boundary conditions
and hydrologic stresses in the model are presented in the
"Parameterization" section of this report.
Simulation of Groundwater Flow 41
40°15 N
40°04'N
40°02'N
49°N
39°58'
104°52'W
104°50'W
104°40'W
104°4s'W
—
—
— Model domain
co
co
a
0 5,009 10,000 FEET
I 171 I I —
�_-..
_....
..
-r
■.
■___
__
�..
■w �•
L�
,...
�o'1■E
...
Li
11
I-
l_
■
---
_lI.._:.�:._.1......_
-r•'--.
_
.. T
.}�I
.•_�--
��
—II'
.T
-
■■oo■
■u:
-
fi
1scYuI;
4{
..
rr
__
■IM
..
�■■
■■�■
_—.
E i..■S
__ ..■■■
_
■A
■MMw
r�iw>r
I
_
l
i
�.
I
M €
_
.,.
■.
-.:.
-_..-
I.
-
■.
■a�rr■■w■
■■,.
E
_�::
_
T,._
r
■emu■
U. .■C�■...
!r
—
'
I1
r■
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—��
...
�Ji
a
_
-
._,�..:.,
._L_l
I I
0 1,000 2,000 3,000 METERS
r i
_i
:E
_ ,
°
±i
EXPLANATION
1 Active cell
"" -" General -head boundary cell
Specified-flowcell
River cell
F.57 Municipal -well cell
Damn cell
Evapotranspiration cell
Inactive cell
Figure 21. Model grid and boundary conditions of the simulated South Platte alluvial aquifer,
Brighton to Fort Lupton, Colorado,
42 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
104°52'W
40°06'N
40°04'N
40°02'N
40°N
39°53'N
104°50'W
104°48'W
EXPLANATION
Layer thickness, in feet
10-20 t:i �� 40-5D
20-30 50-60
30--40 60-70
104°46'W
— Model domain
Inactive cell
Figure 22. Layer thickness of the simulated South Platte alluvial aquifer, Brighton to Fort
Lupton, Colorado.
Simulation of Groundwater Flow 43
Recharge. Recharge from infiltration at the land surface
is simulated by using the MODFLOW Recharge package and
is distributed in the mode! based on land -use conditions in
1957, 1977, and 2000 as shown in figures 17A C. Recharge
beneath non -irrigated land and urban areas is simulated as
having the same value during both the irrigation and non -
irrigation seasons. Recharge beneath irrigated agricultural
areas during the irrigation season is simulated as greater than
during the non -irrigation season to reflect increased recharge
from infiltration of water applied to irrigated fields and seep-
age losses from irrigation ditches. Recharge beneath irrigated
agricultural areas during the non -irrigation season is simulated
to be the same as recharge beneath non -irrigated areas.
Streams and ditches. The South Platte River and Big
Dry Creek are simulated by the MODFLOW River pack-
age, which contributes water to or releases water from the
aquifer at river cells as determined by the hydraulic gradi-
ent between the aquifer and the river and as a function of
streambed conductance. Locations of the South Platte River
and Big Dry Creek were determined from USGS 7.5 -minute
topographic quadrangle maps (1994) at 1:24,000 scale, River
stage of the South Platte River and Big Dry Creek is defined
2 ft below land surface to represent the lower altitude of the
stream channel relative to the mean altitude of each 500 -ft -
wide cell, Streambed conductance is defined as a function of
the stream -channel width and length, streambed thickness,
and hydraulic conductivity of the streambed material. Average
stream -channel width was estimated from USGS 7.5 -minute
topographic quadrangle maps, Stream -channel length within
each model cell was calculated by the MODFLOW Graphical
User Interface (Winston, 2000) on the basis of stream loca-
tions. A streambed thickness of 1 ft and a streambed hydraulic
conductivity of l ft/d were assumed for initial values.
Little Dry Creek and Third Creek are simulated by the
MODFLOW Drain package rather than the River package
because flow in the creeks is intermittent and is assumed not
to contribute substantially to the alluvial aquifer. However, the
creeks can drain water from the aquifer if the water table rises
to near land surface. Locations of Little Dry Creek and Third
Creek are based on CDWR mapping provided by Schupbach
and Lewis (1996). Drain altitude of Little Dry Creek and
Third Creek is defined 2 ft below land surface to represent the
incised stream -channel altitude. Drain conductance is defined
as described for streambed conductance, assuming the stream
channels are 10 -ft wide with a 1 -ft thick streambed having a
hydraulic conductivity of 1 ft/c1,
irrigation ditches in the model domain are not explicitly
simulated except where Lupton Bottom Ditch intercepts and
conveys drainage from Little Dry Creek (fig. I). In this cir-
cumstance, Lupton Bottom Ditch is simulated as a drain in the
same manner as Little Dry Creek.
Some areas of the model (fig. 21) appear to have an
excessively wide (more than one model cell) simulated stream
feature. This is caused by the meandering stream feature
flowing across a small part of many adjacent model cells.
However, the net hydrologic effect of the stream feature is
representative of a stream flowing across a single model cell
because streambed and drain conductance are apportioned by
the extent of the stream feature within each model cell.
Well pumping. Municipal -well pumping is simulated
by using the MODFLOW Well package to specify flow out
of the model at municipal -well locations, The location and
withdrawal rate for each simulated well is based on data pro-
vided by the cities of Brighton and Fort Lupton as described
in the "Aquifer Outflows" section of this report. Withdrawals
from irrigation wells are not simulated by the model because
of large uncertainties in the location and withdrawal rate of
irrigation wells during the time periods simulated. However,
because irrigation wells generally are collocated with or near
irrigated land and generally are fairly evenly distributed within
irrigated areas, the steady-state effects of irrigation -well with-
drawals are assumed to be approximately accounted for by the
term used to represent net recharge beneath irrigated areas.
Evapotranspiration. Evapotranspiration by shallow -
rooted vegetation over large areas is not explicitly simulated in
the model because such evapotranspiration is considered in the
simulation of net recharge for each land -use category. How-
ever, phreatophyte evapotranspiration is explicitly simulated
in the model by using the MODFLOW Evapotranspiration
(EVT) package because phreatophytes commonly withdraw
water directly from the water table. The Evapotranspiration
package simulates the removal of water from the aquifer in
proportion to the depth of the water table below a user -defined
altitude. Evapotranspiration occurs at a maximum rate when
the water table is at the user -defined altitude, and evapotrans-
piration decreases linearly to zero at a user -defined extinction
depth, below which evapotranspiration does not occur. Phre-
atophyte evapotranspiration in the model is simulated relative
to DEM-derived land -surface altitude and has an extinction
depth of 10 ft.
.Evapotranspiration cells in the model (fig. 21) are
assigned on the basis of phreatophyte extent mapped by the
Colorado Division of Wildlife (2007a, b) with small (<2,500
ft2), isolated phreatophyte areas removed, Because phreato-
phyte extent within each model cell generally is less than the
total cell area, a multiplier array is used to apportion phre-
atophyte evapotranspiration in each cell based on the ratio of
phreatophyte area to total cell area.
Hydraulic Conductivity
The spatial distribution of hydraulic conductivity in the
simulated South Platte alluvial aquifer is represented by using
four parameter zones as shown in figure 23. Hydraulic conduc-
tivity within each zone is uniform, and each zone is associated
with a hydraulic -conductivity parameter value estimated by
the model. Zone 1 represents areas of least hydraulic con-
ductivity along parts of the aquifer margins and in tributary
valleys, zone 2 represents areas of medium hydraulic conduc-
tivity along most of the aquifer margins, zone 3 represents
areas of greater hydraulic conductivity along the central part
of the aquifer, and zone 4 represents a small area of greatest
44 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
hydraulic conductivity near Brighton. Hydraulic -conductivity
zones were assigned to reflect the spatial distribution of
hydraulic -conductivity values shown on figure 12.
Parameterization
A parameter represents a hydrologic property of the
model such as hydraulic conductivity or recharge. Fifteen
parameters are used in the model. Four parameters are used to
represent the hydraulic -conductivity distribution (LPF Par],
LPF Part, LPF Para, and LPF Par4) (fig, 23) of the alluvial
aquifer, Three parameters are used to represent the distribu-
tion of recharge (figs. 17A —C ) based on land use. Recharge
beneath irrigated agricultural land during the irrigation season
is represented by parameter RCH Irr. Recharge beneath non -
irrigated land and recharge beneath irrigated agricultural land
during the non -irrigation season are represented by parameter
RCH Nonirr. Recharge beneath urban areas is represented
by parameter RCH Urban, Other properties or aspects of the
model that are represented by parameters are (1) hydraulic
conductance per unit length of general -head boundaries at the
upgradient and downgradient ends of the South Platte River
valley (GI-IB Splt), (2) hydraulic conductance per unit length
of general -head boundaries at the upgradient ends of Big Dry
Creek and Little Dry Creek tributary valleys (GHB_Trib),
(3) streambed conductance per unit length of the South Platte
River (RIV_Splt), (4) streambed conductance per unit length
of Big Dry Creek (RIVBdry), (5) drain conductance per
unit length of Little Dry Creek and Third Creek tributaries
(DRN_Trib), (6) subsurface return -flow rate along the east
side of the model (Q_ReturnE), (7) municipal -well with-
drawal rate (Q_Mwells), and (8) maximum phreatophyte
evapotranspiration rate (EVT_Part). Parameters representing
conductance values per unit length (GHB_Splt, GHB_Trib,
RIV_Sp]t, RIV_Bdry, and DRN_Trib) were multiplied by the
length of the represented stream feature within each model
cell to calculate total conductance values. The parameter
Q_IVtwells represents the mean withdrawal rate per munici-
pal well during the 2000 irrigation season. Multipliers were
used to adjust individual well withdrawals to represent flow
conditions during the irrigation and non -irrigation seasons of
each time period simulated, A sixteenth parameter (specific
yield) was added for transient simulations of the effects of
aggregate mining but was not included in model calibration
because specific yield is not relevant for steady-state calibra-
tion. Specific yield was assigned a value of 0,25 in all transient
simulations, The initial and final estimated values for each
parameter are presented in table 6. Initial parameter values are
based on data and estimated hydrologic conditions described
under "Groundwater Hydrology," All final estimated param-
eter values appear reasonable compared to available data and
estimated error associated with the data. The initial value
(0.0038 ft/d) of parameter RCf-I_lrr represents recharge of
about S in. during the irrigation season, and the final esti-
mated value (0.0056 ft/d) represents recharge of about 12 in,
during the irrigation season. The initial value (0.00011 ft/d) of
parameters RCH_Nonirr and RCH_Urban represents recharge
of about 0.5 in/yr, and final estimated values for RCl-l_Nonirr
(0.000082 ft/d) and RCH_Urban (0 ft/d) represent recharge of
about 0.4 in/yr and 0 in/yr, respectively.
Model Calibration
The model was calibrated by using the Observation,
Sensitivity, and Parameter -Estimation Processes of
MODFLOW-2000 (l -fill and others, 2000), which uses
inverse modeling methods to minimize the difference
between measured values and model -simulated values.
MODFLOW-2000 allows individual stress periods in a single
simulation to be either steady-state or transient. The model
is calibrated to six successive steady-state stress periods
in a single simulation that represents hydraulic head and
base -flow conditions during the irrigation seasons and the
non -irrigation seasons in 1957, 1977, and 2000. Calibration
to hydrologic conditions in multiple time periods was used
to facilitate estimation of recharge as it relates to land -use
change. The years 1957, 1977, and 2000 were selected for
model calibration because spatial datasets indicating land -
use conditions (U.S. Geological Survey, 1999, 2001b, c)
were available for these time periods. The spatial distribution
of recharge by land use in the model was changed between
1957 and 1977 and between 1977 and 2000 by using separate
MODFLOW-2000 Instances (Harbaugh and others, 2000),
which allow a parameter to vary spatially between stress
periods. Calibration to seasonal hydrologic conditions was
used to simulate the seasonal, dynamic nature of the alluvial
aquifer. The model was calibrated to successive steady-state
seasonal stress periods, rather than transient stress periods,
because the aquifer has high hydraulic conductivity that allows
the effects of hydrologic stresses to be rapidly distributed
throughout the aquifer. As a general rule, it is reasonable to
use successive steady-state solutions to periodic (seasonal)
stresses when the dimensionless variable T is less than 0.1
(llaitjema, 2006). The variable -r is defined by the following
relation (Townley, 1995):
T = SG2l4KHIP (6)
where
S
K
If
P
is storage coefficient of the aquifer (dimensionless),
is the average distance between boundary conditions,
in feet,
is hydraulic conductivity, in feet per day,
is saturated thickness, in feet, and
is the period of the forcing function (365 days for
seasonal fluctuations).
By using a specific yield value of 0.25 for the storage
coefficient, an average distance between boundary conditions
of 4,000 ft, average hydraulic conductivity of 1,000 ft/d,
average saturated thickness of 30 ft, and a forcing period of
Simulation of Groundwater Flow 45
104'52'W
40'05'N
40°04'N
4o°02'N
40%
3s°5s'N
104°50'W
104'48'W
_
EXPLANATION
:Ti
104'46'W
— Model domain
Study Area Boundary
0 5,000 10,000 FEET
I HI ill I
-i-
�
0 1,000 2,000 0,000 METERS
Hydraulic -conductivity zone
and associated parameter
(Parameter values arc shown in table 6)
Zone 1 —Parameter LPF Pat I
Zone 2 —Parameter LPF Pat 2
Zone 3 —Parameter LPF_ Par3
Zone 4 —Parameter LPF__Par4
Inactive cell
Figure 23. Hydraulic -conductivity zones of the simulated South Platte alluvial aquifer,
Brighton to Fort Lupton, Colorado.
46 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
Table 6. Initial and final estimated parameter values for the calibrated steady-state model of the South Platte alluvial aquifer,
Brighton to Fort Lupton, Colorado.
[H/d, feet per day; tP/d, cubic feet per day]
Parameter
name
Model feature represented by parameter
Initial
value
Final estimated
value
Units
1PEParl
LPF Part
LPI'_ Par3
EPP Par4
RCH Urban
ItCH_1rr
RCH Nonirr
DUB Splt
CHB Trib
RIVSpit
RIVBdry
DRN_Trib
Q_ReturnE
Q_Mwelis
G VT_Par I
Horizontal hydraulic conductivity of zone 1
Horizontal hydraulic conductivity of zone 2
Horizontal hydraulic conductivity of zone 3
Horizontal hydraulic conductivity of zone 4
Recharge rate beneath urban areas
Recharge rate beneath irrigated land during irrigation season
Recharge rate beneath non -irrigated land and beneath irrigated land
during non -irrigation season
Hydraulic conductance per unit length of general -head boundaries at
upgradient and downgradient ends of South Platte River valley
Hydraulic conductance per unit length of general -head boundaries at
upgradient ends of Big Dry Creek and Little Dry Creek tributary
valleys
Stream bed conductance per unit length of South Platte River
Streambed conductance per unit length of Big Dry Creek
Drain conductance per unit length of Third Creek and Little Dry
Creek tributaries
Subsurface return -flow rate along east model boundary
Municipal -well withdrawal rate
Maximum phreatophyte evapotranspiration rate
450
740
1,200
2,100
1,1E-4
3.8E-3
1.1E-4
1,000
10
140
20
10
I 0,000
-68.000
1.4E-2
430
880
I,070
2,110
0
5,6E-3
8,2E-5
ftld
Old
hid
ft/d
ftld
Rid
Old
870 it/d
10 hid
100
2,2
17
8.490
—95,700
1,3E-2
ftid
ftid
ft/d
ft3/d
ftYcl
fi/d
365 days, T for the model is 0.09, which indicates steady-state
calibration to seasonal conditions likely provides a reasonable
representation of the aquifer system. Because the model is
calibrated to multiple steady-state stress periods in a single
simulation, individual parameter values estimated during
calibration represent the overall best fit between measured
values and model -simulated values for all stress periods.
To evaluate the viability of alternative model structures,
model calibration was accomplished by beginning with a
simple model and adding complexity incrementally based on
the contribution of each added feature to improving statisti-
cal measures of model fit and related measures. The aquifer
hydraulic -conductivity distribution and stage of the South
Platte River also were adjusted to evaluate their effect on cali-
bration. Incremental model additions included simulation of
(1) return flow along the east and west model boundaries, (2)
groundwater drainage to Little Dry Creek and Third Creek, (3)
phreatophyte evapotranspiration, (4) irrigation- and municipal -
well withdrawals, and (5) flow in irrigation ditches. inclusion
of return flow along the east model boundary, groundwater
drainage to Little Dry Creek and Third Creek, phreatophyte
evapotranspiration, and municipal -well withdrawals improved
model fit. inclusion of return flow along the east model
boundary had the greatest influence on model fit. Addition of
phreatophyte evapotranspiration had a small effect on model
fit but reduced parameter correlation in the model. Inclusion of
irrigation -well withdrawals caused simulated recharge beneath
irrigated lands to be unrealistically large and model residuals
to have a less -random spatial distribution. Similarly, explicit
simulation of flow in individual irrigation ditches reduced
model fit and contributed to unrealistic values for other param-
eters. During parameter estimation, urban recharge and return
flow along the west model boundary consistently became very
small (near zero), and the model had very little sensitivity to
these parameters (see "Sensitivity Analysis"). Consequently,
return flow along the west boundary was not included in the
final calibrated model. Urban recharge was retained in the
model for comparison to recharge beneath other land uses, but
it was assigned a value of zero to promote model stability and
to simulate the maximum potential effects of converting irri-
gated or non -irrigated land to urban areas. Because the model
has very little sensitivity to urban recharge, the assumption of
zero recharge beneath urban areas had little effect on model
calibration.
Observations and Prior Information
An observation represents a measurement of a physical
aspect of the hydrologic system, such as hydraulic head or
flow, that can be used for comparison to simulated values
Simulation of Groundwater Flow 47
during model calibration. Field measurements of hydrologic
properties that are used to define parameter values are called
prior information, To simulate average hydrologic conditions
during the irrigation and non -irrigation seasons in each
calibration year (1957, 1977, and 2000) and avoid calibrating
to short-term drought or wet conditions, head and flow
observations for 3 years before and after each calibration year
are included in the calibration data set for each stress period.
Therefore, hydraulic head and flow during the irrigation and
non -irrigation seasons in 1957 represent average seasonal
(irrigation or non -irrigation) conditions for the period
1954-60, hydraulic head and flow during 1977 represent
average seasonal conditions for 1974-80, and hydraulic head
and flow during 2000 represent average seasonal conditions
for 1997-2003, If multiple head or flow observations were
available at a single location for a given time period, the
average of the observations was used to represent hydrologic
conditions at that location,
Hydraulic head observations. Hydraulic -head observa-
tions consist of 159 groundwater -level measurements (fig. 24;
table 7) distributed throughout the aquifer during the six
seasonal calibration stress periods. Forty-one groundwater -
level measurements represent the 1957 irrigation season,
11 measurements represent the 1957 non -irrigation season,
6 measurements represent the 1977 irrigation season, 48
measurements represent the 1977 non -irrigation season, 30
measurements represent the 2000 irrigation season, and 23
measurements represent the 2000 non -irrigation season. If
hydraulic -head observations for a location were available for
different calibration stress periods, the change in groundwater
level between periods was used as the observation rather than
hydraulic head to provide for greater observation accuracy at a
specific location. Of the 159 groundwater -level measurements,
40 represent temporal changes in groundwater levels (table 7).
Flow observations. Flow observations consist of six
gain -loss determinations (fig. 24; table 8) based on monthly
mass -balance analysis of all substantial inflows and outflows
to the South Platte River between stream gages located near
Henderson (station number 06720500) and Fort Lupton
(station number 06721000). One flow observation was used
to represent average base -flow conditions during each of the
irrigation and non -irrigation seasons in 1957, 1977, and 2000,
Because the gage near Henderson is south of the study area,
gain -loss between the south end of the model domain and the
gage near Fort Lupton was estimated by linear interpolation
as 78 percent of gain -loss between the two gages. No major
inflows or outflows occur between the Henderson gage and the
south end of the model domain. Because all flow observations
are positive, flow observations represent average seasonal gain
to the South Platte River during each seasonal stress period.
Observation weights. Hydraulic -head and flow observa-
tion weights were assigned based on estimated measurement
accuracy by using statistical methods described by Hill (1998,
p, 45-49), Three different standard -deviation values were used
to represent measurement error for hydraulic -head observa-
tions. The standard deviation of measurement error for most
hydraulic -head observations is estimated to be about 3 ft based
on errors in land -surface datum and measurement method.
However, the standard deviation of measurement error for
hydraulic -head observations was considered to be 1,5 ft if
the land -surface datum for the groundwater -level measure-
ment was indicated with a precision greater than 1 ft. Because
measurements of temporal changes in groundwater levels at
a specific location remove errors associated with land -surface
datum, observations of temporal hydraulic -head change were
estimated to have a standard deviation of measurement error
of1ft.
Because flow observations are determined from the mass -
balance analysis of all inflows and outflows to the South Platte
River between stream gages at the upstream and downstream
ends of the observation reach, flow -observation weights
include measurement errors associated with each inflow and
outflow measurement, The standard deviation of measurement
error for each flow observation was calculated by summing the
variances of measurement error associated with each inflow or
outflow and taking the square root of the total variance. The
accuracy rating of streamflow measurements was assumed to
be good, in which 95 percent of the measured flows are within
10 percent of their true values (Rantz and others, 1982).
Prior information. Use of prior information allows
direct measurements of model input values to be included in
the calibration regression (Hill, 1998). Prior information was
used to constrain estimates of parameters LPF_Parl, LPF
Part, LPF_Par3, LPF_Par4, RCH_Irr, RCH Nonirr, GHB_
Splt, GHB_Trib, DRN_Trib, Q_Mwells, and EVT—Par1.
Prior -information values were assigned on the basis of initial
parameter values (table 6), and weights for prior -information
values generally were assigned on the basis of the standard
deviation of estimated prior -information measurement en -or as
described by [-till (1998, p. 45-49),
Calibration Assessment
Statistical Measures of Overall Model Fit
The difference between an observation and a simulated
value or between a prior -information value and a parameter
estimate is called a residual. Overall model fit was evaluated
by using the standard error of the model regression and the
mean absolute error (MAE) of unweighted residuals. The
standard error of the regression is an indicator of the overall
magnitude of weighted residuals and is expressed as (Hill,
I998)
where
(ND + NPR— NP)
s' is the standard error of the regression,
SO is the value of the weighted least -squares
objective function, calculated as
(7)
48 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
70°0E'N
90°19'N
,•v
10'N
39°5B'N
101°52'W
4
Slrcarns modified floor U S Cicological Survey National Hydrogrophy Dalasel; I .100,000
North American Datum of 1983
06720500
184'50'W
Henderson
:3,
"95
130 ASllghton
� • • •199
•131, II? 151
,59
• •190
to, 172 Atss
• •154
.'.155. 159 153 135
• 4.
136, 127
•192
IM°70'W
99,95•
•
1 r:, It,
•
34
29,11,95
Ills !.4I:. •112
*113
•1,1
0[72151"$ 1,2
•
E. Fort
O LtiNon
• 15,
8.3 IB
•
••
Id 18,20,21
15 .. 43,24
32
• 39
S
*25 25
•
09■ 26 :1
•
t4 2
99
• •
�
•lt
90
• 95 •
7/
50, 51.62 ••
G3 94
• • 54 * •49
55
"111 66 •5r
no••
5p
•57 62
12
173 129
•122
:23 121•
•
* OM
rre
•
A
EXPLANATION
Loom inn of hydraulic -herd
observatlou---Numhor is observation
nu robot (given in labia 7)
Location or stream gage
used for strenitfnw gain
or loss observation --Number is
station number
Approximate extent of Om odnted aquifer
0 5,000 10,000 FEET
H__.�1 t L -l- 11
0 1,000 2,000 3,000 METERS
Figure 24. Location of hydraulic -head observations and stream gages used to estimate streamflow gain -
loss observations for the simulated South Platte alluvial aquifer, Brighton to Fort Lupton, Colorado.
Simulation of Groundwater Flow 49
Table 7. Hydraulic -head observations used to calibrate the steady-state model of the South Platte alluvial aquifer, Brighton to Fort
Lupton, Colorado.
[Hydraulic head in feel above National Geodetic Vertical Datum of 1929; Observation date(s) in min/dd/yyyy; --, no data; CDWR, Colorado Division of
Water Resources; USGS NW[S. lJ S Geological Survey National Water Information System; CDMRS, Colorado Division of Mining, Reclamation, and
Safety]
Observation
number
Observation
name
Hydraulic
head'
Observation
date(s)
Well
identifier'
Source
1 B16605BC1]_ 1
2 B16605BCD2
3 B16606BB
4 316606BDC _l
5 B16606BDC_2
6 B16606DBD
7 B16607AA
8 B16607ACD.,1
9 316607ACD_2
10 B16607AC)
11 B16607BBB
12 B16607BBB1
13 B16607BC
14 B16607BD
15 B16607DD
16 B16608BAD _1
17 B16608BAD_2
18 B16608BC
19 B16608BCD..1
20 B16608BCD_ 2
21 B16608BCD_ 3
22 B16608CC
23 B 16608CCD„i
24 B 16608CCD2
25 1316617CB
26 B 16617CCD1
27 1316617CCD
28 B 16618ACD -1
29 1316618ACD.2
30 B16618AD
31 B 16618BB _1
32 B16618BB_2
4893.8 10/25/1956, 11/04/1957
4893.8 03/14/1974
4885 03/08/2002
4888 11/--/1957
4888.8 03/17/1974, 01/05/1975
4887.2 03/17/1974
4889 04/16/1957
4898.6 11/04/1957
4895.2 03/14/1974, 01/05/1975
4899 09/19/1977
4892.8 03/1711974,01/05/1975
4894.5 08/30/1956
4899 04/30/1997
4897 10/03/1997
4908 04/21/1977
4914.3 11/04/1957
4906.6 03/17/1974, 01/05/1975
4901 09/04/1998
4906.8
04/04/1955, 04/17/1956,
05/07/1957, 04/15/1958,
04/01/1959, 04/05/] 960
SB00106605BCD
SB00106605BCD
239289A
SB00I06606BDC
SB00106606DBD
SB00106607AA
SB00106607ACD 1
5B00I06607ACD
93440A
SB00106607B13B
SB00106607BBB 1
13697R
197906
19791RF
SB0010660SBAD1
SB00106608BAD2
123510A
SB00106608BCD
USGS NWIS
USGS NWIS
CDWR
Smith and others (1964)
USGS N WIS
USGS NWIS
CDWR
USGS NWIS
USGS NWIS
CDWR
USGS NWIS
USGS NWIS
CDWR
CDWR
CDWR
USGS NWIS
USGS NWIS
CDWR
IISGS NWIS
4910.6 11/08/1955, 11/13/1956, SB00106608BCD USGS NWIS: Smith and others
1l/--/1957, 11/13/1958,
11/11/1959, 11/02/1960
4907.6 03/01/1977
4906 08/25/1978
4924 11/04/1957
4914.3 03/14/1974.01/05/1975
4917 11/19/1999
4930.5 11/04/1957
4920.5 03/14/1974.01/05/1975
4915.6 11/04/1957
4911.8 03/14/1974
4909 11/29/1999
4904.9 02/07/2003
4908.7 07/02/2003
33 B16618DD 4918
34 1316619BAB 4915.5
05/27/1978
11/04/1957
(1964)
SBOOl06608BCD I Schneider and Hillier (1978)
99795A CDWR
SB00106608CCD1 USGS NWIS
SBOO106608CCD USGS NWIS
11384R CDWR
SB001066]7CCDI USGS NWIS
SB00106617CCD2 USGS NWIS
SB00l06618ACD1 USGS NWIS
SBOO106618ACD USGS NWIS
11383R CDWR
250147 CDWR
M2000-016 CDMRS
CI -103-M W03
99034A CDWR
SB00106619BABI USGS NWIS
50 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
Table 7. Hydraulic -head observations used to calibrate the steady-state model of the South Platte alluvial aquifer, Brighton to Fort
Lupton, Colorado.—Contiued
[Hydraulic head in feet above National Geodetic Vertical Datum of 1929; Observation dates) in mre/dd/yyyy; --, no data; CDWR, Colorado Division of
Water Resources; USGS NWIS, U S Geological Survey National Water Information System; CDMRS, Colorado Division of Mining, Reclamation, and
safety]
Observation Observation Hydraulic Observation Well
number name head' date(s) identifier'
Source
35 B16619BCB
36 B16619CA
37 B 16619CB_1
38 B16619CB_2
39 1316619CD..1
40 B16619CD 2
41 B16619DCD
42 B16620BC1
43 B16620BC2
44 B16620CB
45 B16620CBD
46 B16620CCD
47 BI6629BC
48 B I6629CA
49 B16629CCC
50 1116630ADA„1
51 B16630ADA_2
52 B16630ADA3
53 B16630DA
54 13166301)D
55 B16631AA
56 B16631AC
57 BI6631CDD
58 B16631DA1
59 B16631DA2
60 B16631DB
DB
61 B16632BC
62 B16632CCD
63 B16632CDC
4915.2 03/14/1974, 01/05/1975
4920 03/16/1979
4918.2 02/12/2002, 03/27/2003
4920 07112/2002,07/02/2003
4923,9 04/09/2002, 03/27/2003
S130010661911CB
13699F
M2000-016 RI01-
MW08
M2000-016 R10I-
MW08
M2000-016 R101-
MW02
4925.6 07/12/2002. 07/02/2003 M2000 -0I6 R101 -
M W 02
4926,2 08/08/1956 SB00106619DCD1
USGS 4001
4922 04/22/1976 83526A
4921 07/10/1998 46066A
4927 10/16/1997 20138R
4925 03/14/1974 SB00106620CBD
4936.2 11/04/1957 SB00106620CCD1
4930 02/18/2002 232950A
4935 04/07/1998 205941A
4944 11/04/1957 SB00106629CCC1
USGS 4000
4933,8 04/05/1955, 04/17/1956, SB00106630ADA
05/07/1957, 04/07/1958,
04/01/1959, 04/12/1960
4936.9 11/11/1954, 11/08/1955, SB{]0106630ADA
11/13/1956, 11/12/1957,
11/13/1958, 11/11/1959,
11/02/1960
4932.2 03/01/1977
4930
4935
4932
4952
4947.2
SB00106630ADA1
04/13/2002 57300F
10/16/1997 17900A
03/27/1998 181391A
09/18/1998 214916A
11/04/1957 SB00106631 CDD I
USGS 4000
4944 05/15/1975 79073
4945.5 05/01/1997 212525
4945 10/07/1977 81358
4940 11/25/2000 2253IA
4955.3 11/04/1957 SB00106632CCD1
4950.6 03/14/1974, 01/05/1975 SB00106632CDC
USGS NWIS
CDWR
CDMRS
CDMRS
CDMRS
CDMRS
USGS NWIS
CDWR
CDWR
CDWR
USGS NWIS
USGS NWIS
CDWR
CDWR
USGS NWIS
USGS NWIS
USGS NWIS
Schneider and Hillier (1978)
CDWR
CDWR
CDWR
CDWR
USGS NWIS
CDWR
CDWR
CDWR
CDWR
USGS N WIS
USGS N WIS
Simulation of Groundwater Flow 57
Table 7. Hydraulic -head observations used to calibrate the steady-state model of the South Platte alluvial aquifer, Brighton to Fort
Lupton, Colorado, —Continued
[Hydraulic head in feet above National Geodetic Vertical Datum of 1929; Observation date(s) in mmldd/yyyy, --, no date; CDWR, Colorado Division of
Water Resources, USGS NW1S, U S Geological Survey National Water Information System; CDMRS, Colorado Division of Mining, Reclamation, and
Safety]
Observation
number
Observation
name
Hydraulic
head'
Observation
dates)
Well
identifier'
Source
64 B16701DA
65 B 16701 DAA
66 B16701DBC_1
67 B16701DBC 2
68 1316712ACC_1
69 B16712ACC 2
70 1316712BD _1
71 B16712BD_2
72 B16712CD_1
73 B16712CD._2
74 B16712DB_1
75 B16712DB 2
76 B16713AC_I
77 B16713AC 2
78 B16713ADD
79 B16713BDD _1
80 1316713131)0..2
81 1316724AB
82 1316724A13 2
83 1316724BBB
84 B 16725ACC I
85 B16725AOC 2
86 B1672513A
87 B16725CD
88 13167250D_1
89 B16725DD_2
90 B16736ACA
91 B16736CD[)_1
92 B16736CDD 2
93 B26629AB
94 B26629ABC1
95 B26629ABC 2
4885
4888,8
4893
4898.4
4901.6
4899.1
4901.8
03/05/2002
03/17/1974, 01/05/1975
02/04/2003
07/02/2003
1 I/04/1957
03/17/1974, 01/05/1975
03/27/2003
4905.7 08/14/2003
4910 01/29/2003
4914,3 08/18/2003
4903,1 02/05/2003
4904.8 07/02/2003
4907.2 02/07/2003
4912.6 07/02/2003
4911.7
4916
4914.1
4919.6
4922
4929,2
4930.9
4929.9
4929
4930
4930.9
4932,2
4936,7
4948
4948.1
4873
4872,7
03/01/1977
11/--/1957
03/17/1974, 01/05/1975
02/14/2003
07/02/2003
04/19/1977
11/04/1957
03/17/1974, 01(05/1975
07/12/1979
03/21/1997
03/14/2002,03/04/2003
10/3/02,7/9/03
03/17/1974, 01/05/1975
11/--/1957
03/01/1977
07/15/1977
11/08/1957
4868.3 03/14/1974,01/05/1975
236955
SB00106701 DAA2
250149
M2000-016 DS03-
MWOI
SB00106712ACC1
SB00106712ACC
M2000-016 DS03-
MW03
M2000-016 DS03-
MWO3
250142
M2000-016 MY03-
MWOI
250139
M2000-016 MF03-
MWO2
250146
M2000-016 CI103-
MWO1
CDWR
USGS NWIS
CDWR
CDMRS
USGS NWIS
USGS NWIS
CDMRS
CDMRS
CDWR
CDMRS
CDWR
CDMRS
CDWR
CDMRS
SB00106713ADD1 Schneider and Hillier (1978)
Smith and others (1964)
SB00106713BDO USGS NWIS
250144 CDWR
M2000-016 NO03- CDMRS
MWO2
SB00I06724BBB 1 Schneider and Hillier (1978)
SH00106725ACC1 USGS NWIS
SB0OI06725ACC USGS NWIS
I3320RF CDWR
202399A CDWR
M2004051 MW -5 CDMRS
M200405I MW -5 CDMRS
SB00106736ACA USGS NWIS
Smith and others (1964)
SB00106736CDD1 Schneider and Hillier (1978)
177498 CDWR
SB00206629ABC2 USGS NWIS
USGS 4006
SB00206629ABC2 USGS NWLS
USGS 4006
52 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
Table 7. Hydraulic -head observations used to calibrate the steady-state model of the South Platte alluvial aquifer, Brighton to Fort
Lupton, Colorado. —Continued
[Hydraulic head in feet above National Geodetic Vertical Datum of 1929; Observation dates) in mm/dd/yyyy; --, no data; CDWR, Colorado Division of
Water Resources', USGS NWIS, U S Geological Survey National Water Information System; CDMRS, Colorado Division of Mining, Reclamation, and
Safety]
Observation
number
Observation
name
Hydraulic Observation
head' date(s)
Well
identifier'
Source
96 B26629CCD_1 4873.7
97 B26629CCD_2 4875.5
98 B26629CCD_3 4875.5
99 B26629CDB 4877,1
100 B26630ADD_ 1 4870,3
101 B26630ADD 2 4871,3
102 B26630BA 4863
103 B26630BD 4863
104 B26631AB 4872
105 B26631AC 4880
106 B26631ACD_ 1 4874.3
107 B26631ACD 2 4874.6
108 B26631AD 4876
109 B26631BDA, 1 4876.8
110 B26631BDA 2 4876,7
111 B26631DCD 4886.2
112 B26632BCC 4883,9
113 B26632CB 4883
114 B26725CDC_1 4877.2
115 B26725CUC 2 4874.8
116 B26735DAA 4898
117 B26736BDB 4881,1
118 B26736CAC 4885.1
119 B26736DBB 1 4880.8
120 B26736DBB 2 4879.4
121 C16605CC 4964.2
122 C16606AC 4945
123 C16606ACD 4954.9
124 C16606ADC 4956.6
125 C16606CBD 4950.6
04/04/1955, 04/17/1956, SB00206629CCD USGS NWIS
05/06/1957, 04/02/1958,
04/01/1959, 04/05/1960
11/10/1954, 11/08/1955, SB00206629CCD USGS NWIS
11/13/1956,11/12/1957,
11/12/1958, 11/11/1959,
11/02/1960
03/01/1977 SB00206629CCD i
11/08/1957 SB00206629CDB
11/08/1957 SB00206630ADD I
03/14/1974, 01/05/1975 SB00206630ADD
08/01/1997 202995
08/14/1975 80437A
10/19/2000 226861
03/19/1976 20306RF
04/14/1997, 04/29/1998, 6818R
03/24/1999, 04/11/2000,
03/08/2001, 04/02/2002,
04/28/2003
10/15/1997, 10/01/1998, 6818R
11/01/2001
11/30/1999 6818RR
11/04/1957 SI300206631 BDA I
03/17/1974, 01/05/1975 5B0020663IBDA
11/27/1957 SB00206631DCD1
11/04/1957 SB 00206632BCC 1
02/20/1979 103272
11/04/1957 SB00206725CDC1
03/17/1974.01/05/1975 SB00206725CDC
3/--/1974 65643
11/04/1957 SB00206736BDB1
04/19/1977 SB00206736CAC
11/04/1957 SB00206736CBB1
03/01/1977 SB00206736DBBI
05/09/2002 40808MH
02/28/2001 227721 A
03/17/1974.02/28/1976 SC00106606ACD
11/04/1957 SC00106606ADC I
USGS 3955
11/04/1957 SC00106606CBD1
Schneider and Hillier (1978)
USGS NWIS
USGS NWIS
USGS NWIS
CDWR
CDWR
CDWR
CDWR
CDWR
CDWR
CDWR
USGS NWIS
USGS NWIS
USGS NWIS
USGS NWIS
CDWR
USGS NWIS
USGS NWIS
CDWR
USGS NWIS
Schneider and Hillier (1978)
USGS NWIS
Schneider and /Hillier (1978)
CDWR
CDWR
USGS NWIS
USGS NWIS
USGS NWIS
Simulation of Groundwater Flow 53
Table 7, Hydraulic -head observations used to calibrate the steady-state model of the South Platte alluvial aquifer, Brighton to Fort
Lupton, Colorado. —Continued
[Hydraulic head in feet above National Geodetic Vertical Datum of 1929; Observation date(s) in nun/dd/yyyy; --, no data; CDWR, Colorado Division of
Water Resources; USGS NWIS, U S Geological Survey National Water Information System; CDMRS, Colorado Division of Mining, Reclamation, and
Safety]
Observation Observation
number name
Hydraulic
head'
Observation
date(sl
Well
identifier'
Source
126
127
128
129
130
131
CI6606CDB2
C 16606DA
C16607AB
C 16607ABB
CI6607CB
CI6607CCB 1
132 C]6607CCB_2
4946.8 03/15/1955
4947 11/10/2000
4956 09/30/1999
4964 04/20/1959
4957 08/17/2000
4966.3
4967.5
133 C16607CCB_3 4966.6
134 C16607DAC 4971.8
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
C16617CBC1
C16617CBD„1
C16617CBD2
01661 SAC
C16618BA
C16G18BAB1
C16618BAC 1
C16G18BAC_2
C16618DBC1
C16618D1)
C1 G701 AD
C 16701 CA
C16712AB
C16712AD17_1
C16712ADD 2
C1671213B
C16712CA
C16712CCD 1
C16712CCD_2
154 C16713AA
155 C16713AC
5004
5007.5
5005,4
4979
4970
4967
4978,4
4977.1
4986.8
5002
4944
4950
4950
4961
4957
4955
4957
4965
4962.6
4972
4978,5
04/05/1955, 04/17/1956,
05/07/1957, 04/08/1958,
04/01/1959, 04/05/1960
SC00106606CDB2
227512A
47474A
SC00106607AB131
21337A
SC00106607CCB
11/] 1/1954, 11/08/1955, SC00106607CCB
11/13/1956, 11/12/1957,
11/13/1958, 11/11/1959,
11/02/1960
03/03/1977 SC00106607CCB1 Hillier and others (1979)
11/04/1957 SC00106607DAC1 USGS NWIS
USGS 3958
03/08/1955 SC00106617CBC1 USGS NWIS
11/04/1957 SC00106617CBD 1 USGS NWIS
03/14/1974, 02/28/1976 SC00106617CBD USGS NWIS
10/07/1999 220074 CDWR
07/28/2000 13744R CDWR
03/05/1956 SC00106618BAB] LTSGSNWIS
11/04/1957 SC00106618BAC1 USGSNWIS
03/14/1974, 02/28/1976 SC00106618BAC USGS NWIS
09/29/1955 SC00106618DBC1 USGS NWIS
05/08/1997 24161A CDWR
04/24/1975 49687F CDWR
04/16/1960 SC00106701CA CDWR
09/24/1997 202569A CDWR
11/05/1957 SC00106712ADD USGS NWIS
03/11/1976 SC00106712ADL)2 USGS NWIS
03/23/1980 101310 CDWR
07/20/1998 M1998-036 TH-15 CDMRS
11/--/1957 -- Smith and others (1964)
03/17/1974, 03/11/1976 SC00106712CCD USGS NWIS
USGS 39582
08/09/2002 242942A CDWR
01/15/1997 M1987049 Well4 CDMRS
USGS NWIS
CDWR
CDWR
USGS NWIS
CDWR
USGS NWIS
USGS NWIS
54 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
Table 7. Hydraulic -head observations used to calibrate the steady-state model of the South Platte alluvial aquifer, Brighton to Fort
Lupton, Colorado. —Continued
[Hydraulic head in feet above National Geodetic Vertical Datum of 1929; Observation date(s) in mnt/dd/yyyy; --, no data; CDWR, Colorado Division of
Water Resources; USGS NWIS, US Geological Survey National Water Information System; CDMRS, Colorado Division of Mining, Reclamation, and
Safety]
Observation
number
Observation
name
156
157
158
159
C16713CR
C16713CDA1
C16713D13D_ l
C16713DBD 2
Hydraulic
head'
Observation
datels)
Well
identifier'
Source
4965
4980
4976.6
4979.5
05/15/1975
05/11/1956
10/04/1956
03/03/1977
78907A
SCOO1O6713CDA1
SC00106713D13D 1
SC00106713DBD 1
CDWR
LISGSNWIS
USGS NWIS
Hillier and others (1979)
'1-tydiaulic head is the average of observations on the dates indicated hydraulic -head values in bold were used to compute observations as the temporal
change in hydraulic head from the previous observation at the same location
'Identifier for USGS NWIS wells, Schneider and Hillier (1978), and Hillier and others (1979) is the local well number. Identifier for CDWR wells is the
well permit number Identifier for CDMRS wells is the pit permit number followed by local well number.
Table 8. Flow observations used to calibrate the steady-state model of the South Platte
alluvial aquifer, Brighton to Fort Lupton, Colorado.
[fd/d, cubic feet per day]
Observation Years included in
name observation
Months included in
observation
Sp1t_57dry 1954-60
SpIt_57wet 1954-60
Splt_77dry 1974-110
S plt_77wet 1974-80
Splt_00dry 1997-2003
Splt_00wet 1997-2003
November —April
May —October
November —April
May —October
November —April
May OO oho.
Average gain to
South Platte River.'
(W/d)
449,000
3,797,000
710,000
6,622,000
985,000
10,623,000
'Gain is average of mean monthly gains tar years and months indicated
ND AT/i 2
C°r[yl—y'r(1 )12+ , Wr) [P
t=1 p=1
ND is the number of observations,
NPR is the number of prior -information values,
NP is the number of estimated parameters.
y, is the ith observation being matched by the
regression,
y' ,(b) is the simulated value that corresponds to
the ith observation (a function of b),
!'n is the pth prior estimate included in the
regression,
P' (b) is thepth simulated value,
P
CO, is the weight for the ith observation, and
con is the weight for thepth prior
estmate.
Smaller values of standard error indicate better fit of simulated
values to observations. The value of standard error should
be close to 1,0 if weighting used on observations and prior
information represents true data accuracy. In practice, standard
error commonly is greater than 1.0 because of model error or
greater -than -expected measurement error. The standard error
of regression (table 9) for the calibrated model is 1.7,
The MAE (Anderson and Woesner, 1992) is another gen-
eral measure of model fit. The MAE is calculated as the mean
of the absolute value of residuals in the model. Therefore, the
MAE indicates average deviation of the simulated values from
the observed values, whether the deviation is positive or nega-
tive. The MAE of unweighted hydraulic -head residuals in the
calibrated model is 3.3 ft (table 9), which is about 2 percent
of the 145 -ft range of observed hydraulic -head values across
the model domain. The MAE of unweighted flow residuals
in the calibrated model is 2,550,000 ft3/d, which is about 25
percent of the 10,170,000 ft'/d range of observed base -flow
values. Although the MAE of unweighted residuals provides
a general measure of model fit, it does not consider observa-
tion measurement error. Similarly, the standard error of the
Simulation of Groundwater Flow 55
Table 9. Statistics used to assess calibration of the steady-state model of the
South Platte alluvial aquifer, Brighton to Fort Lupton, Colorado.
[Unweighted hydraulic -head residuals in feel; unweightecl low residuals in cubic feel per day; all
other values dimensionless]
Stl'slic Value
Minimum unweighted hydraulic -head residual
Maximum unweighted hydraulic -head residual
Mean absolute error of unweighted hydraulic -head residuals
Minimum unweighted Clow residual
Maximum unweighted flow residual
Mean absolute error of unweighted flow residuals
Standard error of regression, s
Minimum weighted residual
Maximum weighted residual
Mean weighted residual
Correlation coefficient, R2N
Critical value of RNN at the 5 percent significance level
Modified Beale's measure'
—12.1
10.7
—8.170.000
1.230.000
2.550.000
1.7
— 4,1
4.4
— 0.39
0.988
0.985
4.2
'Cool ey and Naff (1990)
model regression and the MAD of unweighted residuals do not
indicate the spatial distribution of error or the validity of the
model regression.
Randomness, Independence, and Normality of Residuals
A valid regression requires observation and prior -
information errors used in the regression to be random and
weighted errors to be uncorrelated (Draper and Smith, 1981).
In addition, observation errors need to be normally distributed
for use in calculating inferential statistics (Helsel and Hirsch,
1992) such as confidence intervals on parameters and predic-
tions. If the model reasonably represents the groundwater
system and the foregoing error conditions are met, weighted
residuals should either be random, independent, and normal or
have predictable correlations (Hill, 1998).
Weighted residuals relative to weighted simulated
values and unweighted residuals relative to unweighted
hydraulic -head observations. To evaluate the randomness
and independence of weighted residuals, weighted residuals
are plotted relative to weighted simulated values (figs. 25,4 and
25B). For a valid regression, weighted residuals are randomly
distributed above and below the zero line for all weighted
simulated values (Draper and Smith, 1981), A nonrandom
distribution or systematic trend in weighted residuals might
indicate weighted residuals are not random or independent
(Hill, 1994). Hydraulic -head residuals in figure 25.4 plot in
three distinct bands because three different weights were used
to calculate simulated equivalents for hydraulic head based
on estimated accuracy of hydraulic -head observations, and
the range of hydraulic -head observations is small relative to
the range of all observation data (hydraulic head and flow)
and prior information. The distribution of weighted residu-
als relative to weighted simulated hydraulic head for values
between ],620 and 1,670 is provided in figure 25B to show
the distribution of weighted residuals relative to weighted
simulated values over a smaller range of values. For compari-
son, the distribution of unweighted head residuals relative to
unweighted hydraulic -head observations is shown in figure 26.
The slight negative bias in residuals in figures 25B and 26
might indicate that simulated heads near the lower and tipper
limits of the head range in the model generally are higher than
observed head values. Because the hydraulic -head gradient of
the water table in the aquifer generally is from south to north
(fig. 9), the lower limit of the head range represents conditions
near the north end of the simulated aquifer, and the upper limit
of the head range represents conditions near the south end of
the simulated aquifer. Although figures 25B and 26 display a
slight negative bias, figure 25A indicates weighted residuals
likely are random and independent when all calibration data
are considered. A summary of the minimum, maximum, and
mean weighted and unweighted residuals for the final cali-
brated model are presented in table 9.
Normal probability of weighted residuals and cor-
relation coefficient R25. To evaluate normality of weighted
residuals and further test for their independence, a normal
probability graph of weighted residuals (fig, 27) is used.
Because weighted residuals fall approximately on a straight
line in the graph, weighted residuals can be considered
independent and normally distributed, The correlation coef-
ficient R2N provides another statistical measure of residual
independence and normality by determining the correlation
56 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Cola.
A
5
WEIGHTED RESIDUAL, DIMENSIONLESS
5
Ej
— 5
— 5,006 20,000
WEIGHTED SIMULATED VALUE, DIMENSIONLESS
O
O
O 0
O
P
{]] O 0
O Hydraulic head
00 Flow
A Prior information
B
45,000
O
0
0
0 0
0
o
0 0 O 0
0 0
—5
1,610
O° )o.. 000 c.7
°O GZ�; 0
0 0 � O 01J0
0
O
O 0 0
0
V 0
O 00
O 00 o O °
O
0C
O
0
T
8°
1,620 1,630 1,640 1,650 1,660 1,670 1,680
WEIGHTED SIMULATED HYDRAULIC HEAD; DIMENSIONLESS
UNWEIGHTED RESIDUAL, IN FEET
Figure 26. Relation of unweighted residuals
to unweighted hydraulic -head observations.
15 •
R
Figure 25. Relation of weighted
residuals to weighted simulated
values for (A) hydraulic head,
flow, and prior information and
(B) weighted hydraulic -head
values between 1,620 and 1,670.
I 1 Tl1 I
0
-15 J I I_I .1 h�
•11 TTTrT I I I I 'i '* , -Y r I' 1
0
0
r O
O
O • 0 O 0
8 O ° O
O
1_ L. 1. 1 l.L I I I
0
4.840 4.660 4,890 4,900 4,620 4,940 4,960
I I
8
4,980
O
UNWEIGHTED HYDRAULIC -HEAD OBSERVATION, IN FEET ABOVE
NATIONAL GEODETIC VERTICAL DATUM OF 1929
I I I
5,000 5,020
Simulation of Groundwater Flow 57
A
� I
cc
0
—I
a
J
O Hydraulic haul
2 Flow
t, Prio Onto rrnaiion
.4
n
�4r
ociF
o
OU
O
O
-2 -1 0 I 2
STANDARD NORMALLY DISTRIBUTED NUMBER
Figure 27. Normal probability of weighted residuals.
between ordered weighted residuals and order statistics from
a probability distribution function (Hill, 1998), If the value of
R2N is significantly less than 1.0, weighted residuals are not
likely to be independent and normally distributed, The value
of R0, is calculated by MODFLOW-2000 and presented along
with critical virl4l4•ti for l�` representing significrince levels of
0.05 and 0.10, l he value of R. for the final calibrated model
is 0.988 (table 9), which is greater than the 0.05 significance
level of 0,985 and indicates the probability that weighted
residuals are independent and normally distributed is greater
than 95 percent.
Correlation between weighted observations and
weighted simulated values. Correlation between weighted
observations and weighted simulated values is evaluated by
plotting weighted observations relative to weighted simulated
values (fig, 28A). If weighted simulated values are similar to
weighted observations, points should fall on a straight line
with an intercept of zero (Hill, 1998). Because the range of all
weighted values in figure 28A is much larger than for weighted
hydraulic head and flow alone, hydraulic head and flow values
appear as only a few points on the graph. To better show the
correlation between weighted observations and weighted
simulated values over a smaller range of values, the distri-
bution of weighted hydraulic head observations relative to
weighted simulated values is shown in figure 2813 for weighted
values between 1,620 and 1,670. Because the plots shown in
figures 28A and 2813 are approximately linear, weighted simu-
lated values (and consequently, unweighted simulated values)
00 000
49,000
w
z
�+c
30 000
2
F 20.000
cc
10.000
—10,000
WEIGHTED HYDRAULIC -HEAD OBSERVA110N,DIMENSIONLESS
o Hydrau{ic he ad
P Flow
Prior in forma ion
I I I
—10,000 0 10,000 20,(100 30,000 40.000
WEIGHTED SIMULATED VALUE, DIMENSIONLESS
IS
1,700
1,660
1.000
1.640
1.620
1509 1 I
1,600 L620 1,640 1,660 1,100
WEIGHTED SIMULATED HYDRAULIC HEAD, DIMENSIONLESS
50,200
1,700
Figure 28. Relation of weighted observations to
weighted simulated values for (A) hydraulic head, flow,
and prior information and (B) weighted hydraulic -head
values between 1,620 and 1,670.
can be considered to reasonably approximate weighted (and
unweighted) observations.
Runs statistic and parameter correlation coefficients,
The runs statistic (Draper and Smith, 1981, p. 157-162)
evaluates the spatial and temporal randomness of weighted
residuals. A run consists of an unbroken sequence of positive
or negative residuals, Too few or too many runs might indicate
significant model error that could affect simulated predictions
(Hill, 1998), MODFLOW-2000 uses the order of observa-
tions listed in the observation input file to determine the runs
statistic. The number of runs in the final calibrated model is
58 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Cola.
85 in 176 observations (including prior information), which
equals the number of runs expected for randomly distributed
weighted residuals. Because observations are distributed spa-
tially and temporally in the model, the runs statistic indicates
that residuals likely are random in both space and time.
Parameter correlation coefficients indicate whether esti-
mated parameter values are likely to be unique (Hill, 1998).
Parameter correlation coefficients for all estimated parameters
in the final model were less than 0.95, indicating that final
parameter values likely are unique.
Simulated Steady -State Groundwater Flow in
the Alluvial Aquifer
The simulated steady-state distributions of aquifer
hydraulic head representing water -table conditions during
the 2000 irrigation and non -irrigation seasons are shown in
figure 29, and the water budget for each simulation is provided
in table 10. The simulated water table during the irrigation
season is higher than during the non -irrigation season in most
of the model domain. The simulated water table during the
irrigation season is lower than during the non -irrigation season
only in a small area near Brighton where heavy municipal -
well pumping, which is greater in the irrigation season than
the non -irrigation season, occurs. The difference between
simulated seasonal groundwater levels generally is larger near
aquifer margins than near the South Platte River because of
the stabilizing influence of the river, and differences in simu-
lated seasonal groundwater levels are similar in magnitude to
those indicated by observations, Simulated saturated thickness
within the model area ranges from 10 to 50 ft, except near
model edges where saturated thickness is less than 10 ft in
some places. Simulated saturated thickness generally is 20-40
ft throughout most of the model domain.
Wetland locations mapped as part of this study are shown
in figure 30 relative to areas where simulated depth to water
is less than 5 ft below land surface during the 2000 irrigation
season. Areas of riparian herbaceous vegetation (including cat-
tails, sedges, rushes, and mesic grasses) with moist to water-
logged soils or permanent standing water mapped by CDOW
(2007a, b) also are presented in figure 30 for comparison to
areas where the simulated depth to water is less than 5 ft. Most
wetlands and riparian herbaceous vegetation are located in
areas where the simulated water table is less than 5 ft below
ground surface, indicating that most wetlands and riparian
herbaceous vegetation in the study area might be affected by
changes in groundwater levels, However, determination of the
specific effects of groundwater -level changes on wetlands and
riparian herbaceous vegetation in the study area would require
site -specific investigations beyond the scope of this report.
All components of the simulated groundwater budgets
for the calibrated steady-state model during the irrigation and
non -irrigation seasons represent reasonable values compared
to available data. General -head boundaries at the upgradient
ends of the simulated South Platte River valley and tributaries
provide the largest source of inflow to the model area during
both the irrigation season (28.2 percent) and non -irrigation
season (43.5 percent). During the irrigation season, inflow
along the east model boundary and recharge distributed at
the land surface account for 25.5 percent and 25 2 percent,
respectively, of inflow to the model area. During the non -
irrigation season, inflow along the east model boundary and
recharge distributed at the land surface are a smaller compo-
nent of the groundwater budget (18.4 percent and 1.0 percent,
respectively), consistent with the conceptual understanding
of groundwater flow, Groundwater discharge to the South
Platte River and Big Dry Creek constitutes the largest outflow
from the simulated aquifer during the irrigation season (54.6
percent) and non -irrigation season (55.9 percent) with most
outflow occurring to the South Platte River, Discharge to the
South Platte River and Big Dry Creek during the irrigation
season is greater than that during the non -irrigation season
because of increased recharge and steeper water -table gradi-
ents toward the South Platte River during the irrigation season.
During the non -irrigation season, outflow at the general -head
boundary at the downgradient end of the model exceeds net
discharge (outflow minus inflow) to the river and represents
24.9 percent of the total outflow. Phreatophyte evapotranspi-
ration and groundwater discharge to tributaries simulated as
drains (Little Dry Creek and Third Creek) during the irrigation
season represent small (3.1 percent and 1.8 percent, respec-
tively) components of the total aquifer outflow. Phreatophyte
evapotranspiration during the non -irrigation season is assumed
zero in the model input. Groundwater discharge to tributary
drains is slightly greater during the non -irrigation season
(3.7 percent) than during the irrigation season likely because
greater municipal -well withdrawals during the irrigation sea-
son reduce groundwater flow to Third Creek. Municipal -well
withdrawals during the irrigation season are about twice as
large as municipal -well withdrawals during the non -irrigation
season. Municipal -well withdrawals represent 21,2 percent
of total aquifer outflow during the irrigation season and 15,5
percent of outflow during the non -irrigation season.
Sensitivity Analysis
Composite scaled sensitivities were calculated for each
parameter using the Sensitivity Process (Hill and others,
2000) ofMODPLOW-2000, Composite scaled sensitivities
are dimensionless quantities that indicate the total amount of
information provided by all observations for estimation of a
parameter (Hill, 1998). When parameter correlation is not a
problem, parameters with high sensitivity generally can be
more precisely estimated front available observations than
parameters with low sensitivity. Parameters with high sen-
sitivity also have greater influence on quantities (head and
flow) simulated by the model and can be more important to
accurately define for model simulations than parameters with
low sensitivity. Parameters with very low sensitivity have little
effect on simulated values and accurate definition of these
Simulation of Groundwater Flow 59
104°501/V
40°06'N
40°04A
40°02'N
40°N
39°56'N
104°45"ii
Streams modified from U S Geological Survey National
Hydiagraphy Dataset; 1:100,000
Roads modified from Colorado Depnrhnertof Tinnsportalion
North Antoricuui Datum of 1983
EXPLANATION
Line of equal simulated hydraulic head during
the irrigation season
[Joe of equal simulated hydraulic head during
the non -irrigation season
Contours show altitude of water table Contour interval 20 feet
National Geodetic Vertical Datum of 1929
Limit of simulated aquifer
0 5,000 10,000 FEET
I r_I L i
l i
o 1,500 3,000 METERS
Figure 29. Simulated steady-state distributions of hydraulic head in the South Platte alluvial aquifer during the
irrigation and non -irrigation seasons in 2000, Brighton to Fort Lupton, Colorado.
Table 10. Groundwater budgets for the calibrated model and simulations of the hydrologic effects of predicted land -use change and reclaimed pits in 2020 and 2040,
Brighton to Fort Lupton, Colorado.
[All values are in cubic feet per day; totals reflect sum of all rounded components, --, not applicable]
Budget
component
Calibrated
model
2000
irrigation
season
Calibrated
model
2000 non -irri-
gation season
Simulation
11
Simulation
22
Simulation Simulation Simulation
3' 44 55
Groundwater inflow from general -head boundaries
at upgradient end of South Platte River valley
and tributaries
Subsurface irrigation return flow along east model
boundary
Distributed recharge at the land surface
Leakage to aquifer from South Platte River and Big Dry Creek
Leakage to aquifer from unlined pits
Groundwater released from storage
1969.000
1,782,000
1.763,000
L469.000
Aquifer inflows
2,103,000 2.022.000
891,000 1,782,000
50,000 1,448.000
1,789,000 1,526,000
2,051,000 2,057,000 2,042,000
1,782,000 1,782,000 1,782, 040
1,206.000
1,574.000
1,467,000
1,748,000
3,719,000
107,000
1,253,000
1,752,000
2.868.000
108.000
2,057,000
1,782,000
1,253,000
2,016,000
11,317,000
110,000
Total
6,983,000 4,833,000 6,778,000 6,613,000 10,880,000 9,805,000 18,535,000
Aquifer outflows
Groundwater outflow to general -head boundary at
downgradient end of South Platte River valley
Groundwater discharge to South Platte River and
Big Dry Creek
Groundwater discharge to Little Dry Creek and
Third Creek
Groundwater discharge to unlined pits
Phreatophyte evapotranspiration
Municipal -well withdrawals
Groundwater entering storage
1,345,000 1,205,000 1,323,000 1,301,000 1,15 7,000 1,152,000 1,168.000
3,810,000 1702,000 3,632,000 3,495,000 3,644.000 3,388.000 3,787.000
127,000 178,000 12 2, 000 116.000 144,000 155,000 141,000
219,000
1,483,000
0 218,000 217.000
748,000 1.483.000 1,483.000
3,757,000
191,000
1,421,000
570,000
1955.000
191,000
1,421,000
545,000
1 1,52 7,000
186,000
1,421,000
425,000
Total 6,984,000 4,833,000 6,778.000 6,612,000 10,884,000 9,807,000 18,655.000
Percent discrepancy (Recharge —Discharge)
'Land -use conditions in 2020
2Land-use conditions in 2040
'Reclaimed pits in 2020
°Reclaimed lined pits in 2040
'Reclaimed unlined pits in 2040
—0.01
0.00 0.00
0.02 —0.04
—0.02 —0.65
0
Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Culo
Simulation of Groundwater Flow 61
104°50' W
tD 06'N
49°04'N
40°02'N
40°N
39°50'N
104°4a'W
Stieams modified from US Geological Survey National
Nydi ography Damsel; 1 c 100,000
Roads modified from Colorado Department of Transportation
Noith Ainci ican Daum of 1983
EXPLANATION
Wetland mapped as part of this study
Rip:nnil ILRllcceu is vegetation Indicated
by.1'nlor.+JiuDivision of Wildliui 120070. b)
Simulated depth to water less than 5 feet
below land surface
Limit of simulated aquifer
o 5,000 10,000 FEET
_1i 1I I I i1i I
0 1,000 2,000 0,000 METERS
Figure 30. Location of largest wetlands mapped as part of this study and riparian herbaceous
flora indicated by Colorado Division of Wildlife relative to areas where simulated depth to water
is less than 5 feet below land surface in the South Platte River valley, Brighton to Fort Lupton,
Colorado.
62 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley. Colo.
parameters is less important to model calibration, However,
parameters with low sensitivity could be important to predic-
tions if predictions are distant from observation locations or
occur under conditions substantially different than those used
to calibrate the model. Because composite scaled sensitivities
depend on model structure and the number and location of
observations, the absolute magnitude of composite scaled sen-
sitivity for a parameter is less meaningful than its magnitude
relative to that of other parameters. Composite scaled sensitiv-
ity of each parameter in the calibrated steady-state model is
presented in figure 31. The parameter representing recharge
beneath irrigated areas (RCH_Irr) has the highest composite
scaled sensitivity in the model, Other parameters with rela-
tively high composite scaled sensitivities represent (1) inflow
along the east side of the model (Q_ReturnE), (2) hydraulic
conductivity of zones 2 (LPF_Par2) and 3 (LPF Par3), and
(3) municipal well pumping (Q_Mwells). Parameters with
relatively low composite sensitivity represent (1) phreatophyte
evapotranspiration (EVT_Parl), (2) drain conductance of
Little Dry Creek and Third Creek (DRN_Trib), (3) recharge
beneath non -irrigated areas (RCH_Nonirr), (4) hydraulic
conductance of general -head boundaries at upgradient and
dowugradient ends of the South Platte River valley (GHB
Splt), and (5) recharge beneath urban areas (RCH_Urban).
Other model parameters (LPF Par!, LPF_Par4, GHB_Trib,
REV Spit, and RIV_Bdry) have moderate sensitivity.
Model Nonlinearity
The post -processing program BEALE-2000 (Hill and
others, 2000) provided with MODPLOW-2000 was used to
evaluate model nonlinearity, BEALE-2000 uses the modified
Beale's measure (Cooley and Neff, 1990) to test model nonlin-
earity near the optimized parameter values. The model needs
to be approximately linear near optimized parameter values
for linear confidence intervals on parameters and predictions
to be valid (Hill, 1994). Interpretation of the modified Beale's
measure is different for each model and is provided as output
from BEALE-2000, For the current study, the model is consid-
ered effectively linear near optimized parameter values if the
Beale's measure is less than 0.051. The model is considered
nonlinear if the Beale's measure is between 0.051 and 0.56,
and highly nonlinear if the Beale's measure is greater than
0.56. Because the Beale's measure for the calibrated model is
4.2 (table 9), the model is considered highly nonlinear, and lin-
ear confidence intervals on predictions of the effects of land -
use change and aggregate mining likely would not accurately
represent prediction uncertainty. The model is most nonlinear
with respect to (1) non -irrigation recharge (RCH_nonirr), (2)
stream bed conductance of the South Platte River (R[V_Splt),
and (3) irrigation recharge (RCH_irr). Because the model is
highly nonlinear, linear confidence intervals are not presented
for parameters or predictions concerning the effects of land -
use change and aggregate mining on groundwater flow,
Simulated Effects of Land -Use Change
and Aggregate Mining on Groundwater
Flow
Simulated Hydrologic Effects of Land -Use
Change
Land -use conditions in 2020 and 2040 (figs, I8A1 and
I8B) predicted by the SLEUTH urban -growth model are used
to simulate the hydrologic effects of converting non -irrigated
and irrigated land to impervious urban area, Two numerical
simulations of the potential hydrologic effects of land -use
change are presented as follows:
Simulation 1 —The hydrologic effects of predicted land -
use conditions in 2020.
Simulation 2 --The hydrologic effects of predicted land
use conditions in 2040.
All hydrologic stresses in the simulations, except the dis-
tribution of recharge based on land use, are the same as in the
calibrated model for 2000 in order to determine the individual
effect land -use change might have on the aquifer. Hydraulic
head simulated by the calibrated model for the 2000 irrigation
season is used to represent initial hydraulic -head conditions
for predictions of the effects of land -use change. Because
conversion of non -irrigated and irrigated land to urban areas
are likely to be relatively permanent, the long-term hydrologic
effects of land -use change are of interest and are predicted
using steady-state simulations.
Simulation 1 —Land -Use Conditions in 2020
Simulation 1 represents the potential hydrologic effects
of converting irrigated and non -irrigated land to impervious
urban area using land -use conditions predicted for 2020. The
simulated hydrologic effects of land -use conditions in 2020
relative to water -table conditions during the 2000 irrigation
season are shown in figure 32, and the simulated groundwater
budget for simulation l is provided in table 10, Groundwater -
level declines relative to the 2000 irrigation -season water
table are as much as 1.2 ft in areas converted to urban land
use. Declines are greatest where irrigated land is converted
to urban areas because of the large difference (about 12 in.
during the irrigation season) in recharge between the two land
uses, Because recharge beneath non -irrigated areas (about
0,4 iii,) is only slightly greater than the zero recharge value
assigned to impervious urban areas and because the aquifer
has relatively high hydraulic conductivity, the conversion of
non -irrigated areas to urban areas in the model has little effect
on the simulated aquifer. Groundwater -level declines relative
to the 2000 non -irrigation season water table (which does not
receive recharge from irrigation) are less than 0.04 ft at all
locations in 2020 and are not presented graphically. Because
simulated groundwater -level declines are small and wetlands
Simulation of Groundwater Flow 63
COMPOSITE SCALED SENSITIVITY
1.0
0.9
0.8
0,7
9.6
0.5
0.4
0,3
0.2
0,1
0
0.855
0.946
— 0016
—
0.751
0984
0 I
0.729
D163
—
0134
0L 6
0016
t—,
0,00Q D012
=
0035
n
�0.-0-3-91
I l
- d
J
LPF_Parl
LPF_Par2
LPF_Par3
LPF_Par4
RCH_Nonirr
RCH _Irr
RCH _Urban
GHB_Splt
GHB_trib
RIV_SpIt
RIV_Bdry
DRNtrib
0._ReturnE
Q .Mwells
EVT,Parl
0
TI
cc
x
cc
uJ
s
EXPLANATION
Horizontal hydraulic conductivity of zone 1
Horizontal hydraulic conductivity of zone 2
Horizontal hydraulic conductivity of zone 3
Horizontal hydraulic conductivity of zone 4
Recharge rate beneath non -irrigated land
Recharge rate beneath irrigated land
Recharge rate beneath urban areas
General -head boundary conductance at upgradient and
downgradient ends of South Platte River valley
General -head boundary conductance at upgradient
ends of tributary valleys
Riverbed conductance of the South Platte River
Riverbed conductance of Big Dry Creek
Drain conductance of Third Creek and Little Dry Creek
Inflow rate along east model boundary
Municipal -well withdrawal rate
Maximum phreatophyte evapotranspiration rate
Figure 31. Composite scaled sensitivities of parameters for the calibrated steady-state model of the
South Platte alluvial aquifer, Brighton to Fort Lupton, Colorado.
64 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
104°50'W
40°00'N
40°04'N
40°02'N
40°N
29°50'N
104°48'W
Streams modified from U S. Geological Survey National
liydio .iaphy Datasel; 1:100,000
Rands modified from Cala'ado Department ofTransportation
North Aram icon Detain of 1903
EXPLANATION
Urban
❑ Irrigated agriculture
Lill Non -irrigated
Wetland mapped as part of this study
n Riparian herbaceous vegetation indicated
by Colorado Division of Wildlife (2007a, h)
— Line of aqua! drawdown--Contain interval 0 5 feet
— Limit of simulated aquifer
0
0
5,000 10,000 FEET
.1 I I I .I �r
1,000 2,000 3,000 METERS
Figure 32. Simulation 1 Steady-state drawdown resulting from predicted land -use conditions in
2020, Brighton to Fort Lupton, Colorado.
Simulated Effects of Land -Use Change and Aggregate Mining on Groundwater How 65
mapped by this study and areas of riparian herbaceous vegeta-
tion mapped by CDOW (2007a, b) generally are located where
little or no decline occurs (fig. 32), wetlands and riparian
herbaceous vegetation in the study area likely would not be
substantially affected by declines resulting from the predicted
land -use conditions in 2020.
Distributed recharge at the land surface in simulation 1
(table 10) is 17.9 percent less than in the 2000 irrigation sea-
son because of the larger urban extent in simulation 1, which
is assumed not to contribute recharge to the aquifer. Ground-
water inflow at the upgradient ends of South Platte River
valley and its tributaries is 2.7 percent greater than in the 2000
irrigation season, and leakage from the South Platte River and
Big Dry Creek is 3.9 percent greater. Similarly, discharge to
the South Platte River and Big Dry Creek is 4.7 percent less,
discharge to Little Dry Creek and Third Creek is 3.9 percent
less, and outflow at the downgradient end of the South Platte
valley is 1,6 percent less, Phreatophyte evapotranspiration and
irrigation return flow along the east model boundary are not
substantially affected by the lower water table beneath urban
areas in simulation 1.
Simulation 2 —Land -Use Conditions in 2040
Simulation 2 represents the potential hydrologic effects
of converting irrigated and non -irrigated land to impervious
urban area using land -use conditions predicted for 2040. The
hydrologic effects of land -use conditions in 2040 relative to
water -table conditions during the 2000 irrigation season are
shown in figure 33, and the simulated groundwater budget
for simulation 2 is provided in table 10. Groundwater -level
declines relative to the 2000 irrigation -season water table
are as much as 1.9 ft in areas converted to urban land use.
Declines in 2040 are greater than in 2020 because a larger part
of the model has been converted to impervious urban area. As
with declines in 2020, declines in 2040 are greatest where irri-
gated land is converted to urban area and near the east model
boundary. Groundwater -level declines relative to the 2000
non -irrigation season water table are less than 0.05 ft at all
locations and are not presented graphically. As with declines
resulting from land -use conditions in 2020, simulated declines
resulting from land -use conditions in 2040 appear to have little
effect on mapped wetlands and areas of riparian herbaceous
vegetation because they are mostly located where little or no
decline occurs (fig. 33).
The effects of land -use change on the groundwater
budget (table 10) in simulation 2 are similar but greater than
the effects indicated by simulation 1 because a larger urban
area is simulated, which reduces recharge to a greater extent.
Distributed recharge at the land surface in simulation 2 is
31.6 percent less than in the 2000 irrigation season.
Groundwater inflow at the upgradient ends of South Platte
River valley and its tributaries is 4.2 percent greater than in
the 2000 irrigation season, and leakage from the South Platte
River and Big Dry Creek is 7.! percent greater. Similarly,
discharge to the South Platte River and Big Dry Creek is
8.3 percent less, discharge to Little Dry Creek and Third Creek
is 8.7 percent less, and outflow at the downgradient end of
the South Platte valley is 3.3 percent less in 2040 relative to
the 2000 irrigation season. As in simulation 1, phreatophyte
evapotranspiration and irrigation return flow along the east
model boundary are not substantially affected by the lower
water table beneath urban areas in simulation 2,
Summary of Land -Use Change Simulations
Steady-state simulations of the hydrologic effects of
land -use conditions in 2020 and 2040 indicate groundwater -
level declines resulting from conversion of irrigated and
non -irrigated land to urban areas are less than 2 ft relative to
the irrigation -season water table in 2000. Groundwater -level
declines are largest where irrigated agricultural land is con-
verted to urban area because of the large difference in recharge
between the two land uses. Groundwater levels change little
where non -irrigated land is converted to urban area because
estimated recharge beneath non -irrigated land is only slightly
greater than that assumed for urban areas, Groundwater -level
declines resulting from land -use conditions in 2020 and 2040
are predicted to not substantially affect wetlands and riparian
herbaceous vegetation in the study area because the declines
are small and mapped wetlands and areas of riparian her-
baceous vegetation generally are located where little or no
simulated decline occurs. The larger urban extent in simula-
tions of land -use change in 2020 and 2040 decreases recharge
to the simulated aquifer by 17.9-31.6 percent, and the result-
ing lower water table increases groundwater inflow from the
upgradient ends of South Platte River valley and its tributaries
by 2.7-4.2 percent and leakage from the South Platte River
and Big Dry Creek by 3.9-7.1 percent. Similarly, discharge
to the South Platte River and Big Dry Creek decreases by
4.7-8.3 percent, discharge to Little Dry Creek and Third Creek
decreases by 3.9-8.7 percent, and groundwater outflow at
the downgradient end of the South Platte valley decreases by
1,6-3.3 percent.
Simulated Hydrologic Effects of Aggregate
Mining
Simulated Cumulative Hydrologic Effects of
Reclaimed Aggregate Pits
Cumulative effects of reclaimed aggregate pits on
groundwater flow in the study area are simulated for the poten-
tial extent of mining shown in figures 19.4 and 198 for 2020
and 2040, respectively. Three numerical simulations of the
66 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo,
104°5tYW
104°48W
Streams modified from U,S Geological Survey National
Hyde ogi aptly Dc tuset; I ;100,000
Roads modified from Colorado Department of Transportation
Nrn Ih American Datum of 1983
EXPLANATION
2 Urban
I Irrigated agriculture
r Non -irrigated
® Wetland mapped as part of this study
Li
Riparian herbaceous vegetation indicated
by Colorado Division of Wildlife (2007a, b)
— Line of equal drawdown—Contour interval 0 5 feet
- Limit of simulated aquifer
0 5,000 10,000 FEET
1 I
0 1,000 2,000 3,000 METERS
Figure 33. Simulation 2 —Steady-state drawdown resulting from predicted land -use
conditions in 2040, Brighton to Fort Lupton, Colorado.
Simulated Effects of Land -Use Change and Aggregate Mining on Groundwater Flow 67
potential cumulative hydrologic effects of reclaimed pits are
presented as follows:
Simulation 3 — The cumulative hydrologic effects of
multiple reclaimed pits representing the
potential extent of aggregate mining in
2020.
Simulation 4 — The cumulative hydrologic effects of
multiple reclaimed pits representing the
potential extent of aggregate mining in
2040. All pits added after 2020 are
simulated as lined.
Simulation 5 — The cumulative hydrologic effects of
multiple reclaimed pits representing the
potential extent of aggregate mining in
2040. All pits added after 2020 are
simulated as unlined.
Simulation Design
All reclaimed pits are simulated either as (1) lined,
(2) unlined, or (3) backfilled with fine-grained sediments
based on review of reclamation plans on file with CDMRS.
Pits indicated as backfilled with unspecified overburden in
CDMRS records are assumed to have hydraulic conductiv-
ity similar to the surrounding aquifer and are not explicitly
simulated. Pits for which reclamation information was not
found are assumed backfilled with unspecified overburden and
also are not explicitly simulated. Lined pits are simulated by
using inactive model cells at pit locations, thereby simulating
no -flow barriers at pit edges where slurry walls or clay liners
would be present, Unlined pits are simulated using the Lake
package (Merritt and Konikow, 2000) of MODFLOW-2000.
The Lake package simulates water exchange between a
surface -water body and the aquifer based on lakebed leakance
(hydraulic conductivity divided by lakebed thickness) and the
relative difference between lake stage and adjacent ground-
water levels. Unlined pits in the model have lakebed leakance
defined using the value of aquifer hydraulic conductivity at
the pit location and a lakebed thickness of 1 ft to simulate the
open hydraulic connection between the pit and the aquifer.
Simulation of lake stage in unlined pits considers the effects
of seasonal direct precipitation and evaporation at the lake sur-
face based on long-term climate records (see "Physiography
and Climate" section), Simulated precipitation is about 9 in,
during the irrigation season and 4 in. during the non -irrigation
season. Simulated evaporation is about 41 in. during the irriga-
tion season and 7 in. during the non -irrigation season. Pits
backfilled with fine-grained sediments are simulated by using
model cells with a hydraulic conductivity value of 10 ft/d
at backfilled pit locations. In some locations, reclaimed pits
overlap Third Creek and phreatophyte areas. Drain cells used
to simulate Third Creek and evapotranspiration cells used to
simulate phreatophyte evapotranspiration at areas overlapped
by pits are deactivated.
Because reclaimed pits commonly are spaced less than
500 ft apart, the cell size (500 ft by 500 ft) of the calibrated
model is too large to simulate groundwater flow between pits
in some places. To enable simulation of groundwater flow
between closely spaced pits and more accurately simulate the
effects of reclaimed pits on groundwater flow and wetlands, a
revised model grid with 289 rows, 96 columns, and a uniform
cell size of 200 ft by 200 ft is used for simulations 3-5. Initial
hydraulic head for simulations of reclaimed pits was deter-
mined by rerunning the steady-state calibrated model using
the revised model grid to avoid attributing groundwater -level
changes caused by the finer grid to the effects of pits. Hydrau-
lic head simulated using the revised model grid generally
differed from that simulated by the original grid by less than
1 ft throughout the model domain for conditions represent-
ing the 2000 irrigation season. Hydraulic -head differences
between simulations using the two model grids generally are
less than 0.5 ft in the vicinity of simulated pits for conditions
representing the 2000 irrigation season. All hydrologic stresses
(including recharge distributed by land use) in simulations of
reclaimed pits are the same as in the calibrated model except
for the addition of pits.
Considerations for Transient Simulations
Because of uncertainties concerning the order and extent
to which individual pits will be developed and because the
cumulative effects of multiple reclaimed pits are of inter-
est, the hydrologic effects of reclaimed pits are simulated as
though pits are added simultaneously to the aquifer in either
2020 or 2040. Because wetlands may be able to adapt to
slowly changing water -table conditions, transient simulations
are used to indicate how quickly the aquifer responds to the
addition of reclaimed pits. The transient aquifer response is
simulated for 15 years after the pits are added by using 30
seasonal stress periods that represent hydrologic conditions
during the irrigation and non -irrigation seasons. The aquifer
response to pit development in 2020 therefore is simulated for
the period 2020-2035, and the aquifer response to pit devel-
opment in 2040 is simulated for the period 2040-2055. The
irrigation season is simulated as 183 days, and the non -irri-
gation season is simulated as l82 days. The first stress period
(non -irrigation season) is divided into six time steps of about
30 days each, The second stress period (irrigation season) is
divided into two time steps of 91.5 days each. All subsequent
stress periods have a single time step equal to the stress -period
length.
Groundwater -level declines and rises resulting from the
addition of reclaimed pits are calculated relative to simulated
water -table conditions for the 2000 irrigation season for all
simulations, Groundwater -level declines and rises are calcu-
lated relative to the water table during the irrigation season
because groundwater levels during the irrigation season gener-
ally are higher and are therefore more likely to support wet-
lands at the land surface. Groundwater -level declines relative
to the 2000 non -irrigation season would be less than that indi-
cated for the irrigation season because groundwater levels gen-
erally are lower during the non -irrigation season. Conversely,
68 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
groundwater -level rises resulting from pits would be greater
relative to the 2000 non -irrigation season. For the purposes of
this report, areas where simulated groundwater levels change
more than 2 ft indicate locations where groundwater -supported
wetlands might be vulnerable to the cumulative hydrologic
effects of reclaimed pits. Groundwater -level declines and rises
resulting from a single pit would be different than that shown
for the multiple pits simulated.
Simulation 3 —Reclaimed Pits in 2020
Simulation 3 indicates the potential cumulative hydro-
logic effects of pit development in 2020. Simulated ground-
water -level changes in 2035 resulting from the potential extent
of pit development in 2020 are shown on figure 34 relative to
locations of wetlands and riparian herbaceous vegetation, and
the simulated groundwater budget for simulation 3 is provided
in table 10. Positive values of groundwater -level change in
figure 34 indicate areas of groundwater -level rise, whereas
negative values indicate areas of groundwater -level decline.
Groundwater -level changes represent potential hydrologic
conditions 15 years after pits are reclaimed.
The results of simulation 3 show that groundwater
levels rise on the upgradient side of lined pits (fig. 34) and
decline on the downgradient side of lined pits. Groundwa-
ter levels rise upgradient from lined pits because the low -
permeability pit lining creates a barrier to groundwater flow,
which causes groundwater to mound against the upgradient
pit wall, Groundwater levels decline downgradient from lined
pits because groundwater is diverted around the pit, creat-
ing a hydrologic shadow behind the pit. Unlined pits have an
opposite but generally lesser effect on groundwater levels in
simulation 3, Unlined pits cause groundwater levels to decline
on the upgradient side of pits (fig. 34) and rise on the down -
gradient side because the lake inside the pit creates an area
where the water level is flat within the sloping water table.
Because the water level in the unlined pit reaches equilibrium
somewhere between the groundwater -table altitude at the
upgradient and downgradient ends of the pit, the water table
at the upgradient end slopes toward the pit, whereas the water
table at the downgradient end slopes away from the pit. Pits
hackfilled with fine-grained material are simulated as having
lower hydraulic conductivity than the surrounding aquifer and
result in hydrologic effects similar to those for lined pits, but
the effects are less because the backfilled pits are less of a bar-
rier to groundwater flow than lined pits. The hydrologic effects
of lined, unlined, and fines-backfilled pits interact to increase
the magnitude of groundwater -level changes at some locations
and decrease it at others, depending on the relative position of
the pits.
The area of greatest groundwater -level decline that occurs
near Big Dry Creek (fig. 34) results from the combined effects
of the lined water -storage facility and unlined pits downgradi-
ent from the facility. Maximum simulated groundwater -level
decline at this location is about 9 ft. The areas of greatest
groundwater -level rise (fig. 34) occur on the upgradient side of
lined pits on the west side of the South Platte River, where the
maximum simulated rise is about 6 ft, and near location B on
the east side of the river, where the maximum simulated rise
is about 5 ft. Groundwater -level changes near the South Platte
River generally are less than 2 ft because the river lessens the
hydrologic effects of pits by contributing or receiving water as
groundwater levels change.
Wetlands mapped in the study area generally are located
where simulated groundwater levels change less than 2 ft
(fig. 34) and might not be substantially affected by groundwa-
ter -level declines or rises resulting from the potential extent
of reclaimed pits in 2020. However, substantial areas of
riparian herbaceous vegetation (primarily to the south of Big
Dry Creek) mapped by CDOW (2007a, b) are located where
groundwater levels change more than 2 ft (fig. 34), indicat-
ing that riparian herbaceous vegetation in these areas might
be affected by groundwater -level declines or rises resulting
from the potential extent of reclaimed pits in 2020. Because
most areas where groundwater levels are simulated to rise
more than 2 ft on the east side of the South Platte River occur
where the simulated premining depth to water is more than 5
ft (fig. 30), higher groundwater levels resulting from reclaimed
pits ill 2020 are not likely to create conditions favorable to
the formation of new wetlands on the east side of the river,
However, most areas where groundwater levels were simu-
lated to rise more than 2 ft on the west side of the South Platte
River (except near Big Dry Creek upgradient from the lined
water -storage facility) occur where the simulated premining
depth to water is less than 5 ft, and groundwater -level rises ill
these areas could create conditions favorable to the formation
of new wetlands.
The transient response of groundwater -level declines and
rises in the simulated aquifer varies by location depending
on the proximity of reclaimed pits, aquifer properties,
hydrologic boundaries, and seasonal water -table fluctuations.
Because the aquifer is highly transmissive, groundwater
levels change most during the first year, and groundwater
levels generally change by successively smaller amounts in
subsequent years. Examples of transient groundwater -level
decline and rise are provided in figures 35A and 35B for two
locations in the simulated aquifer. The groundwater -level
decline at location A (fig. 34) occurs smoothly over time
(fig. 35.4) because seasonal water -table fluctuations at this
location are small (about 1.5 ft), and groundwater levels are
controlled primarily by the combined hydrologic effects of the
surrounding pits. The groundwater -level decline at location
A occurs rapidly during the first half of the year because
of declining groundwater levels throughout the simulated
aquifer during the non -irrigation season in the first stress
period. Groundwater levels continue to decline during the
second half of the first year, but the rate of decline decreases
because of increased recharge during the irrigation season.
By contrast, the groundwater -level rise at location B (fig. 34)
oscillates about 2.5 ft between the irrigation and non -irrigation
seasons (fig. 35B) because seasonal fluctuations of the water
table at this location are relatively large (about 5 ft). The
Simulated Effects of Land -Use Change and Aggregate Mining on Groundwater Flow 69
i04°501/11
40°O6'N
40°04'N
40°.12"N
40°N
39°58`N
104°49'W
Streams modified from U S. Geological Survey National
Etych agraplry Dalaset; 1: 100,000
Roads modified from Coloi ado Department of Transportation
P crIh American Datum of 1983
EXPLANATION
LLined pit
Unlined pit
Fines-backfilled pit
Wetland mapped as part of this study
1,
Riparian herbaceous vegetation indicated by
Colorado Division of Wildlife (2007a, b)
Simulated groundwater -level change, in feet
Positive values indicate groundwater -level rise
Negative values indicate groundwater -level decline
4to6
1 2to4
1 —2 10 —2
—4 to —2
ti to •I
—4 En
—I (I
Limit of simulafed aquifer
A Location of hydro -graph presented in
figures 35A and 3513
0 5,900 10,000 FEET
11 I I I I J
I I I I I
0 1,000 2,000 3,000 METERS
Figure 34. Simulation 3 —Groundwater -level changes in 2035 resulting from the potential extent of
reclaimed aggregate pits in 2020, Brighton to Fort Lupton, Colorado.
70 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
0
GROUNDWATER -LEVEL DECLINE, IN FEET
GROUNDWATER -LEVEL RISE, IN FEET
1
2
3
4
5
6
7
8
4
2
4 6 8 10 12 14 16
TIME, IN YEARS
-
12
14
6 8 10
TIME, IN YEARS
16
Figure 35. A. Simulated transient groundwater -level decline at location resulting from the
potential extent of reclaimed pits in 2020, Brighton to Fort Lupton, Colorado. B. Simulated transient
groundwater -level rise at location resulting from the potential extent of reclaimed pits in 2020,
Brighton to Fort Lupton, Colorado. The time period represents 15 years after pit reclamation. Location
of groundwater -level decline is shown in figure 34.
Simulated Effects of Land -Use Change and Aggregate Mining on Groundwater Flow 71
groundwater -level rise during the first half year at location
B is reduced by declining groundwater levels throughout
the simulated aquifer during the non -irrigation season, The
groundwater level then rises substantially throughout the
second half of the first year because of increased recharge
during the subsequent irrigation season. Groundwater levels
during subsequent years continue to oscillate with the seasons
but remain relatively stable within each season from year to
year after the initial rise in groundwater levels. Groundwater -
level rise on the upgradient side of lined pits generally is
largest within 1-3 years (fig. 35/3) after reclaiming the pits and
then decreases slightly as groundwater flows around the pit
and reaches new equilibrium conditions.
Simulated seasonal groundwater -level changes result-
ing from reclaimed pits cease to change substantially in most
areas of the aquifer within about 10 years (figs. 35A and 35B).
Groundwater -level declines and rises (15 years after pits are
reclaimed) shown in figure 34, therefore, generally represent
the approximate maximum predicted magnitude and extent of
groundwater -level changes resulting from reclaimed pits over
time. Because reclaimed pits can result in long-term changes
in groundwater levels, groundwater -supported riparian her-
baceous vegetation located where groundwater -level decline
exceeds 2 ft might be permanently altered by lower groundwa-
ter levels, Because a large proportion of groundwater -level rise
resulting from reclaimed pits occurs rapidly within the first
year, groundwater -supported riparian herbaceous vegetation
located in areas affected by higher groundwater levels might
be susceptible to flooding.
Total aquifer inflow and outflow in simulation 3
(table 10) is larger than in the 2000 irrigation season and
in simulations l and 2 because the groundwater budget is
accounting for inflow and outflow at unlined pits in simula-
tion 3, and the simulation is transient, which considers aquifer
storage in computing the groundwater budget. Although
groundwater inflow and outflow at unlined pits are the larg-
est components of the groundwater budget, net groundwater
discharge to unlined pits (groundwater discharge to pits minus
pit leakage) represents only about 0.3 percent of the total
groundwater budget of simulation 3, Therefore, the larger
groundwater budget of simulation 3 does not represent a large
increase in available water in the study area, Other than leak-
age to the aquifer from unlined pits, distributed recharge at the
land surface and leakage from the South Platte River and Big
Dry Creek represent the greatest changes to aquifer inflow in
simulation 3 relative to the 2000 irrigation season. Distributed
recharge at the land surface decreased by 16.8 percent relative
to the 20O0 irrigation season because substantial land areas
contributing recharge to the model are converted to pits, which
do not contribute to distributed recharge. Leakage from the
South Platte River and Big Dry Creek increased by 19,0 per-
cent relative to the 2000 irrigation season because of ground-
water -level declines near streams in some areas. Groundwater
declines also caused groundwater inflow at the upgradient
end of the South Platte River valley to increase slightly (4.5
percent) in simulation 3 relative to the 2000 irrigation season,
Other than groundwater discharge to unlined pits, groundwater
outflow at the downgradient end of the South Platte River val-
ley and discharge to the South Platte River and Big Dry Creek
represent the greatest changes to aquifer outflow in simulation
3. Groundwater outflow at the downgradient end of the South
Platte River valley decreased by 14,0 percent relative to the
2000 irrigation season likely because the large lined pit simu-
lated at the north (downgradient) end of the aquifer obstructs
groundwater outflow from the simulated aquifer. Although dis-
charge to the South Platte River and Big Dry Creek decreased
by only 4.4 percent relative to the 2000 irrigation season, the
decrease (166,000 ft"/d) represents the second largest absolute
change to the outflow water budget. The decreased discharge
likely is caused primarily by lined pits obstructing groundwa-
ter flow to the streams. The other components of the outflow
water budget in simulation 3 also change relative to the 2000
irrigation season. Groundwater discharge to Little Dry Creek
and Third Creek increased by 13.4 percent, phreatophyte
evapotranspiration decreased by 12.8 percent, and municipal -
well withdrawals decreased by 4.2 percent. The increase in
groundwater discharge to Little Dry Creek and Third Creek in
simulation 3 likely is the result of higher groundwater levels
near Third Creek, whereas the decrease in phreatophyte evapo-
transpiration likely is the result of lower groundwater levels in
areas of phreatophyte vegetation. The decrease in municipal -
well withdrawals is the result of one well (located near the
center of the simulated aquifer) (fig. 21) being removed from
the simulation because a lined pit was added at that location.
Simulation 4 —Reclaimed Lined Pits in 2040
Simulation 4 indicates the potential cumulative hydro-
logic effects of pit development in 2040, when mining in the
study area has been approximately fully developed and all
pits excavated after 2020 are lined with slurry walls or clay.
All hydrologic conditions in simulation 4 are the same as in
simulation 3 except for the addition of lined pits after 2020.
Simulated groundwater -level changes in 2055 resulting from
the potential extent of pit development in 2040 are shown
in figure 36 relative to locations of wetlands and riparian
herbaceous vegetation, and the simulated groundwater budget
for simulation 4 is provided in table 10, Groundwater -level
changes represent potential hydrologic conditions 15 years
after the pits are reclaimed. As with simulation 3, most
groundwater -level change occurs during the first year because
of the aquifer's high transmissivity, and groundwater levels
cease to change substantially in most areas of the simulated
aquifer within about 10 years.
The general pattern of simulated groundwater -level
decline and rise resulting from the addition of lined pits in
2040 is similar to that resulting from pits in 2020, Simulated
groundwater levels rise on the upgradient side of lined pits
and decline on the downgradient side of lined pits. However,
the extent and magnitude of rise on the upgradient side of
lined pits is greater in 2040 than in 2020 because groundwater
flow is obstructed over a larger part of the simulated aquifer.
72 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
Maximum groundwater -level rise is about 9 ft upgradient from
lined pits near Little Dry Creek. The addition of lined pits
in 2040 results in greater groundwater -level declines than in
2020 in some areas of the simulated aquifer and lesser declines
in other areas. Areas of greater decline occur downgradient
from added pits. Areas of lesser decline occur where ground-
water -level rises resulting from additional lined pits offset
declines resulting from unlined pits. Maximum groundwater -
level declines resulting from lined pits in 2040 is about 9 ft
near Big Dry Creek downgradient from the lined water -storage
facility,
Wetlands mapped in the study area generally are not
located where simulated groundwater levels decline inure
than 2 ft (fig. 36), but substantial wetland areas are located
where groundwater levels rise more than 2 ft between Big
Dry Creek and Little Dry Creek on the west side of the South
Platte River, Wetlands located where the depth to water is less
than 5 ft (fig. 30) in this area might be flooded by the higher
groundwater levels, or additional wetlands might form. As in
simulation 3, substantial areas of riparian herbaceous vegeta-
tion are located where groundwater levels decline or rise more
than 2 ft, indicating that riparian herbaceous vegetation in
these areas might be affected by groundwater changes result-
ing from the extent of reclaimed lined pits in 2040.
Total aquifer inflow and outflow in simulation 4 is less
than in simulation 3 because the additional lined pits obstruct
groundwater inflow and outflow in the model. Leakage from
unlined pits and distributed recharge at the land surface
represent the largest changes to aquifer inflow in simulation 4
relative to simulation 3. Leakage from unlined pits decreased
22.9 percent relative to simulation 3 because groundwater flow
from unlined pits is obstructed to a greater extent by the addi-
tion of lined pits. Distributed recharge at the land surface in
simulation 4 decreased by 14.6 percent relative to simulation 3
because a larger land area contributing recharge to the model
is converted to aggregate pits. Groundwater discharge to
unlined pits and to the South Platte River and Big Dry Creek
represent the largest changes to aquifer outflow in simula-
tion 4. Groundwater discharge to unlined pits decreased 21.3
percent relative to simulation 3, and groundwater discharge to
the South Platte River and Big Dry Creek decreased by
7.0 percent.
Simulation 5 —Reclaimed Unlined Pits in 2040
Simulation 5 indicates the potential cumulative hydro-
logic effects of pit development in 2040, when mining in the
study area has been approximately fully developed and all pits
excavated after 2020 are unlined, All hydrologic conditions
in simulation 5 are the same as in simulation 3 except for the
addition of unlined pits after 2020, Simulated groundwater -
level changes in 2055 resulting from the potential extent of
pit development in 2040 are shown on figure 37 relative to
locations of wetlands and riparian herbaceous vegetation, and
the simulated groundwater budget for simulation 5 is provided
in table 10. Comparison of simulation 5 to simulation 4 (lined
pits added after 2020) allows for assessment of which recla-
mation method would likely minimize the hydrologic effects
of reclaimed pits. Groundwater -level changes represent poten-
tial hydrologic conditions 15 years after pits are reclaimed. As
with simulation 3, most groundwater -level change occurs dur-
ing the first year because of the aquifer's high transmissivity,
and groundwater levels cease to change substantially in most
areas of the simulated aquifer within about 10 years.
Groundwater -level declines in simulation 5 generally
are greater than in simulations 3 and 4 because the addition
of unlined pits creates larger areas where groundwater levels
decline to newly imposed pit -lake levels. The magnitude and
extent of groundwater -level rise in simulation 5 generally
is less than in simulations 3 and 4 because the presence of
additional unlined pits lessens groundwater -level rises result-
ing from nearby lined pits. The maximum groundwater -level
decline in simulation 5 is about 11 ft near Big Dry Creek
downgradient from the lined water -storage facility and is
about 2 ft greater than the maximum decline in simulations
3 and 4. The maximum groundwater -level rise is about 5 ft
in small areas along the upgradient walls of lined pits near
the south end of the simulated aquifer. Large groundwater -
level rises resulting from pits on the west side of the South
Platte River in simulations 3 and 4 are greatly reduced by the
addition of unlined pits adjacent to the lined pits in simula-
tion 5. Comparison of simulation 5 to simulation 4 indicates
the overall effect of adding lined pits to the simulated aquifer
is to increase the magnitude and extent of groundwater -level
rise, whereas the effect of adding unlined pits is to increase the
magnitude and extent of groundwater -level decline.
Wetlands mapped in the study area are located where
simulated groundwater levels decline more than 2 ft (fig, 37)
between Little Dry Creek and the South Platte River. Because
the simulated depth to water (fig. 30) in this area generally is
less than S ft, wetlands in this area might be affected by lower
groundwater levels resulting from the extent of reclaimed
unlined pits in 2040. As in simulations 3 and 4, substantial
areas of riparian herbaceous vegetation are located where
groundwater levels decline or rise more than 2 It, indicating
that riparian herbaceous vegetation in these areas also might
be affected by groundwater -level changes resulting from the
extent of reclaimed unlined pits in 2040.
Total aquifer inflow and outflow in simulation 5 is much
greater than in simulation 3 or any of the other simulations
because of groundwater inflow and outflow at additional
unlined pits simulated as lakes in simulation 5, Leakage
from unlined pits and from the South Platte River and Big
Dry Creek represent the largest changes to aquifer inflow in
simulation 5 relative to simulation 3. Leakage from unlined
pits increased about 204 percent relative to simulation 3,
and leakage from the South Platte River and Big Dry Creek
increased 15.3 percent, Groundwater discharge to unlined pits,
groundwater entering storage, and groundwater discharge to
the South Platte River and Big Dry Creek represent the largest
changes to aquifer outflow in simulation 5, Groundwater dis-
charge to unlined pits increased about 207 percent relative to
Simulated Effects of Land -Use Change and Aggregate Mining on Groundwater Flow 73
109°50"IN
40°00'N
I.1rrfr !7
40°04'N
Water
storage _.
facility
40°02'N
40°N
39°5IVN
104°40%V
Streams modified from US Geological Survey National
Hydtoglaphy Datnset; 1:100,020
Roads modified from Colorado Department of Transportation
North Ameiican Datum of 1983
art •
tLtlptnrt-
Brighton
EXPLANATION
ILined pit
� Unlined pit
Fines-backfilled pit
{Mr
Wetland mapped as part of this study
i Riparian herbaceous vegetation indicated by
` Colorado Division at Wildlife (2007a, b)
Simulated grauudvcatcl-level change, in feet
Positive values indicate groundwater -level rise
Negative values indicate groundwater -level decline
•
S to 10
6 to 8
4 to 6
2 to 4
—2 to —2
—4to-2
-fi 141 -4
—% to
—IF, Ohl
Limit of simulated aquifer
0 5,000 10,000 FEET
I I!JrL L— . .fi
0 1 000 2,000 3,000 METERS
Figure 36. Simulation 4 —Groundwater -level changes in 2055 resulting from the potential extent of
reclaimed aggregate pits in 2040, Brighton to Fart Lupton, Colorado. Pits added after 2020 are simulated as
lined.
74 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
104°50'W
40°06'N
40'04'N
40°erN
49°N
39°50'N
104°48'W
Streams modified from U.S Geological Survey National
Hydrography Dataset, 1:100,000
Roads modified fi am Colorado Department of Transportation
North American Datum of 1983
EXPLANATION
JLined pit
Unlined pit
Fines-backfllled pit
Wetlnnd mopped as part of this study
I�Vf tai
1--1 Riparian herbaceous vegetation indicated by
1 Colorado Division of Wildlife (2007a, b)
Simulated groundwater -level change, in feet
Positive values indicate groundwater -level rise.
Negative values indicate groundwater -level decline
0
L57' 4 to
2to4
—2to2
— 4to-2
—6 to —4
Mgt —8 to —6
— 10to-8
Mai —12 to -1O
Limit of simulated aquifer
5.000 10,000 FEET
0 1,000 2,000 3,000 METERS
Figure 37, Simulation 5 —Groundwater -level changes in 2055 resulting from the predicted extent of reclaimed
aggregate pits in 2040, Brighton to Fort Lupton, Colorado. Pits added after 2020 are simulated as unlined.
Simulated Effects of Land -Use Change and Aggregate Mining on Groundwater Flow 75
simulation 3, groundwater entering storage decreased by 25.4
percent, and groundwater discharge to the South Platte River
and Big Dry Creek increased by 3.9 percent,
Summary of Reclaimed Pit Simulations
Transient simulations of the cumulative hydrologic
effects of reclaimed pits in 2020 and 2040 indicate lined pits
cause groundwater levels to rise upgradient from pits and
decline downgradient from pits, whereas unlined pits have an
opposite effect on groundwater levels. The hydrologic effects
of pits backfilled with fine-grained sediments are similar to
but less than those of lined pits because the pits create less of
a barrier to groundwater flow than lined pits. The hydrologic
effects of lined, unlined, and fines-backfilled pits interact to
increase the magnitude of groundwater -level changes at some
locations and decrease it at others, depending on the relative
position of the pits. The maximum simulated groundwater -
level decline in 2035 (15 years after reclamation) resulting
from the extent of reclaimed pits in 2020 is about 9 ft, and
the maximum simulated groundwater -level rise is about 6 ft.
Groundwater -level changes near the South Platte River gener-
ally are less than 2 ft because the river lessens the hydrologic
effects of pits by contributing or receiving water as ground-
water levels change. Groundwater levels change most during
the first year after pits are reclaimed, and groundwater levels
cease to change substantially in most areas of the simulated
aquifer within about 10 years. The addition of lined pits
in 2040 results in a general increase in the magnitude and
extent of groundwater -level rise relative to simulated condi-
tions for 2020 because groundwater flow is obstructed over a
larger part of the simulated aquifer. The maximum simulated
groundwater -level decline and rise resulting from the addition
of reclaimed lined pits in 2040 is about 9 ft in 2055, 15 years
after reclamation. The addition of unlined pits in 2040 results
in a general increase in the magnitude and extent of ground-
water -level decline relative to simulated conditions for 2020
because groundwater levels decline to lake levels in unlined
pits over a larger area of the simulated aquifer. The maximum
groundwater -level decline resulting from the addition of
reclaimed unlined pits in 2040 is about 11 ft in 2055, 15 years
after reclamation.
Wetlands mapped by this study generally are located
where simulated groundwater -level changes resulting from
reclaimed pits in 2020 are less than 2 ft, but some areas of
riparian herbaceous vegetation mapped by the Colorado Divi-
sion of Wildlife are located where simulated groundwater -
level changes are more than 2 ft. Mapped wetlands and areas
of riparian herbaceous vegetation are both located where simu-
lated groundwater -level changes resulting from reclaimed pits
in 2040 are snore than 2 ft, Wetlands and riparian herbaceous
vegetation located where groundwater -level changes are more
than 2 ft might be affected by the changes. Some areas where
groundwater levels are simulated to rise more than 2 ft at
locations where the depth to water is less than 5 ft could create
conditions favorable to the formation of new wetlands.
The groundwater budgets for simulations of reclaimed
pits in 2020 and 2040 are greater than the budgets for the cali-
brated model or simulations of the effects of land -use change
because of groundwater inflow and outflow at additional
unlined pits simulated as lakes in 2020 and 2040. Other than
groundwater inflow and outflow at unlined pits, distributed
recharge at the land surface and leakage from the South Platte
River and Big Dry Creek represent the greatest changes to
aquifer inflow in 2020 relative to the 2000 irrigation season,
and groundwater outflow at the downgradient end of the South
Platte River valley and discharge to the South Platte River and
Big Dry Creek (in terms of absolute flow) represent the great-
est changes to aquifer outflow. Distributed recharge at the land
surface decreases by 16.8 percent because land area that is
converted to pits (lined and unlined) no longer contributes to
distributed recharge. Leakage from the South Platte River and
Big Dry Creek increases by 19.0 percent because of ground-
water -level declines resulting from the extent of reclaimed pits
in 2020. Groundwater outflow at the downgradient end of the
South Platte River valley decreases by 14.0 percent, and dis-
charge to the South Platte River and Big Dry Creek decreases
by 4.4 percent, likely because of lined pits in 2020 obstructing
groundwater flow in the simulated aquifer. The general effect
of adding lined pits in 2040 is to decrease distributed recharge
at the land surface by 14.6 percent relative to simulated condi-
tions for 2020 and to obstruct groundwater flow over a larger
part of the simulated aquifer, which decreases leakage from
unlined pits by 22.9 percent, discharge to unlined pits by 21.3
percent, and discharge to the South Platte River and Big Dry
Creek by 7.0 percent. The general effect of adding unlined
pits in 2040 (relative to simulated conditions for 2020) is to
increase leakage from unlined pits by 204 percent, leakage
from the South Platte River and Big Dry Creek by 15.3 per-
cent, discharge to unlined pits by 207 percent, and discharge to
the South Platte River and Big Dry Creek by 3.9 percent, and
to decrease groundwater entering storage by 25.4 percent.
Simulated Hydrologic Effects of Actively
Devvatered Pits
Simulations of the hydrologic effects of actively dewa-
tered pits represent potential drawdown resulting from pits
as they are dewatered to allow dry -mining of aggregate. The
term drawdown, rather than groundwater -level decline, is
used to describe lowering of the water table in simulations
of actively dewatered pits because the lowering is the result
of withdrawing water from the aquifer rather than obstruct-
ing or altering the direction of groundwater flow, such as in
the case of lined or unlined reclaimed pits. The dewatered pit
locations are hypothetical and were selected to indicate the
general effects of dewatered pits and the potential effects that
hydrologic boundaries, such as the South Platte River, no -flow
boundaries, and reclaimed pits, might have on drawdown near
pits being actively dewatered. Four simulations of actively
dewatered pits are presented as follows:
76 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
Simulation 6 —The hydrologic effects of a single
dewatered pit.
Simulation 7 —The hydrologic effects of two closely
spaced, dewatered pits.
Simulation 8 The hydrologic effects of two widely
spaced, dewatered pits.
Simulation 9 —The hydrologic effects of three closely
spaced, dewatered pits.
The model grid, time diseretization, and all hydrologic
conditions in simulations 6-9 are the same as those in simu-
lation 3 (reclaimed pits in 2020) except for the addition of
actively dewatered pits in 2020. Dewatering of active pits is
simulated by using the Flow and Head Boundary package
(Leake and Lilly, 1997) ofMODFLOW-2000. Initial head at
actively dewatered pits is set equal to that of the water table
during the 2000 irrigation season, and final head after dewater-
ing is set 1 ft above bedrock at the base of the simulated aqui-
fer. Lowering of the water level within the pit is simulated to
occur linearly over a period of 90 days as the pit is dewatered
and the final water level is reached.
As with simulations of reclaimed pits (simulations 3-5),
drawdown in all active -pit simulations is determined rela-
tive to the 2000 irrigation season rather than the 20O0 non -
irrigation season because groundwater -supported wetlands are
more likely affected by groundwater -level changes during the
irrigation season when groundwater levels are highest. Square
pits (1,200 ft by 1,200 ft), representing the average size (about
34 acres) of a mining phase, are used to simulate the general
hydrologic effects of actively dewatered pits. Drawdown
resulting from pits having different size and shape would be
somewhat different than that indicated by the simulations
provided. However, Arnold and others (2003) found pit size
(radius) to have relatively small effect on steady-state draw -
down extent compared to other factors, such as horizontal
hydraulic conductivity, recharge, and pit depth below the
water table in hypothetical sand -and -gravel aquifers having
conditions similar to those of the South Platte alluvial aquifer.
Simulation 6 —One Actively Dewatered Pit
Simulation 6 represents the potential hydrologic effects of
dewatering a single, average -size pit in the South Platte allu-
vial aquifer, Simulated drawdown resulting from the pit rela-
tive to water -table conditions during the 2000 irrigation season
is shown in figure 38, and the simulated groundwater budget
for simulation 6 is provided in table 11. Although the location
of the pit is hypothetical, locations of wetlands mapped by the
study and locations of riparian herbaceous vegetation mapped
by CDOW (2007a, b) are shown in figure 38 for comparison to
drawdown extent. Drawdown represents hydrologic conditions
1 year and ] 5 years after the start of pit dewatering. Because
the simulated aquifer has high transrnissivity, most drawdown
occurs during the first year, and drawdown extent after l year
is almost as large as after 15 years (fig. 38), .Drawdown ceases
to increase substantially after 15 years, Maximum drawdown
in simulation 6 is about 37 ft at the pit wall, where dewatering
has lowered groundwater levels to 1 ft above bedrock at the
base of the simulated aquifer, Drawdown decreases rapidly
away from the pit to a value of 20 ft about 100 ft from the
pit wall and decreases more gradually farther from the pit.
Drawdown extent is affected by the presence of hydrologic
boundaries near the pit, such as the South Platte River, the
no -flow west model boundary, and reclaimed pits. Drawdown
between the pit and the South Platte River is less than between
the pit and the west model boundary because the river supplies
water to lessen drawdown, whereas the no -flow west bound-
ary limits the supply of water. Unlined reclaimed pits located
north and south of the dewatered pit also lessen drawdown and
limit drawdown extent. Drawdown of 10 ft occurs about 200
ft from the pit wall in the direction of the South Platte River,
whereas drawdown of I 0 ft occurs up to about 1,000 ft from
the pit wall in the direction of the west model boundary. Draw -
down of 2 ft occurs at a maximum distance of about 9,500
ft downgradient from the pit center and is affected by the
presence of reclaimed pits. The full drawdown extent defined
by the limit of 2 -ft drawdown has a maximum width of about
12,400 ft. Because dewatering typically occurs for multiple
years and most drawdown occurs rapidly during the first year,
groundwater -supported wetlands within the limit of 2 -ft draw -
down could be affected by lower groundwater levels resulting
from pit dewatering.
Leakage from unlined pits, leakage from the South Platte
River and Big Dry Creek, and groundwater inflow from the
upgradient ends of the South Platte valley and tributaries are
the largest sources of inflow to the aquifer in simulation 6
(table II ). Combined inflow from these sources represents
about 70 percent of total inflow in simulation 6. Groundwater
discharge to unlined pits and groundwater discharge to the
South Platte River and Big Dry Creek are the largest sources
of outflow in simulation 6. Combined outflow from these
sources represents about 60 percent of the total outflow in
simulation 6. Groundwater discharge to the actively dewatered
pit represents about 9 percent of the total simulated outflow,
Relative to baseline conditions represented by the water
budget of simulation 3 (reclaimed pits in 2020 without an
actively dewatered pit), the primary effect of pit dewatering is
to increase leakage from the South Platte River and Big Dry
Creek by 21 percent and to decrease discharge to the South
Platte River and Big Dry Creek by 15 percent.
Simulation 7 —Two Closely Spaced, Actively Dewatered
Pits
Simulation 7 represents the potential hydrologic effects
of simultaneously dewatering two closely spaced (400 ft apart)
average -'size pits or pit phases of a single larger pit in the
South Platte alluvial aquifer, Simulated drawdown resulting
from the pits after 1 year and after 15 years of dewatering is
shown in figure 39 relative to water -table conditions of the
2000 irrigation season, and the simulated groundwater budget
for simulation 7 is provided in table 11, Wetlands and areas
of riparian herbaceous vegetation also are shown in figure 39
Simulated Effects of Land -Use Change and Aggregate Mining on Groundwater Flow 77
Table 11. Groundwater budgets for simulations of the hydrologic effects of actively dewatered pits rn the South Platte alluvial
aquifer.
[All values are in cubic feet per day; totals reflect sum of all rounded components]
Budget
component
Simulation Simulation Simulation Simulation
61 72 83 94
Aquifer inflows
Groundwater inflow from general -head boundaries 2,065,000 2,071,000 2,075,000 2,077,000
at upgradient end of South Platte River valley
and tributaries
Subsurface irrigation return flow along east model 1,782,000 1,782,000 1,782,000 1,782,000
boundary
Distributed recharge at the land surface 1,457,000 1,449,000 1,447,000 1,438,000
Leakage to aquifer from South Platte River and 2,108,000 2,305,000 2,428,000 2,500,000
Big Dry Creek
Leakage to aquifer from unlined pits 3,750,000 3,635,000 3,467,000 3,452,000
Groundwater released from storage 108.000 108,000 108,000 108,000
Total 11,270,000 11,350,000 11,307,000 11,357,000
Groundwater outflow to general -head boundary at
dowagradient end of South Platte River valley
Groundwater discharge to South Platte River and
Big Dry Creek
Groundwater discharge to Little Dry Creek and
Third Creek
Groundwater discharge to unlined pits
Phreatophyte evapotranspiration
Municipal -well withdrawals
Groundwater discharge to actively dewatered pits
Groundwater entering storage
Total
Percent discrepancy (Recharge —Discharge)
Aquifer outflows
1,150,000 1,145,000 1,140,000 1,139,000
3,097,000 2,916,000 2,798,000 2,759,000
144,000 144,000 144,000 144,000
3,690,000 3,541,000 3,360,000 3,332,000
181,000 178,000 177,000 176,000
1,421,000 1,421,000 1,421,000 1,421,000
1,021,000 1,442,000 1,705,000 1,827,000
570.000 567,000 566,000 563,000
11,274,000 11,354,000 11,311,000 11,361,000
—0.04 —0.04 —0.04 —0.04
'One actively dewatered pit
'Two closely spaced, actively dcwatered pits
'Two widely spaced, actively dewatered pits
'Three closely spaced, actively dewatered pits
78 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
40°06'N
40'04'N
0
5,000 10,000 FEET
I I
I I r I f I I iI I I I ly I I
1,000 2,000 3,000 METERS
[n
104°50'W
0
EXPLANATION
Actively dewateied pit
Lined pit
Unlined pit
I I
Wetland mapped as part of this study
Riparian herbaceous vegetation indicated by
Colorado Division of Wildlife (2007a, b)
Line of equal drawdown after 15 years, in feet
Line of equal drawdown afteu 1 year, in feet
Limil of simulated aquifer
104'48'W
Figure 38. Simulation 6—Drawdown resulting from a single actively dewatered pit in the South Platte
alluvial aquifer after 1 year and after 15 years of dewatering.
Simulated Effects of Land -Use Change and Aggregate Mining on Groundwater Flow 79
for comparison to drawdown extent, As in simulation 6, most
drawdown occurs during the first year (fig. 39) and increases
only slightly after the first year. Maximum drawdown in simu-
lation 7 is the same as in simulation 6 with a value of about
37 ft at the wall of the southern pit. Drawdown in simulation
7 decreases rapidly away from the pits to a value of20 ft at
a distance of 100-200 ft from pit walls and decreases more
gradually farther from the pits. Drawdown of 10 ft occurs
at a maximum distance of about 200 ft from the pits in the
direction of the South Platte River and at a maximum distance
of about 3,200 ft from the pits in the direction away from
the river. Drawdown near the pits is irregular because of the
overlapping effect of each dewatered pit. Farther from the
pits, the aquifer responds as though the two pits are a single
larger dewatered pit. Drawdown of 2 ft occurs at a maximum
distance of about 8,800 ft downgradient from the midpoint of
the pits (at the west model boundary) and is affected by the
presence of reclaimed pits. The full drawdown extent defined
by the limit of 2 -ft drawdown has a maximum width of about
12,800 ft.
The overall groundwater budget for simulation 7 is simi-
lar to that for simulation 6, but the magnitude of some water -
budget components is slightly different (table 11). Combined
leakage from unlined pits, leakage from the South Platte River
and Big Dry Creek, and groundwater inflow from the upgradi-
ent ends of the South Platte valley and tributaries represent
about 71 percent of the total inflow in simulation 7. Combined
groundwater discharge to unlined pits and groundwater dis-
charge to the South Platte River and Big Dry Creek represent
about 57 percent of the total outflow in simulation 7. Ground-
water discharge to the actively dewatered pits in simulation 7
represents about 13 percent of the total outflow.
Simulation 8 —Two Widely Spaced, Actively Dewatered
Pits
Simulation 8 represents the potential hydrologic effects of
simultaneously dewatering two widely spaced (2,000 ft apart)
average -size pits in the South Platte alluvial aquifer. Simulated
drawdown resulting from pits after 1 year and after 15 years of
dewatering is shown in figure 40 relative to water -table condi-
tions of the 2000 irrigation season, and the simulated ground-
water budget for simulation 8 is provided in table 11. Wetlands
and areas of riparian herbaceous vegetation also are shown in
figure 40 for comparison to drawdown extent. As in simulation
7, most drawdown occurs during the first year (fig. 40) and
increases only slightly after the first year. Maximum draw -
down in simulation 8 is the same as in simulation 7 with a
value of about 37 ft at the wall of the southern pit. As in simu-
lation 7, drawdown in simulation 8 decreases rapidly away
from the pits to a value of 20 ft at a distance of about 100-200
ft from the pits and decreases more gradually farther from the
pits, However, the areal extent of drawdown in simulation 8
generally is larger than in simulation 7 because the pits are
farther apart and the water table is effectively lowered over
a larger area by the combined effects of the separated pits. In
addition, the independent drawdown effects of each pit are
evident where drawdown is more than about 20 ft. Drawdown
of 10 ft occurs at a maximum distance of about 500 ft from
the pits in the direction of the South Platte River and about
3,500 ft from the pits in the direction away from the river. The
line of 10 -ft drawdown is irregular near the pits because of
the overlapping effect of each dewatered pit. Farther from the
pits, the aquifer responds as though the two pits are a single
larger dewatered pit. Drawdown of 2 ft occurs at a maximum
distance of about 8,100 ft downgradient from a point midway
between the pits (at the west model boundary) and is affected
by the presence of reclaimed pits. The full drawdown extent
defined by the limit of 2 -ft drawdown has a maximum width
of about 12,900 ft.
The overall groundwater budget for simulation 8 is
similar to those for simulations 6 and 7, but the magnitude of
some water -budget components is slightly different (table 11).
Combined leakage from unlined pits, leakage from the South
Platte River and Big Dry Creek, and groundwater inflow from
the upgradient ends of the South Platte valley and tributaries
represent about 70 percent of the total inflow in simulation 8.
Combined groundwater discharge to unlined pits and ground-
water discharge to the South Platte River and Big Dry Creek
represent about 54 percent of the total outflow in simulation 8.
Groundwater discharge to the actively dewatered pits in simu-
lation 8 represents about 15 percent of the total outflow.
Simulation 9 —Three Closely Spaced, Actively Dewatered
Pits
Simulation 9 represents the potential hydrologic effects
of simultaneously dewatering three closely spaced (400 ft
apart) average -size pits or pit phases of a single larger pit in
the South Platte alluvial aquifer. Simulated drawdown result-
ing from the pits after 1 year and after 15 years of dewatering
is shown in figure 41 relative to water -table conditions of the
2000 irrigation season, and the simulated groundwater budget
for simulation 9 is provided in table 11. Wetlands and areas
of riparian herbaceous vegetation also are shown in figure
41 for comparison to drawdown extent. Drawdown extent in
simulation 9 (fig, 41) is similar to that of simulation 8, which
indicates that the addition of a third pit between two widely
spaced pits has relatively little effect on drawdown. The
primary difference in drawdown between simulations 8 and 9
is that slightly greater drawdown occurs in the direction of the
west model boundary, especially near Little Dry Creek. Draw -
down in simulation 9 decreases rapidly away from the pits to a
value of 20 fl at a maximum distance of about l00 ft from the
pits in the direction of the South Platte River and about 600 ft
from the pits in the direction away from the river. Drawdown
decreases more gradually farther from the pits. Drawdown of
10 ft occurs at a maximum distance of about 600 ft from pits
in the direction of the South Platte River and about 3,900 ft
from the pits in the direction away from the river, Drawdown
near the pits is irregular because of the overlapping effect of
each dewatered pit. Farther from the pits, the aquifer responds
80 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
90°96'N
40°04'N
104°50'W
104°48'W
a
l
Union Pacific Railroad
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65
ti
--, _LW Nt' ,
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1 4-,S,14
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Fort
Lupton
Streams modified horn U S Geological SuiveyNational Hydrography Datasel: 1:100,000
Roads modified from Colorado Department of Transportation
North American Datum of 1983
a
1
EXPLANATION
Actively dcwateied pit
Lined pit
Unlined pit
Welland mapped as pail of this study
Riparian hetbaecous vegetation indicated by
Colorado Division of Wildlife (2007a, h)
Line of equal drawdown aftei 15 years, in feet
Line of equal drawdown after I year, in feet
Limit of simulated aquifer
0
5,000
0
I I
1 I
10,050 FEET
1,000 2,000 3,000 METERS
Figure 39. Simulation 7—Drawdown resulting from two closely spaced, actively dewatered pits in the
South Platte alluvial aquifer after 1 year and after 15 years of dewatering,
Simulated Effects of Land -Use Change and Aggregate Mining on Groundwater Flow 81
46°06'N —
40°04'N —
Streams modified from U S Geological Survey National Hydrogiaphy [Maser; 1.100,000
Roads modified from Colorado Department of Transportation
North American Datum of 1901
•
r
ut� i
l_7
107°5e'W
EXPLANATION
Actively dewatered pit
Lined pit
Unlined pit
Wetland mapped as pail of this study
Riparian herbaceous vegetation indicated by
Calm ado Division of Wildlife (2007a, b)
I,ine of equal diawdown after 15 years, in feet
Line of equal drawdown after I ytletr, in feel
Ii nit of sii ulmiled atluifer
104°40'W
0 0,000 10,000 FEET
1-1—t1 i 11 t1 11
0 1,000 2,000 3,000 METERS
Figure 40. Simulation 8—Drawdown resulting from two widely spaced, actively dewatered pits in the
South Platte alluvial aquifer after 1 year and after 15 years of dewatering,
82 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
40°06'N
40°04'N
Streams modifiedliom U S Geological SmveyNational HydrographyDalaset; 11100,000
Roads Inadified from Colorado Department of Transportation
North Amei icon Datum of 1983
1
r
l04°50'W
EXPLANATION
Actively dewatered pit
Lined pit
Unlined pit
Weiland mapped as part of his study
I Riparian herbaceous vegetation indicated by
I Colorado Division of Wildlife (2007a, b)
Line of equal diawdown after 15 years, in feel
Line of equal diawdown after 1 year, in feet
Limit of simulated nquifei
104°48'W
0 5,000
1 I I I-4- 1 i I i i 1 Hi I I I II
0 1,000
10,000 FEET
2,000 3,000 METERS
Figure 41. Simulation 9—Drawdown resulting from three closely spaced, actively dewatered pits in the
South Platte alluvial aquifer after 1 year and after 15 years of dewatering.
Simulated Effects of Land -Use Change and Aggregate Mining on Groundwater Flow 83
as though the three pits are a single larger dewatered pit.
Drawdown of 2 ft occurs at a maximum distance of about
8,100 ft downgradient from the center of the middle pit (at
the west model boundary) and is affected by the presence of
reclaimed pits, The full drawdown extent defined by the limit
of 2 -ft drawdown has a maximum width of about 13,100 ft.
Combined leakage from unlined pits, leakage from the
South Platte River and Big Dry Creek, and groundwater inflow
from the upgradient ends of the South Platte valley and tribu-
taries represent about 71 percent of the total inflow in simula-
tion 9 (table 1 l). Combined groundwater discharge to unlined
pits and groundwater discharge to the South Platte River and
Big Dry Creek represent about 54 percent of the total outflow
in simulation 9. Groundwater discharge to the actively dewa-
tered pits in simulation 9 represents about 16 percent of the
total outflow.
Summary of Actively Dewatered Pit Simulations
Comparison of the hydrologic effects of actively dewa-
tered pits in simulations 6-9 indicates that the extent of
drawdown increases as the number of pits increases because
the drawdown effects of each pit are additive. Increasing the
distance between two actively dewatered pits from 400 ft to
2,000 ft increased the areal extent of drawdown, rather than
reduced it, because drawdown resulting from each pit com-
bined to create a single larger drawdown extent. Maximum
drawdown in all simulations of actively dewatered pits was
37 ft. Drawdown decreased to 20 ft within about 100-600 ft of
actively dewatered pits and decreased more gradually farther
from pits, The full drawdown extent defined by the limit of
2 -ft drawdown has a maximum width of about 12,400 ft for a
single dewatered pit, 12,800 ft for two pits spaced 400 ft apart,
12,900 ft for two pits spaced 2,000 ft apart, and 13,100 ft for
three pits spaced 400 ft apart. Drawdown was affected by the
presence of the simulated South Platte River, the no -flow west
model boundary, and nearby unlined pits. Simulated flow from
the South Platte River and unlined pits decreased drawdown
resulting from dewatered pits, whereas the no -flow model
boundary increased drawdown. Most drawdown occurs during
the first year, and drawdown extent after 1 year is almost as
large as after 15 years. Because dewatering typically occurs
for multiple years and most drawdown occurs rapidly during
the first year, groundwater -supported wetlands within the limit
of 2 -ft drawdown might be affected by the lower groundwater
levels resulting from pit dewatering.
Groundwater budgets for simulations 6-9 indicate that
the general effect of actively dewatered pits is to increase
groundwater inflow from the upgradient ends of the South
Platte River valley and leakage from the South Platte River
and Big Dry Creek and to decrease or not affect most other
components of the water budget. Simulated groundwater dis-
charge to actively dewatered pits increased as the number of
pits increased and as the distance between two pits increased
from 400 ft to 2,000 ft.
Simulated Effects of Pit Spacing and
Configuration on Groundwater Levels Near Pits
Simulations of different hypothetical pit spacings and
configurations are used to assess the effect that pit spacing
and configuration might have on groundwater levels near
reclaimed pits. Lined pits are used in all simulations related
to pit spacing and configuration because lined pits generally
resulted in greater groundwater -level changes than unlined pits
in simulation 3 (see "Simulation 3 —Reclaimed Pits in 2020").
Unlined pits would be expected to have an opposite but pos-
sibly lesser effect on groundwater levels near the pits. The
model grid, time discretization, and all hydrologic conditions
in simulations related to pit spacing and configuration are the
same as those in simulation 3 (reclaimed pits in 2020) except
for the addition of lined pits and different initial hydrau lic-
head conditions. Initial hydraulic -head conditions are taken
from the final water table simulated at the end of simulation
3 so that simulated groundwater -level changes reflect only
effects related to pit spacing and configuration. As with simu-
lations of the cumulative hydrologic effects of reclaimed pits
(simulations 3-5), lined pits are simulated by using inactive
model cells at added pit locations, which act as barriers to
groundwater flow. The direction of groundwater flow under
initial head conditions is about N.15°E. at the location of
simulated pits.
The effects of pit spacing and configuration on ground-
water levels are simulated by using three pits in seven dif-
ferent configurations (fig. 42). Five configurations simulate
different pit sizes aligned approximately cross gradient to the
direction of horizontal groundwater flow, and two configura-
tions simulate average -size pits (1,200 ft by 1,200 ft) offset
upgradient or downgradient from each other, The distance
between pits in each aligned configuration varies from 0 to
1,000 ft in 200 -ft increments, depending on pit size. Pits
aligned approximately cross gradient to groundwater flow
obstruct groundwater flow to different extents based on pit
size, Pit width in aligned configurations ranges from 1,000
to 1,800 ft in increments of 200 ft, and the combined width
of the three aligned pits ranges from 3,000 to 5,400 ft, which
represents 50-90 percent of the 6,000 -ft distance between the
west model boundary and the South Platte River at the loca-
tion of simulated pits. Because the pits extend to bedrock and
saturated thickness varies little at the aligned pit locations,
the combined cross-sectional area of the pits consequently
obstructs groundwater flow through about 50-90 percent of
the aquifer on the west side of the river.
84 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
(a)
(i0
(c)
(d)
(e►
(0
(g)
I
General direction of groundwater flow
Aligned 1,000 -ft -wide pits
Aligned 1,200 -ft -wide pits
Aligned 1,400 -ft -wide pits
Aligned 1,600 -ft -wide pits
Aligned 1,800 -ft -wide pits
1,200 -ft -wide pits with center pit offset upgradient
0 l 1,500 L 3,000 FEET
I I f I l I I I I I I
0 500 1,000 METERS
1,200 -ft -wide pits with center pit offset downgradient
Figure 42. Seven configurations used to simulate the effect of pit spacing and configuration on groundwater
levels near reclaimed lined pits.
Simulated Effects of Land -Use Change and Aggregate Mining on Groundwater Flow 85
Simulated Effects of Three Aligned Pits
The magnitude and extent of groundwater -level decline
and rise resulting from three 1,400 -ft -wide pits (configuration
c in figure 42) aligned approximately cross gradient to ground-
water flow are shown in figures 43A —C to illustrate the gen-
eral effect of pit spacing on groundwater levels, The maximum
groundwater -level decline resulting from contiguous pits (no
space between pits) is about 3.1 ft (fig. 43A) near the midpoint
of the downgradient pit wall. The maximum groundwater -
level rise resulting from contiguous pits is about 3.0 ft (fig.
43.4), but the maximum occurs slightly west of the upgradi-
ent wall's midpoint because groundwater flow is not exactly
perpendicular to pit alignment. The limits of 2 -ft decline and
rise occur about 1,000 ft downgradient and upgradient, respec-
tively, of the contiguous pits (fig. 43A). The areal extent of
groundwater -level decline decreases as the pits are separated,
and the pattern of decline reflects the hydrologic effects of
individual pits more so than when the pits are contiguous. The
limit of 2 -ft decline occurs about 500 ft downgradient from the
center pit when pits are 200 ft apart (fig. 43B) and about 200
ft downgradient when the pits are 400 ft apart (fig, 43C). The
limit of 2 -ft groundwater -level rise resulting from groundwater
mounding upgradient from pits exhibits similar reduction as
the spacing between pits increases. Groundwater -level changes
are less than 2 ft at nearly all locations when the pits are
spaced 600 ft apart or more and are therefore not presented.
Pits of different sizes that are aligned approximately
cross gradient to groundwater flow result in general patterns
of groundwater decline and rise similar to those shown in
figures 434—C, but the magnitude and extent of groundwater -
level change are different. The magnitude of groundwater -
level decline and rise relative to pit spacing for five different
pit widths is presented in figures 44.4 and 44B for locations
100 ft downgradient and 100 ft upgradient from the center
pit, Fewer values are presented for larger pits because the
distance between the South Platte River and the west model
boundary limits the range of spacing available for larger pits.
Groundwater -level decline and rise near the center pit repre-
sent maximum groundwater -level changes resulting from lined
pits that are located in areas minimally affected by hydrologic
boundaries. Groundwater -level decline and rise near the east
pit generally are less than that indicated by figures 44.4 and
4413 because of the hydrologic influence of the South Platte
River. Groundwater -level decline and rise near the west pit
generally are greater than that indicated by figures 44A and
44B because of the influence of the no -flow boundary on the
west side of the simulated aquifer. Groundwater -level changes
become greater as pit width increases because larger pits cre-
ate larger obstructions to groundwater flow. The groundwater -
level decline resulting from three contiguous 1,000 -ft -wide
pits is about 2.3 ft, whereas the decline resulting from three
contiguous 1,800 -ft -wide pits is about 3.9 ft. Similarly,
the groundwater -level rise resulting from three contiguous
1,000 -ft -wide pits is about 2.1 ft, whereas the rise resulting
from three contiguous 1,800 -ft -wide pits is about 3.8 ft.
The magnitude of groundwater -level decline and rise near
pits decreases most as pit spacing is increased from 0 to 200
ft, and the magnitude of decline and rise decreases less as the
distance between pits becomes greater. For average -size pits
(1,200 -ft wide), the magnitude of groundwater -level decline
decreases by about 0.6 ft (from 2.7 to 2.1 ft) as the distance
between pits is increased from 0 to 200 ft, but the magnitude
of the decline decreases only by about 0.1 ft as pit spacing is
increased front 800 to 1,000 ft. Because pit spacing has a pro-
gressively lesser affect on groundwater levels as the distance
between pits increases, about 50 percent (0.6 ft) of the total
reduction (1.2 ft) in the magnitude of the decline that occurs
for a pit spacing of 1,000 ft is achieved by a pit spacing of
only 200 ft, and about 75 percent (0,9 ft) of the total reduction
in the magnitude of the decline is achieved by a pit spacing
of 400 ft. Similarly, the magnitude of groundwater -level rise
that occurs upgradient from 1,200 -ft -wide pits decreases by
about 0,5 ft (from 2.5 to 2.0 ft) as the distance between pits is
increased from 0 to 200 ft, but it decreases only by about 0.1
ft as pit spacing is increased from 800 to 1,000 ft. About 42
percent (0.5 ft) of the total reduction (1.2 ft) in the magnitude
of the rise that occurs for a pit spacing of 1,000 ft is achieved
by a pit spacing of 200 ft, and about 67 percent (0.8 ft) of the
total reduction in the magnitude of the rise is achieved by a pit
spacing of 400 ft.
Simulated Effects of Three Offset Pits
Groundwater -level declines and rises resulting from
average -size pits (1,200 -ft wide) that are offset upgradient
or downgradient (fig. 42, configurations f and g) from each
other are simulated for five pit distances ranging from 0 to
800 ft (figs. 45.4 and 45B), Groundwater -level declines and
rises represent conditions 100 ft downgradient and 100 ft
upgradient from the center pit near the midpoint of the pit
wall. Similar to pits aligned approximately cross gradient
to groundwater flow, the magnitude of decline and rise
resulting from offset pits decreases most as pit offset spacing
increases from 0 to 200 ft, and the magnitude of decline and
rise changes progressively less as the distance between pits
increases. However, the magnitude of groundwater -level
decline and rise decreases to a greater extent when pits are
offset than when aligned pits are moved farther apart because
the hydrologic effects of the offset pit are counteracted by
the hydrologic effects of the other pits. In the configuration
where the center pit is offset upgradient from other pits, the
groundwater -level decline downgradient from the center pit
is counteracted by the groundwater -level rise upgradient from
other pits. Similarly, in the configuration where the center
pit is offset downgradient from other pits, the groundwater -
level rise upgradient from the center pit is counteracted by
the groundwater -level decline downgradient from other
pits. Offsetting the center pit 200 ft upgradient reduces the
magnitude of groundwater -level decline by about 1,7 ft (from
2.7 to 1.0 ft), which is about 7I percent of the total reduction
(2.4 ft) in the magnitude of the decline achieved by offsetting
86 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
40°06'N
40°04"N
too°50'W
Sheaths modified from U S Geological Survey National Hydrogtaphy Datasel; 1:1D0,000
Road modified From Colorado Department of Transportation
North American Datum of 1983
EXPLANATION
Lined pit
0
0
Unlined pit
Limit of 2 -fl groundwater -level decline after 15 year s
Limit of 2 -fl gf oundwater-level rise after 15 years
Limit of simulated aquifer
Gene.' al diieclion of gi oundwaler flow
III
5,000
III 1 1 1 1
1,030
too°48'W
10,000 FEET
J 1 t it
2,000 3,000 METERS
Figure 43A. Simulated groundwater -level changes resulting from three 1,400 -ft -wide lined pits with a spacing
of Oft when pits are aligned approximately crass gradient to groundwater flow in the South Platte alluvial
aquifer.
Simulated Effects of Land -Use Change and Aggregate Mining on Groundwater Flow 87
40°gfi'N
40°C4'N
04°50'W
52
Streams modif ed fiom U S Geological Survey National Hydiogiaphy Dataset; 1:100,000
Road modified fiom Coto' ado Deportment of Ti an sportalion
North American Datum of 1983
EXPLANATION
104°4B'W
0 5.000 10,000 FEET
I I t l ? I I r III r I ji f
0 1,000 2,000 3,000 METERS
I Lined oil
[! Unlined pit
Limit of 2-R gioandwater-level decline after 15 years
Limit of 2 -ft groundwater -level rise after 15 years
iI of simulated aquifer
tGeneral diieclion of groundwater now
Figure 438. Simulated groundwater -level changes resulting from three 1,400 -ft -wide lined pits with a
spacing of 200 ft when pits are aligned approximately cross gradient to groundwater flow in the South Platte
alluvial aquifer.
88 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo,
40°06'N
40°04'N
04°50'W
Sh cams modified from U S Geological Survey National 1-lydrogiaplry Datasei; 1;100,000
Road modified front Colorado Department ofTrauspmtahon
North American Datum of 1983
EXPLANATION
Lined pit
Unlined pit
laa°aa'w
a 5,000 10,000 FEET
II l l I I l I I I LI I I It II t It
0 t 000 I 2,000 3,000 METERS
Limit of 2 -ft groundwater -level decline after 15 years
Limit of 2 -ft groundwater -level rise after 15 years
Limit of simulated aqui£et
lGeneral direction ot'groundwaler flow
Figure 43C, Simulated groundwater -level changes resulting from three 1,400 -ft -wide lined pits with a
spacing of 400 ft when pits are aligned approximately cross gradient to groundwater flow in the South
Platte alluvial aquifer.
Simulated Effects of Land -Use Change and Aggregate Mining on Groundwater Flow 89
A
1.0
GROUNDWATER -LEVEL DECLINE, IN FEET
1,5
2.0
25
30
35
40
35
LL
U-
30
cc
w
w 2
0 2.0
z
0
cc
co
t.5
200 400 600
PIT SPACING, IN FEET
PIT WIDTH
—A— 1,o00feet
—41D— 1,200feet
1,400 feet
—31(-- 1,600 feet
f 1,800 feet
800 1,000 1,200
1n _
200
1 t 1
J. — —t
400 600
PIT SPACING, IN FEET
800
PIT WIDTH
# 1,000 feet
1,200feet
1,400 feet
—F 1,600feet
—I— 1,800feat
1,000
1,20- 0
Figure 44. A. Relation of groundwater -level decline to pit spacing for five different pit sizes aligned
approximately cross gradient to groundwater flow in the South Platte alluvial aquifer at a location 100
feet dcwngradient 15 years after pit reclamation, B. Relation of groundwater -level rise to pit spacing
for five different pit sizes aligned approximately cross gradient to groundwater flow in the South Platte
alluvial aquifer at a location 100 feet upgradient 15 years after pit reclamation.
90 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
the pit 800 ft upgradient. Increasing the offset distance of
the upgradient pit from 600 to 800 ft reduces the magnitude
of decline by less than 0.1 ft. Offsetting the center pit 200 ft
downgradient reduces the magnitude of the decline by about
l.0 ft, which is about 91 percent of the total reduction (1.1 ft)
in the magnitude of the decline achieved by an offset distance
of 800 ft. Increasing pit offset distance more than 200 ft
downgradient has little effect on reducing the magnitude of the
decline (fig. 45A) because the downgradient end of the pit is
beyond substantial hydrologic influence of the other pits.
The magnitude of groundwater -level rise upgradient
from the center pit responds to offset distance in a manner
similar to that for groundwater -level declines. The magnitude
of groundwater -level rise decreases most as the center pit is
offset from 0 to 200 ft and decreases less as pit offset distance
increases beyond 200 ft. Offsetting the center pit 200 ft upgra-
dient reduces the magnitude of groundwater -level rise by 1,3
ft (from about 2,5 to 1,2 ft), which is about 93 percent of the
total reduction (1.4 ft) in the magnitude of the rise achieved by
offsetting the pit 800 ft upgradient. Increasing pit offset farther
than 200 ft upgradient has relatively little effect on reducing
the magnitude of groundwater -level rise because the upgradi-
ent end of the pit is beyond substantial hydrologic influence
of the other pits. Offsetting the center pit 200 ft downgradi-
ent reduces the magnitude of groundwater -level rise by 1.2 ft
(from about 2.5 to 1.3 ft), which is about 71 percent of the
total reduction (about 1.7 ft) in the rise achieved by an offset
distance of 800 ft.
Summary of Simulated Pit Spacing and Configuration
Effects
Comparison of simulations related to pit spacing and
configuration indicates groundwater -level declines downgra-
dient from lined pits and groundwater -level rises upgradient
from lined pits increase as pit size increases and decrease as
pit spacing increases. The magnitude of groundwater -level
decline and rise decreases most as the spacing between pits is
increased from 0 to 200 feet, and the magnitude of decline and
rise decreases by successively lesser amounts as the distance
between pits is increased beyond 200 feet. For 1,200 -ft -wide
pits aligned approximately cross gradient to groundwater flow,
about 75 percent of the total reduction in the magnitude of
groundwater -level decline obtained by a pit spacing of 1,000
ft was achieved by a spacing of 400 ft. About 67 percent of
the total reduction in the magnitude of groundwater -level rise
obtained by a pit spacing of 1,000 ft was achieved by a pit
spacing of 400 ft. Offsetting the center pit upgradient or down -
gradient from other pits decreases the magnitude of groundwa-
ter -level decline and rise more so than increasing the distance
between aligned pits. For 1,200 -ft -wide pits, offsetting the
center pit 200 ft upgradient from adjacent pits achieves about
71 percent of the total reduction in the magnitude of the
decline and about 93 percent of the total reduction in magni-
tude of the rise that was obtained by offsetting the pit 800 ft
upgradient. Offsetting the center pit 200 ft downgradient from
adjacent pits achieves about 91 percent of the total reduction
in the magnitude of the decline and about 71 percent of the
total reduction in magnitude of the rise that was obtained by
offsetting the pit 800 ft downgradient. Offset pits with a spac-
ing of 200-400 ft provided a configuration that reduced the
hydrologic effects of lined pits by the greatest amount while
minimizing the distance between pits.
Model Limitations and Transferability
of Results
The numerical groundwater flow model presented by
this study is a representation of a real hydrologic system. The
accuracy of model simulations depends on the accuracy of
parameter values input to the model and the extent to which
important aspects of the hydrologic system are appropriately
represented. Because parameters were estimated based on
available data, measurement errors associated with the data are
incorporated in the model. Because parameters representing
recharge beneath irrigated land (RCH_irr), inflow along the
east side of the model (Q ReturnE), and hydraulic conductiv-
ity of zone 2 (LPF_Par2) have the highest composite scaled
sensitivities in the model, errors associated with these param-
eters likely have the largest influence on simulation results.
Errors associated with these parameters are estimated as small
to moderate based on available data. The following limitations
also need to be considered when interpreting simulation results
provided by the model;
Model input parameters (such as hydraulic conductiv-
ity and recharge) and boundaries (such as the altitude of
the top and base of the aquifer) are simulated as uniform
within each model cell. Variability in parameters and
boundaries smaller than the model cell size are not rep-
resented. Simulations that incorporate smaller -scale vari-
ability could produce results different than those shown,
2. Hydraulic head is computed at the center of each model
cell as the average of head conditions within the cell.
Analysis of the hydrologic effects of land -use change and
aggregate mining at a scale finer than the model cell size
(500 ft by 500 ft and 200 ft by 200 ft, respectively) used
in this study would require simulations with a finer grid
spacing.
3. The model is calibrated to steady-state average seasonal
water -table and groundwater -flow conditions by using
average seasonal parameter values. Although the model
simulates the dynamic seasonal nature of the aquifer, the
South Platte alluvial aquifer is a system with a continuous
response to hydrologic stresses. A fully transient calibra-
tion to hydrologic conditions of the aquifer could produce
results somewhat different than those shown.
4. The predicted effects of land -use change and aggregate
mining assume hydrologic conditions such as precipita-
Simulated Effects of Land -Use Change and Aggregate Mining on Groundwater Flow 91
A
GROUNDWATER -LEVEL DECLINE, IN FEET
GROUNDWATER -LEVEL RISE, IN FEET
0
05
10
15
20
25
30
0
T I
! ■
CENTER PIT OFFSET DIRECTION
Upgradient
f Downgradient
100 200 300 400 500 600 700 800 900
PIT OFFSET DISTANCE, IN FEET
300 400
500 600
,
100
1
200
r
CENTER PIT OFFSET DIRECTION
Upgradient '
—.— Downgradiont
I I
PIT OFFSET DISTANCE, IN FEET
r
700 000 95'3
Figure 45. A. Relation of groundwater -level decline to pit offset distance for three
1,200 -ft -wide lined pits in the South Platte alluvial aquifer at a location 100 feet downgradient
15 years after pit reclamation. B. Relation of groundwater -level rise to pit offset distance for
three 1,200 -ft -wide lined pits in the South Platte alluvial aquifer at a location 100 ft upgradient
15 years after pit reclamation.
92 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
tion, streamflow, and water use are the same during future
simulation periods as they were during 2000. Substantial
changes to these hydrologic conditions in the study area
could alter the effects of land -use change and aggregate
mining.
5. Simulations of the hydrologic effects of land -use change
and aggregate mining indicate areas where groundwater -
level changes might affect groundwater -supported
wetlands. However, determination of the specific effects
of groundwater -level changes on wetlands in the study
area would require site -specific investigation beyond the
scope of this report.
Simulation results presented by this study indicate the
potential hydrologic effects of land -use change and aggregate
mining on groundwater flow in the South Platte River val-
ley between Brighton and Fort Lupton and are specific to the
conditions described for each simulation, However, some of
the simulation results might be transferable to other areas hav-
ing hydrologic conditions similar to those of the study area.
Simulation results also provide general information about the
hydrologic effects of land -use change and aggregate mining
that could be applicable to other hydrologic systems. The
extent to which simulation results are transferable to other
areas depends on the extent to which conditions in other areas
are similar to those simulated. Knowledge of parameter sensi-
tivity can be used to help determine transferability of results.
Similarity between aquifers for parameters with high sensitiv-
ity, such as irrigation recharge, likely is more important than
similarity for parameters with low sensitivity.
Summary and Conclusions
To improve understanding of land -use change and the
potential effects of land -use change and aggregate mining on
groundwater flow, the U.S. Geological Survey, in cooperation
with the city of Fort Lupton and the city of Brighton, ana-
lyzed socioeconomic and land -use trends and constructed a
numerical groundwater flow model of the South Platte alluvial
aquifer in the Brighton —Fort Lupton area. As part of the study,
wetlands in the Brighton -Fort Lupton area were mapped using
false -color aerial photography and field investigations to indi-
cate locations where groundwater -level changes resulting from
land -use change or aggregate mining might affect wetlands.
Comparison of land use in 1957, 1977, and 2000 indi-
cated little change in the general distribution of irrigated
agricultural land and non -irrigated land over time, but both
land uses decreased slightly between 1957 and 2000, irri-
gated land use decreased 6 to 7 percent between 1957 and
[977 and between 1977 and 2000, whereas non -irrigated land
use decreased about 4 percent during each time period, By
contrast, urban land use increased about [65 percent between
1957 and 1977 and about 56 percent between 1977 and 2000.
In 2000, urban land represented about 13 percent of the total
Brighton —Fort Lupton area, whereas irrigated agricultural land
represented about 38 percent and non -irrigated land repre-
sented about 50 percent. Urban growth of Brighton and Fort
Lupton between 1957 and 2000 primarily has been east of the
South Platte River and along major transportation corridors.
As of late 1999, five aggregate mining operations were evident
in the Brighton —Fort Lupton area,
The urban -growth modeling program SLEUTH (Slope,
Land cover, Exclusion, Urbanization, Transportation, and
Hillshade) was used to predict future urban extent in the study
area in 2020 and 2040 based on historical urban extent and
growth in 1937, 1957, 1977, and 2000. SLEUTH simulations
predicted urban growth will continue to occur predominantly
to the east of Brighton and Fort Lupton and along major
transportation routes. However, substantial growth also was
predicted to the south and west of Brighton as areas of low
urban density are filled in. The potential extent of aggregate
mining in 2020 was estimated on the basis of existing min-
ing and reclamation plans. The potential extent of aggregate
mining in 2040 was estimated on the basis of the size, spacing,
and density of pit development in 2020 and represents condi-
tions when aggregate mining in the Brighton —Fort Lupton area
is approximately fully developed.
Wetlands in the study area were mapped by the U.S.
Geological Survey and U.S. Bureau of Reclamation during
July —August, 2004. False -color infrared aerial photographs
were obtained at 18 locations in the central and western parts
of the study area at a scale of 1:24,000. Preliminary photo
interpretation was performed to identify probable wetland
areas, and those areas were subsequently verified by field
inspection.
The numerical groundwater flow model used to simulate
the hydrologic effects of land -use change and aggregate min-
ing was calibrated to steady -slate groundwater -level and flow
conditions of the South Platte alluvial aquifer during the irri-
gation and non -irrigation seasons of 1957, 1977, and 2000 by
using the inverse modeling capabilities of MODFLOW-2000.
The calibrated model was used to simulate (I) steady-state
hydrologic effects of predicted land -use conditions in 2020
and 2040, (2) transient cumulative hydrologic effects of the
potential extent of reclaimed aggregate pits in 2020 and 2040,
(3) transient hydrologic effects of actively dewatered aggre-
gate pits, and (4) the effects of different hypothetical pit spac-
ings and configurations on groundwater levels,
Calibration statistics indicated residuals likely are
independent, random, and normal, and the calibrated model
regression likely is valid. However, the model is highly non-
linear, and linear confidence intervals on predictions would not
accurately represent prediction uncertainty. Composite scaled
sensitivities were calculated for parameters used in the cali-
brated model to evaluate the influence of each parameter on
the calibrated model. Composite scaled sensitivity was largest
for recharge beneath irrigated areas and lowest for recharge
beneath urban areas. All components of the simulated ground-
water budgets for the calibrated model represent reasonable
values compared to available data,
Summary and Conclusions 93
Steady-state simulations of the hydrologic effects
of land -use conditions in 2020 (simulation 1) and 2040
(simulation 2) indicated groundwater -level declines resulting
from conversion of irrigated and non -irrigated land to urban
areas were less than 2 ft relative to the irrigation -season water
table in 2000. Groundwater -level declines were largest where
irrigated agricultural land is converted to urban area because
of the large difference (about 12 in. during the irrigation
season) in recharge between the two land uses. Groundwater
levels changed little where non -irrigated land was converted
to urban area because estimated recharge beneath non -
irrigated land (about 0.4 in,) was only slightly greater than that
assumed for urban areas (0 in.). Groundwater -level declines
resulting from land -use conditions in 2020 and 2040 were
predicted to not substantially affect wetlands mapped by this
study or areas of riparian herbaceous vegetation mapped by
the Colorado Division of Wildlife in the study area because
the declines were small and mapped wetlands and areas of
riparian herbaceous vegetation generally are located where
little or no simulated decline occurred. The larger urban extent
ill simulations of land -use change in 2020 and 2040 decreased
recharge to the simulated aquifer by 17.9-31.6 percent, and
the resulting lower water table increased groundwater inflow
from the upgradient ends of South Platte River valley and
its tributaries by 2.7-4.2 percent and leakage from the South
Platte River and Big Dry Creek by 3.9-7.1 percent. Similarly,
discharge was decreased to the South Platte River and Big Dry
Creek (4.7-8,3 percent), Little Dry Creek and Third Creek
(3.9-8.7 percent), and the downgradient end of the South
Platte valley (1.6-3.3 percent).
Transient simulations of the cumulative hydrologic
effects of reclaimed pits in 2020 (simulation 3), reclaimed
lined pits in 2040 (simulation 4), and reclaimed unlined pits in
2040 (simulation 5) indicated lined pits caused groundwater
levels to rise upgradient from pits and decline downgradi-
ent from pits, whereas unlined pits had an opposite effect on
groundwater levels, The hydrologic effects of pits backfilled
with fine-grained sediments were similar to but less than
those of lined pits because the pits created less of a barrier to
groundwater flow than lined pits. The hydrologic effects of
lined, unlined, and fines-backfilled pits interacted to increase
the magnitude of groundwater -level changes at some locations
and decrease it at others, depending on the relative posi-
tion of the pits. The maximum simulated groundwater -level
decline resulting from the extent of reclaimed pits in 2020 was
about 9 ft in 2035, 15 years after reclamation. The maximum
simulated groundwater -level rise upgradient from lined pits
was about 6 ft in 2035, 15 years after reclamation. Ground-
water levels changed most during the first year after pits were
reclaimed, and groundwater levels ceased to change substan-
tially in most areas of the simulated aquifer within about 10
years. The addition of lined pits in 2040 resulted in a general
increase in the magnitude and extent of groundwater -level rise
relative to simulated conditions for 2020 because groundwater
flow was obstructed over a larger part of the simulated aquifer.
The maximum simulated groundwater -level decline and rise
resulting from the addition of reclaimed lined pits in 2040 was
about 9 ft in 2055, 15 years after reclamation. The addition
of unlined pits in 2040 resulted in a general increase in the
magnitude and extent of groundwater -level decline relative
to simulated conditions for 2020 because groundwater levels
declined to lake levels in unlined pits over a larger area of the
simulated aquifer. The maximum groundwater -level decline
resulting from the addition of reclaimed unlined pits in 2040
was about 11 ft in 2055, 15 years after reclamation. Maximum
groundwater -level rise resulting from the addition of reclaimed
unlined pits in 2040 was about 5 ft after 15 years.
Wetlands mapped by this study generally are located
where simulated groundwater -level changes resulting from
reclaimed pits in 2020 were less than 2 ft, but some areas
of riparian herbaceous vegetation mapped by the Colorado
Division of Wildlife are located where simulated groundwater -
level changes were more than 2 ft. Mapped wetlands and areas
of riparian herbaceous vegetation are both located where simu-
lated groundwater -level changes resulting from reclaimed pits
in 2040 were more than 2 ft. Wetlands and riparian herbaceous
vegetation located where groundwater -level changes are more
than 2 ft might be affected by the changes. Some areas where
groundwater levels were simulated to rise more than 2 ft at
locations where the depth to water was less than 5 ft could cre-
ate conditions favorable to the formation of new wetlands.
The groundwater budgets for simulations of reclaimed
pits in 2020 and 2040 were greater than the budget for the cali-
brated model or simulations of the effects of land -use change
because of additional groundwater inflow and outflow at
unlined pits simulated as lakes in 2020 and 2040. The conver-
sion of land area to pits (lined and unlined) in 2020 decreased
distributed recharge at the land surface by 16.8 percent relative
to the 2000 irrigation season because the simulated pits do not
contribute distributed recharge. Groundwater -level declines
resulting from the extent of reclaimed pits in 2020 increased
leakage from the South Platte River and Big Dry Creek by
19.0 percent, and lined pits obstructed groundwater outflow at
the downgradient end of the South Platte River valley, caus-
ing outflow to decrease by 14.0 percent. The general effect
of adding lined pits in 2040 was to decrease leakage from
unlined pits (22.9 percent), distributed recharge at the land
surface (14.6 percent), groundwater discharge to unlined pits
(21.3 percent), and groundwater discharge to the South Platte
River and Big Dry Creek (7.0 percent) because the additional
lined pits obstruct groundwater flow in the simulated aqui-
fer. The general effect of adding unlined pits in 2040 was to
increase leakage from unlined pits (204 percent), leakage
from the South Platte River and Big Dry Creek (15.3 percent),
discharge to unlined pits (207 percent), and discharge to the
South Platte River and Big Dry Creek (3.9 percent) and to
decrease groundwater entering storage (25.4 percent).
Transient simulations of the hydrologic effects of actively
dewatered pits (simulations 6-9) indicated that the magnitude
and extent of drawdown increased as the number of dewa-
tered pits increased because the drawdown effects of each pit
were additive. Increasing the distance between two actively
94 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
dewatered pits from 400 ft to 2,000 ft did not reduce draw -
down extent, because the drawdown resulting from each pit
combined to create a single larger drawdown extent. Maxi-
mum drawdown in all simulations of actively dewatered pits
was 37 ft. Drawdown decreased to 20 ft within 100-600 ft of
actively dewatered pits and decreased more gradually farther
from the pits. Maximum drawdown extent defined by the limit
of 2 -ft drawdown was about 12,400 ft for a single dewatered
pit, 12,800 ft for two pits spaced 400 ft apart, 12,900 ft for
two pits spaced 2,000 ft apart, and 13,100 ft for three pits
spaced 400 ft apart. Drawdown was affected by the presence
of the South Platte River, the no -flow west model boundary,
and nearby reclaimed pits. Simulated flow from the South
Platte River and unlined reclaimed pits decreased drawdown
resulting from dewatered pits, whereas the no -flow west model
boundary increased drawdown. Most drawdown occurred
during the first year, and drawdown extent after 1 year was
almost as large as after 15 years. Because dewatering typically
occurs for multiple years and most drawdown occurred during
the first year, groundwater -supported wetlands overlying areas
of drawdown resulting from dewatered pits might be affected
by drawdown resulting from the pits. The general effect of
actively dewatered pits was to increase groundwater inflow
from the upgradient ends of the South Platte River valley and
its tributaries, increase leakage from the South Platte River
and Big Dry Creek. and decrease most other components of
the water budget.
Transient simulations of the hydrologic effects of differ-
ent hypothetical pit spacings and configurations indicated that
groundwater -level declines and rises resulting from reclaimed
lined pits increased with pit size and decreased as the distance
between pits increased, Groundwater -level decline and rise
resulting from three contiguous 1,000 -ft -wide pits aligned
approximately cross gradient to groundwater flow were about
2 ft near the center pit, whereas groundwater -level decline
and rise resulting from three contiguous 1,800 -ft -wide pits
were about 4 ft. The magnitude of groundwater -level change
near pits decreased most as pit spacing was increased from
0 to 200 ft, and the magnitude decreased less as the distance
between pits became larger. For 1,200 -ft -wide pits aligned
approximately cross gradient to groundwater flow, about 50
percent of the total reduction in the magnitude of groundwa-
ter -level decline that was obtained by a pit spacing of 1,000 ft
was achieved by a pit spacing of only 200 ft, and about 75
percent of the total reduction in the magnitude of the decline
was achieved by a pit spacing of 400 ft. About 42 percent of
the total reduction in the magnitude of groundwater -level rise
that was obtained by a pit spacing of 1,000 ft was achieved by
a pit spacing of 200 ft, and about 67 percent of the total reduc-
tion in the magnitude of the rise was achieved by a pit spacing
of 400 ft.
Offsetting the center pit upgradient or downgradient of
other pits decreased groundwater -level declines and rises to
a greater extent than increasing the distance between aligned
pits. For 1,200 -ft -wide pits, offsetting the center pit 200 ft
upgradient from adjacent pits achieved about 71 percent of the
total reduction in the magnitude of groundwater -level decline
and about 93 percent of the total reduction in the magnitude of
groundwater -level rise that was obtained by offsetting the pit
800 ft upgradient. Offsetting the center pit 200 ft downgradi-
ent from adjacent pits achieved about 91 percent of the total
reduction in the magnitude of groundwater -level decline and
about 71 percent of the total reduction in the magnitude of
groundwater -level rise that was obtained by offsetting the pit
800 ft downgradient. Offset pits with a spacing of 200-400 ft
provided a configuration that reduced the hydrologic effects
of lined pits by the greatest amount while minimizing the
distance between pits.
Acknowledgments
Many individuals contributed to the completion of this
study. Thanks are extended to Jim Sartoris of USGS and David
Salas of the U.S. Bureau of Reclamation for their efforts
mapping and verifying wetlands for the study. Thanks also are
extended to USGS geographers Susan Guthrie, for compiling
numerous Geographic Information System data sets that pro-
vided information for the numerical groundwater flow model,
and Mark Feller, for running the SLEUTH urban -growth
model to predict future urban extent in the study area, Special
thanks are extended to Lafarge Mining Company for provid-
ing a tour of an aggregate mine site where operations could
be observed directly. Carl Mount of the Colorado Division of
Mining, Reclamation, and Safety provided useful informa-
tion about mining regulations and the pit permitting process,
as well as aggregate -mine records for the study area. Carl
Eiberger of Black Bear Water Resources provided ground-
water -level data and information concerning the location of
lined water -storage facilities in the study area. Ned Banta of
the USGS provided technical assistance that was helpful to
the completion of the study. The contributions of each of these
individuals are gratefully acknowledged.
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p.331-341.
Townley, L.R., 1995, The response of aquifers to periodic
forcing: Advances in Water Resources, v. 18, p. 4,
795-4812,
98 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
Trimble, D.E., and Machette, M.N„ 1979, Geologic map
of the greater Denver area, Front Range Urban Corridor,
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Investigations Map 1-856—H, scale 1:100,000.
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the United States: Environmental Science Services
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gov/regional/reist
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Resolution Land Use and Land Cover 1996/1997 Front
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Resolution Land Use and Land Cover 1937/1938 Front
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19, 2004, athilp./'rockyweb crusgs.gov/fronirange/datai-
ets.htm.
U.S. Geological Survey, 2001 b, Coverage LU50—High-
Resolution Land Use and Land Cover 1953-1958 Front
Range Infrastructure Resources Project Demonstration
Area: U.S. Geological Survey spatial dataset,
accessed May [9, 2004, at http://rockyveb.cr.usgs.gov/
frontrange/datasets him.
U.S. Geological Survey, 2001c, Coverage LU70—High-
Resolution Land Use and Land Cover 1977/1978 Front
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accessed May 19, 2004, at htlp://rockyrveb.cr:usg.s.gov/
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datasets. him.
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h ttp: //i vaterdata. usgs. gov/co/mvis/s w.
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wrs. html.
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CLIMATi/DATA him!.
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Appendix 99
Appendix —Color infrared aerial
photographs used by this study to map
wetlands and surface water in the
South Platte River valley, Brighton to
Fort Lupton, Colo.
(Locations of aerial photograph centroids are shown in
figure 20.)
The date (mmrdd—yy), time (photographs l —I, 1-9, 2—I, and
2-9 only), scale at which photographs were taken, photograph
project name, and photograph number are shown at the top of
each photograph.
100 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
Photograph 1-1.
Appendix 101
Photograph 1-2.
102 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
Photograph 1-3.
Appendix 103
Photograph 1-4.
104 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo,
Photograph 1-5,
Appendix 105
Photograph 1-6.
106 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
Photograph 1-7.
Appendix 107
Photograph 1—B.
10B Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
Photograph 1-9.
Appendix 109
Photograph 2-1.
110 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
Photograph 2-2.
Appendix 111
Photograph 2-3.
112 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
Photograph 2-4.
Appendix 113
Photograph 2-5.
114 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
Photograph 2-6.
Appendix 115
Photograph 2-7.
116 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
Photograph 2-8.
Appendix 117
Photograph 2—g.
Publishing support provided by:
Denver Publishing Service Center
For more information concerning this publication, contact:
Director, USGS Colorado Water Science Center
Box 25046, Mail Stop 415
Denver, CO 80225
(303)236-4882
Or visit the Colorado Water Science Center Web site at
http://co.waterusgs.gov/
ATTACHMENTS
36 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Cola,
10492W
1Q'4°5D'W
104' 46"w
104'46'W
Bachofer Property]
Simms modified from U S Geological Survey National Hydrography Datasai, I; (00,000
Roads modified from Colorado Department of Transportation
Noarth American Daum of 1983
EXPLANATION
Pit Completion
Lined
I unlined.
13ackfilled with fines
Overburden backill
or unknown
Approximate extent of
alluvial aqui£ot
-
a
1
I
Study Area Boundary
2 MILES
I I 1 l
1 ' 1
4 I 2 KILOMETERS
Figure 19A, Predicted extent of aggregate mining in 2020, Brighton to Fort Lupton, Colorado.
Simulated Effects of Land•Use Change and Aggregate Mining on Groundwater Flow 69
Sneatns modified from U S Geological Survey National
l°iyd'rngrsily Datasel; I : E °COO)
Roth modified from Colorado Department ofTrsnspertatian
Nmlii American Tatum of [983
lB achole r Prcapel*
EXPLANATION
Lined pit
Untitled pit
Fines-hackflled pit
IIWetland mapped as part of this elutdy
Riparian herbaceous vegetation indicated by
Colorado Division ofWildlife (2007a, b)
Sirnulsftcd groundwater -level change, in feet
Positive vaaluea indicate groundwater-levet rise.
Negative values indicate groundwater -level decline.
4to6
2 ter4
to -2
j— to -2
to -4
—8, to
—IQ to -8
�••••••Limit oi"simulated aquifer
Locations of hydregraph presented in
egnres'35A and 33D
0 5,000 10,000 FEET
rat li' 1
0 10000 2,t'>X0 0,000 METERS
Figure 34. Simulation 3 —Groundwater -level changes in 2035 resulting from the potential extent of
reclaimed aggregate pits in 2020, Brighton to Fort Lupton, Colorado.
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USES Surface Water for Colorado: Peak Streamflaw
1940 Jul. 03, 1940
1941 Jun. 23, 1941
1942 Apr. 26, 1942
1943 May 09, 1943
1944 May 18, 1944
1945 Aug. 06, 1945
1946 Sep. 08, 1946
3.99
4.41
7.24
2.47
4.26
5.70
3.83
1,8806
2,4106
9,0006
1,0106
2,5206
4,2406
1,9706
tt
M'(( age 2 of 2
2003 May 11, 2003
2004 Aug. 19, 2004
2005 Aug. 05, 2005
2006 Oct. 11, 2005
2007 Apr. 25, 2007
2008 Aug. 16, 2008
2009 Jun. 02, 2009
7.526
9.93
9.12
7.81
10.44
8.99
9.70
3,2106
6,4406
5,2406
3,5506
8,5406
5,7406
7,0406
1i Peak Gage -Height Qualification Codes.
• 6 -- Gage datum changed during this year
Peak Streamflow Qualification Codes.
• 2 -- Discharge is an Estimate
. 6 -- Discharge affected by Regulation or Diversion
Questions_about sites/data?
Feedback on this web site
Automated retrievals
Help,
Top
Explanation of terms
Subscrtie for system changes
News
Accessibility FOIA Privacy Policies and Notices
Department of the interior I €J.S. Geological Survey
Title: Surface Water for Colorado: Peak Streamflaw
URL: http: / /waterdata.usgs.gav/ca/nwis/peek?
Page Contact Information: Colorado Water -Data Support Team,
Page Last Modified: 2010-06-06 18:35:05 EDT
L44 1.45 nadwwOl
sA.gov.,
TAX Prone:
�n�HigR�GA
USGS Surface Water for Colorado: Peak Strearnfl
Page 2 of 3
1937 Jun. 02, 1937
1938 May 30, 1938
1939 Mar, 11/ 1939
1940 Jul. 03, 1940
1941 Jun. 22, 1941
1942 Apr. 26, 1942
1943 May 09, 1943
1944 May 17, 1944
1945 Aug. 06, 1945
1946 Sep. 08, 1946
1947 Jun. 22, 1947
1948 May 31, 1948
1949 Jun. 14, 1949
1950 Jun. 17, 195Q
1951 Aug. 03, 1951
1952 Jun. 10, 1952
1953 Jul. 09, 1953
1954 Jul. 14, 1954
1955 Aug. 28, 1955
1956 Aug. 01, 1956
1957 May 09/ 1957
1958 May 25, 1958
1959 May 21, 1959
1960 Jul. 04, 1960
1961 Jul. 31, 1961
1962 Jun. 29, 1962
1963 Jun. 16, 1963
1964 May 29, 1964
1965 Jun. 17, 1965
1966 Sep. 02, 1966
3,2006
4,4806
5,7306
2,4106
3,7106
10,7006
9626
2,7206
5,7206
2,1906
5,6706
7,9206
8,8506
3,5206
3,8906
2,5006
2/8406
4.05 1,8806
4.78 3,2806
5.53
11.35
6.06
4.96
4.736
5.40
4.14
5.97
^6.17
1 2.93
4.19
3,9706
14,8006
4,5606
3,2006
2,2406
3,1606
1,7506
3,9406
4,4606
29,6006
1,7506
1978 May 01, 1978
1979 May 02, 1979
1980 May 16, 1980
1981 Jun. 03, 1981
1982 May 13, 1982
1983 Jun. 27, 1983
1984 Aug. 21, 1984
1985 Jul. 20, 1985
1986 Jul. 20, 1986
1987 Jury. 09, 1987
1988 Aug. 04, 1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2O00
2001
2002
2003
2004
2005
2006
2007
Jun. 03, 1989
May 29, 1990
Jun. 01, 1991
Aug. 24, 1992
Jun. 18, 1993
Aug. 11, 1994
May 17, 1995
May 26, 1996
Jul. 29, 1997
Jul. 26, 1998
Apr. 30, 1999
Jul. 17, 2000
Jul. 08, 2001
Sep. 12, 2002
May 10, 2003
Aug. 19, 2004
Aug. 04, 2005
Oct. 10, 2005
Apr. 25, 2007
5.52 4,1906
7.41 7,6006
7.022 7,3106
4.67 2,9106
6,19 5,0506
7.58 12,3006
6.73 10,8006
6.74 12,1006
7.396 6,2106
9.50 11,0006
9.12 9,3906
8.89 8,6006
8.87 8,2106
9.06 9,7206
9.80 11,4006
6.88 4,2606
6.84 4,1806
9.91 11,6006.
8.22 7,5606
8.46 7,9606
8.81 9,3606
8.84 9,7206
8.58 8,4606
7.88 7,0806
7.42 4,6706
7.57 5,1306
9.98 9,0506
8.58 6,0606
7.31 4,0406
9.49 8,1206
i Peak Gage -Height Qualification Codes.
• 2 -- Gage height not the maximum for the year
July 9, 2020
Petitioner:
BACHOFER ROSS (BN)
7525 US HIGHWAY 85
FORT LUPTON, CO 80621-8809
CLERK TO THE BOARD
PHONE (970) 400-4226
FAX (970) 336-7233
WEBSITE: www.weldgov.com
1150 O STREET
P.O. BOX 758
GREELEY CO 80632
Agent (if applicable):
RE: THE BOARD OF EQUALIZATION 2020, WELD COUNTY, COLORADO
NOTIFICATION OF HEARING SCHEDULED
Docket 2020-2029, AS0106 Appeal 2008226216 Hearing 7/27/20202:00 PM
Account(s) Appealed:
R5270886
Dear Petitioner(s):
The Weld County Board of Equalization has set a date of July 27, 2020, at or about the hour of 2:00
PM, to hold a hearing on your valuation for assessment. This hearing will be held at the Weld
County Administration Building, Assembly Room, 1150 O Street, Greeley, Colorado.
You have a right to attend this hearing and present evidence in support of your petition. The Weld
County Assessor or his designee will be present. The Board will make its decision on the basis of
the record made at the aforementioned hearing, as well as your petition, so it would be in your
interest to have a representative present. If you plan to be represented by an agent or an attorney
at your hearing, prior to the hearing you shall provide, in writing to the Clerk to the Board's Office, an
authorization for the agent or attorney to represent you. If you do not choose to attend this hearing,
a decision will still be made by the Board by the close of business on August 5, 2020, and mailed to
you within five (5) business days.
Because of the volume of cases before the Board of Equalization, most cases shall be limited to 10
minutes. Also due to volume, cases cannot be rescheduled. It is imperative that you provide
evidence to support your position. This may include evidence that similar homes in your area are
valued less than yours or you are being assessed on improvements you do not have. Please note:
The fact that your valuation has increased cannot be your sole basis of appeal. Without
documented evidence as indicated above, the Board will have no choice but to deny your appeal.
If you wish to discuss your value with the Assessor's Office, please call them at (970) 400-3650. If
you wish to obtain the data supporting the Assessor's valuation of your property, please submit a
written request to assessor@weldgov.com. Upon receipt of your written request, the Assessor will
notify you of the estimated cost of providing such information. Payment must be made prior to the
Assessor providing such information, at which time the Assessor will make the data available within
three (3) working days, subject to any confidentiality requirements.
Please advise me if you decide not to keep your appointment as scheduled. If you need any
additional information, please call me at your convenience.
Very truly yours,
BOARD OF EQUALIZATION
az,L.,d „rc,G
Esther E. Gesick
Clerk to the Board
Weld County Board of Commissioners
and Board of Equalization
cc: Brenda Dones, Assessor
057 11-XV
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a
GANSER LUJAN & ASSOCIATES
Hydrogeolog►cal and Env►ro►unental Consultants
toward the northeast and discharge into the South Platte River adjacent to the
Bachofer property This discharge will increase surface flows in the channel
• The results of Deere and Ault's HEC-RAS surface model and appear to be unreliable
and unrealistic when compared to actual stream flow data measured at the nearby
Fort Lupton gauging station Peak flows during recent flooding measured 9,200 cfs
at this station on 9/13/2013, while the water level was at 4877 6 feet The model
results produced a 100 year level at 4876 5 feet with flows of 29,000 cfs The model
results should not be used as a basis to re-establish a new 100 -year flood plain map
o Groundwater mounding that occurs on the west side due to the impermeable slurry
wall around L G Everest pits has created a higher heads in this area and as a
consequence , the alluvial aquifer is discharging and contributing more water to
surface flows in the river than what was occurring prior to the slurry wall
construction
o Deere & Ault have misconstrued the results of the USGS Report 2010-5019 The
presence of mounding 2 to 4 feet above normal levels means that in addition to the
actual length of the slurry wall, a hydraulic barrier to flow has formed beyond the
edges of the slurry wall that will block nearly all alluvial flow This means that
essentially no alluvial groundwater can flow northward to discharge areas as in the
past As a result, nearly all of this flow will be routed into the river channel as either
alluvial or surface flow ultimately incieasmg the base -flow and stage level in the
river in this area
o There is some evidence to suggest that higher groundwater levels exist on the east of
the river at or about the time that the pits were lined Data obtained from the
Colorado Division of Water Resources well listing report shows two wells in the
vicinity of the Bachofer property with a continuous monitoring record that dates
back to 1989 and 1994
• Historic aerial photos from Google Earth from 2002 to 2012 show that the liver
morphology has changed A noticeable increase in the formation of sand bars has
occurred in a reach of the river from approximately 1/2 mile upstream of the Bachofer
property to the Fort Lupton gauging station %2 mile downstream meaning that the
river stage has increased and caused flooding of the subject property
o There is ample evidence that indicates construction of the sand and gravel pits
located on the west of the South Platte River from the Bachofer property has
disturbed the natural topographic surface of the area Prior to construction of the
Golden cell, the land was below the 100 year flood plain elevation of 4874 and thus
could mitigate a flood swell or higher stage in the river that was above this value
Based on a pre -mining topographic map developed by Deere and Ault (11/25/2008)
and a survey of the Golden Cell by King Surveyors (5/19/2009), approximately 60
percent of this area that was once part of the flood fringe that could mitigate flood
stage levels has been removed More importantly, the Golden Cell together with
19
GANSER LUJAN & ASSOCIATES
Hydrogeological and Environmental Consultants
I
L G Everest's pits to the west has removed approximately 90 percent of the area that
was historically at or below the 100 year level Removal of this area has
significantly impacted the ability of the river to dissipate stage levels and prevent
flooding on the east side of the nver
Prepared by Reviewed by
yc._e 1)(4,
Joel M. Sobol Donald R Ganser, P G
Senior Hydrogeologist Principal Hydrogeologist
20
I
Simulated Effects of Land -Use Change and Aggregate Mining on Groundwater Flow 73
40°06'N
40°04'N
40°02'N
40°N
39°58'N
104550'W
104°48'W
Streams modified from U.S. Geological Survey National
Hydrography Datasct: I :100,000
Roads modified from Colorado Department of Transportation
North American Datum of 1983
0
i
c
> 1:
L
EXPLANATION
lined pit
Unlined pit
Fines-backfilled pit
Wetland mapped as part of this study
EXHIBIT
1
2s4270 into
Riparian herbaceous vegetation indicated by
Colorado Division of Wildlife (2007a, b)
Simulated groundwater -level change, in feet
Positive values indicate groundwater -level rise.
Negative values indicate groundwater -level decline.
8 t 10
6to8
4to6
2 to 4
—2 to —2
-4 to —2
—6 to 4
—8 to —6
—10 to —8
Limit of simulated aquifer
5,000
l I I I
1
10,000 FEET
0 1,000 2,000 3,000 METERS
Figure 36. Simulation 4 Groundwater -level changes in 2055 resulting from the potential extent of
reclaimed aggregate pits in 2040, Brighton to Fort Lupton, Colorado. Pits added after 2020 are simulated as
lined.
74 Land -Use Analysis and Simulated Effects of Land -Use Change and Aggregate Mining, South Platte River Valley, Colo.
104°50'W
1 34 48 Vi
Streams modified from U.S. Geological Survey National
Hydrography Dataset; 1:100,000
Roads modified from Colorado Department of Transportation
North American Datum of 1983
EXPLANATION
lined pit
Unlined pit
Fines-backfilled pit
Wetland mapped as part of this study
Riparian herbaceous vegetation indicated by -
Colorado Division of Wildlife (2007a, b)
Simulated groundwater -level change, in feet
Positive values indicate groundwater -level rise.
Negative values indicate groundwater -level decline.
4to6
2 to 4
—2to2
—4to-2
—6 to --4
— 8to-6
— 10 to -8
— 12 to -10
a Limit of simulated aquifer
0
i
5,000
ll
10,000 FEET
0 1,000 2,000 3,000 METERS
Figure 37. Simulation 5 Groundwater -level changes in 2055 resulting from the predicted extent of reclaimed
aggregate pits in 2040, Brighton to Fort Lupton, Colorado. Pits added after 2020 are simulated as unlined.
July 28, 2020
Petitioner:
BACHOFER ROSS (BN)
7525 US HIGHWAY 85
FORT LUPTON, CO 80621-8809
CLERK TO THE BOARD
PHONE (970) 400-4226
FAX (970) 336-7233
WEBSITE: www.weldgov.com
1150 O STREET
P.O. BOX 758
GREELEY CO 80632
Agent (if applicable):
RE: THE BOARD OF EQUALIZATION 2020, WELD COUNTY, COLORADO
NOTICE OF DECISION
Docket 2020-2029 Appeal 2008226216 Hearing 7/27/2020 2:00 PM
Dear Petitioner:
On the day indicated above, the Board of County Commissioners of Weld County Colorado convened and
acting as the Board of Equalization, pursuant to C.R.S. §39-8-101 et seq., considered petition for appeal of
the Weld County Assessor's valuation of your property described above, for the year 2020.
Account # Decision
The Assessment and valuation is set as follows:
Actual Value as Actual Value as Set by
Determined by Assessor Board
R5270886 Deny - Denied in Full
$227,000 $227,000
A denial of a petition, in whole or in part, by the Board of Equalization must be appealed within thirty (30)
days of the date the denial is mailed to you. You must select only one of the following three (3)
options for appeal:
1. Appeal to Board of Assessment Appeals: You have the right to appeal the County Board of
Equalization's decision to the Colorado Board of Assessment Appeals. A hearing before that Board
will be the last time you may present testimony or exhibits or other evidence, or call witnesses in
support of your valuation. If the decision of the Board of Assessment Appeals is further appealed to
the Court of Appeals pursuant to C.R.S. §39-8-108(2), only the record of proceedings from your
hearing before the Board of Assessment Appeals and your legal brief are filed with the appellate
court.
All appeals to the Board of Assessment Appeals filed after August 10, 2020, MUST comply with the
following provisions of C.R.S. §39-8-107(5):
(5)(a)(I) On and after August 10, 2020, in addition to any other requirements under law, any petitioner
appealing either a valuation of rent -producing commercial real property to the Board of Assessment Appeals
pursuant to C.R.S. §39-8-108(1) or a denial of an abatement of taxes pursuant to C.R.S. §39-10-114 shall
provide to the County Board of Equalization or to the Board of County Commissioners of the County in the
case of an abatement, and not to the Board of Assessment Appeals, the following information, if applicable:
(A) Actual annual rental income for two full years including the base year for the relevant property tax year;
(B) Tenant reimbursements for two full years including the base year for the relevant property tax year;
(C) Itemized expenses for two full years including the base year for the relevant property tax year; and
(D) Rent roll data, including the name of any tenants, the address, unit, or suite number of the subject
property, lease start and end dates, option terms, base rent, square footage leased, and vacant space for
two full years including the base year for the relevant property tax year.
(II) The petitioner shall provide the information required by subparagraph (I) of this paragraph (a) within
ninety days after the appeal has been filed with the Board of Assessment Appeals.
(b)(I) The Assessor, the County Board of Equalization, or the Board of County Commissioners of the
County, as applicable, shall, upon request made by the petitioner, provide to a petitioner who has filed an
appeal with the Board of Assessment Appeals not more than ninety days after receipt of the petitioner's
request, the following information:
(A) All of the underlying data used by the county in calculating the value of the subject property that is being
appealed, including the capitalization rate for such property; and
(B) The names of any commercially available and copyrighted publications used in calculating the value of
the subject property.
(II) The party providing the information to the petitioner pursuant to subparagraph (I) of this paragraph (b)
shall redact all confidential information contained therein.
(c) If a petitioner fails to provide the information required by subparagraph (I) of paragraph (a) of this
subsection (5) by the deadline specified in subparagraph (II) of said paragraph (a), the County may move
the Board of Assessment Appeals to compel disclosure and to issue appropriate sanctions for
noncompliance with such order. The motion may be made directly by the County Attorney and shall be
accompanied by a certification that the County Assessor or the County Board of Equalization has in good
faith conferred or attempted to confer with such petitioner in an effort to obtain the information without action
by the Board of Assessment Appeals. If an order compelling disclosure is issued under this paragraph (c)
and the petitioner fails to comply with such order, the Board of Assessment Appeals may make such orders
in regard to the noncompliance as are just and reasonable under the circumstances, including an order
dismissing the action or the entry of a judgment by default against the petitioner. Interest due the taxpayer
shall cease to accrue as of the date the order compelling disclosure is issued, and the accrual of interest
shall resume as of the date the contested information has been provided by the taxpayer.
Appeals to the Board of Assessment Appeals must be made on forms furnished by that Board, and must be
mailed or delivered within thirty (30) days of the date the denial by the Board of Equalization is mailed to
you
The address and telephone number of the Board of Assessment Appeals are:
Board of Assessment Appeals
1313 Sherman Street, Room 315
Denver, Colorado 80203
Telephone Number: (303) 864-7710
Email: baa@state.co.us
Fees for Appeal to the Board of Assessment Appeals: A taxpayer representing himself is not charged for the
first two (2) appeals to the Board of Assessment Appeals. A taxpayer represented by an attorney or agent
must pay a fee of $101.25 per appeal.
OR
2. Appeal to District Court: You have the right to appeal the decision of the Board of Equalization to the
District Court of the county wherein your property is located: in this case that is Weld County District
Court. A hearing before The District Court will be the last time you may present testimony or exhibits
or other evidence, or call witnesses in support of your valuation. If the decision of the District Court is
further appealed to the Court of Appeals pursuant to C.R.S. §39-8-108(1), the rules of Colorado
appellate review and C.R.S. §24-4-106(9), govern the process.
OR
3. Binding Arbitration: You have the right to submit your case to binding arbitration. If you choose this
option, the arbitrator's decision is final and you have no further right to appeal your current
valuation. C.R.S. §39-8-108.5 governs this process. The arbitration process involves the following:
a. Select an Arbitrator: You must notify the Board of Equalization that you will pursue
arbitration. You and the Board of Equalization will select an arbitrator from the official list of
qualified people. If you cannot agree on an arbitrator, the District Court of the county in which
the property is located (i.e. Weld) will select the arbitrator.
b. Arbitration Hearing Procedure: Arbitration hearings are held within sixty (60) days from the
date the arbitrator is selected, and are set by the arbitrator. Both you and the Board of
Equalization are entitled to participate in the hearing. The hearing is informal. The arbitrator
has the authority to issue subpoenas for witnesses, books, records documents and other
evidence pertaining to the value of the property. The arbitrator also has the authority to
administer oaths, and determine all questions of law and fact presented to him. The
arbitration hearing may be confidential and closed to the public if you and the Board of
Equalization agree. The arbitrator's decision must be delivered personally or by registered
mail within ten (10) days of the arbitration hearing.
c. Fees and Expenses: The arbitrator's fees and expenses are agreed upon by you and the
Board of Equalization. In the case of residential real property, the fess may not exceed
$150.00 per case. For cases other than residential real property, the arbitrator's total fees
and expenses are agreed to by you and Board of Equalization, but are paid by the parties as
ordered by the arbitrator.
If you have questions concerning the above information, please call me at (970) 400-4226.
Very truly yours,
BOARD OF EQUALIZATION
LeL� i!"lv:yfG C.
Esther E. Gesick
Clerk to the Board
Weld County Board of Commissioners
and Board of Equalization
cc: Brenda Dones, Weld County Assessor
July 28, 2020
Petitioner:
BACHOFER ROSS (BN)
7525 US HIGHWAY 85
FORT LUPTON, CO 80621-8809
CLERK TO THE BOARD
PHONE (970) 400-4226
FAX (970) 336-7233
WEBSITE: www.weldgov.com
1150 O STREET
P.O. BOX 758
GREELEY CO 80632
Agent (if applicable):
RE: THE BOARD OF EQUALIZATION 2020, WELD COUNTY, COLORADO
NOTICE OF DECISION
Docket 2020-2029 Appeal 2008226216 Hearing 7/27/2020 2:00 PM
Dear Petitioner:
On the day indicated above, the Board of County Commissioners of Weld County Colorado convened and
acting as the Board of Equalization, pursuant to C.R.S. §39-8-101 et seq., considered petition for appeal of
the Weld County Assessor's valuation of your property described above, for the year 2020.
Account # Decision
The Assessment and valuation is set as follows:
Actual Value as Actual Value as Set by
Determined by Assessor Board
R5270886 Deny - Denied in Full
$227,000 $227,000
A denial of a petition, in whole or in part, by the Board of Equalization must be appealed within thirty (30)
days of the date the denial is mailed to you. You must select only one of the following three (3)
options for appeal:
1. Appeal to Board of Assessment Appeals: You have the right to appeal the County Board of
Equalization's decision to the Colorado Board of Assessment Appeals. A hearing before that Board
will be the last time you may present testimony or exhibits or other evidence, or call witnesses in
support of your valuation. If the decision of the Board of Assessment Appeals is further appealed to
the Court of Appeals pursuant to C.R.S. §39-8-108(2), only the record of proceedings from your
hearing before the Board of Assessment Appeals and your legal brief are filed with the appellate
court.
All appeals to the Board of Assessment Appeals filed after August 10, 2020, MUST comply with the
following provisions of C.R.S. §39-8-107(5):
(5)(a)(I) On and after August 10, 2020, in addition to any other requirements under law, any petitioner
appealing either a valuation of rent -producing commercial real property to the Board of Assessment Appeals
pursuant to C.R.S. §39-8-108(1) or a denial of an abatement of taxes pursuant to C.R.S. §39-10-114 shall
provide to the County Board of Equalization or to the Board of County Commissioners of the County in the
case of an abatement, and not to the Board of Assessment Appeals, the following information, if applicable:
(A) Actual annual rental income for two full years including the base year for the relevant property tax year;
(B) Tenant reimbursements for two full years including the base year for the relevant property tax year;
(C) Itemized expenses for two full years including the base year for the relevant property tax year; and
(D) Rent roll data, including the name of any tenants, the address, unit, or suite number of the subject
property, lease start and end dates, option terms, base rent, square footage leased, and vacant space for
two full years including the base year for the relevant property tax year.
(II) The petitioner shall provide the information required by subparagraph (I) of this paragraph (a) within
ninety days after the appeal has been filed with the Board of Assessment Appeals.
(b)(I) The Assessor, the County Board of Equalization, or the Board of County Commissioners of the
County, as applicable, shall, upon request made by the petitioner, provide to a petitioner who has filed an
appeal with the Board of Assessment Appeals not more than ninety days after receipt of the petitioner's
request, the following information:
(A) All of the underlying data used by the county in calculating the value of the subject property that is being
appealed, including the capitalization rate for such property; and
(B) The names of any commercially available and copyrighted publications used in calculating the value of
the subject property.
(II) The party providing the information to the petitioner pursuant to subparagraph (I) of this paragraph (b)
shall redact all confidential information contained therein.
(c) If a petitioner fails to provide the information required by subparagraph (I) of paragraph (a) of this
subsection (5) by the deadline specified in subparagraph (II) of said paragraph (a), the County may move
the Board of Assessment Appeals to compel disclosure and to issue appropriate sanctions for
noncompliance with such order. The motion may be made directly by the County Attorney and shall be
accompanied by a certification that the County Assessor or the County Board of Equalization has in good
faith conferred or attempted to confer with such petitioner in an effort to obtain the information without action
by the Board of Assessment Appeals. If an order compelling disclosure is issued under this paragraph (c)
and the petitioner fails to comply with such order, the Board of Assessment Appeals may make such orders
in regard to the noncompliance as are just and reasonable under the circumstances, including an order
dismissing the action or the entry of a judgment by default against the petitioner. Interest due the taxpayer
shall cease to accrue as of the date the order compelling disclosure is issued, and the accrual of interest
shall resume as of the date the contested information has been provided by the taxpayer.
Appeals to the Board of Assessment Appeals must be made on forms furnished by that Board, and must be
mailed or delivered within thirty (30) days of the date the denial by the Board of Equalization is mailed to
you
The address and telephone number of the Board of Assessment Appeals are:
Board of Assessment Appeals
1313 Sherman Street, Room 315
Denver, Colorado 80203
Telephone Number: (303) 864-7710
Email: baa@state.co.us
Fees for Appeal to the Board of Assessment Appeals: A taxpayer representing himself is not charged for the
first two (2) appeals to the Board of Assessment Appeals. A taxpayer represented by an attorney or agent
must pay a fee of $101.25 per appeal.
OR
2. Appeal to District Court: You have the right to appeal the decision of the Board of Equalization to the
District Court of the county wherein your property is located: in this case that is Weld County District
Court. A hearing before The District Court will be the last time you may present testimony or exhibits
or other evidence, or call witnesses in support of your valuation. If the decision of the District Court is
further appealed to the Court of Appeals pursuant to C.R.S. §39-8-108(1), the rules of Colorado
appellate review and C.R.S. §24-4-106(9), govern the process.
OR
3. Binding Arbitration: You have the right to submit your case to binding arbitration. If you choose this
option, the arbitrator's decision is final and you have no further right to appeal your current
valuation. C.R.S. §39-8-108.5 governs this process. The arbitration process involves the following:
a. Select an Arbitrator: You must notify the Board of Equalization that you will pursue
arbitration. You and the Board of Equalization will select an arbitrator from the official list of
qualified people. If you cannot agree on an arbitrator, the District Court of the county in which
the property is located (i.e. Weld) will select the arbitrator.
b. Arbitration Hearing Procedure: Arbitration hearings are held within sixty (60) days from the
date the arbitrator is selected, and are set by the arbitrator. Both you and the Board of
Equalization are entitled to participate in the hearing. The hearing is informal. The arbitrator
has the authority to issue subpoenas for witnesses, books, records documents and other
evidence pertaining to the value of the property. The arbitrator also has the authority to
administer oaths, and determine all questions of law and fact presented to him. The
arbitration hearing may be confidential and closed to the public if you and the Board of
Equalization agree. The arbitrator's decision must be delivered personally or by registered
mail within ten (10) days of the arbitration hearing.
c. Fees and Expenses: The arbitrator's fees and expenses are agreed upon by you and the
Board of Equalization. In the case of residential real property, the fess may not exceed
$150.00 per case. For cases other than residential real property, the arbitrator's total fees
and expenses are agreed to by you and Board of Equalization, but are paid by the parties as
ordered by the arbitrator.
If you have questions concerning the above information, please call me at (970) 400-4226.
Very truly yours,
BOARD OF EQUALIZATION
LeL� i!"lv:yfG C.
Esther E. Gesick
Clerk to the Board
Weld County Board of Commissioners
and Board of Equalization
cc: Brenda Dones, Weld County Assessor
Hello