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EVALUATION OF THE GROUND -WATER RESOURCES
IN THE VICINITY OF GRAVEL MINING BY
STERLING COMPANIES, FORT COLLINS, COLORADO
RESOURCE CONSULTANTS INC
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• TABLE OF CONTENTS
Page
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Ground Water at the Site. . . . . . . . . . . . 1
Depth to Water Estimates. . . . . . . . . . . g
Other Water Rights . . . . . . . . . . . . . . . . . . . . 8
Summary and Recommendations . . . . . . . . . . . . . . 9
' FIGURES
1. Exploration Program of February 1, 1985. . . . . . . . . . . . 3
' 2. Exploration Program of March 8 & 10, 1988. . . . . . . . . 4
' TABLES
1. Summary of Water Level Measurements . . . . . . . . . . . . . . 5
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9LUATION
OF THE GROUND -WATER REORCES
IN THE VICINITY OF GRAVEL MINING BY
STERLING COMPANIES, FORT COLLINS, COLORADO
Introduction
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The recently ongoing ravel extraction in the NE 1 4 SE 1 g g g / � 4 Section /
33, T8N, R69W of the 6th P.M. in Larimer County, Colorado will be expanded
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toward the north into the South 1/2, NE 1/4 of the above described Section
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33. During the mining of the gravel, the ground -water table will be
lowered to bedrock
the underlying to allow dry material removal from the
pit.
The
objective of this investigation is to evaluate the effect of the
proposed pit dewatering through the estimated 15 years of mining on the
ground -water resource of the surrounding area. If any detrimental effect
is found on any other water right in the area, mitigating measures will be
proposed for the duration of the mining.
The effects of the mining operation on the Cache la Poudre River or the
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effects on the river after the mining has been completed, as well as miti-
gation of any negative impacts on the river, are not investigated in this
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study.
Ground Water at the Site
The unconfined ground water of the site, subject to this investiga-
tion, occupies the alluvial deposits of the Cache la Poudre River, extend-
ing into the alluvium of Dry Creek to the north. Ground water is sustained
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by seepage losses from numerous irrigation canals to the north and by deep
percolation and underflows from irrigated agricultural lands. The general
ground -water flow is from the north-northwest to the south-southeast toward
the Cache la Poudre River which serves as a drain to the aquifer.
The aquifer and the local ground -water conditions were explored by two
sets of test -boring programs completed by Empire Laboratories, Inc. The
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first consisted of drilling of nine exploratory holes in the East 1/2 of
Section 33 February 1, 1985. The the
on second exploration consisted of
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drilling of nine more holes on March 8 and 10, 1988, five holes in the East
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• 1/2 of Section 33 and four holes in the West 1/2 of Section 34 on the
east side of Taft Hill Road. The locations of the holes drilled in 1985
and 1988 are shown on Figures 1 and 2 respectively, reproduced from the
' respective Empire Laboratories reports. Water level measurements are
summarized in Table 1.
In the first exploration, bedrock was found at an average depth of
18.5 feet, ranging from 17 to 21 feet. The average saturated thickness was
' 12.8 feet (11.5-15.3 feet) and the average depth to water was found to be
5.7 feet (4.5-8.4 feet).
' The recent exploration program in early 1988 found the average mea-
surement of bedrock at 16.8 feet (15.5-18.0 feet), the saturated thickness
' of 9.2 feet (7.3-11.0 feet), and the depth to water at 7.6 feet (6.0-9.2
feet). The water level measurements were repeated on March 28, 1988, with
results showing minor local adjustments in the water levels, confirming the
measurements taken 14 days earlier.
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The testhole locations of the two exploratory drilling programs allow
2, 1985,
a limited comparison of the ground -water levels of February and
March 14, 1988. In this comparison, the 1988 water levels were generally
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lower along Taft Hill Road than they were observed in 1985. Unfortunately,
no water level data were found between these two measurements, and it is
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unknown whether such records exist.
The lower water levels in 1988 could be due to a combination of fac-
tors. Investigating water imports by ditches and agricultural irrigation
to the north and northwest of the study site could provide some indication,
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since it has a primary role of sustaining ground water in the area. Assum-
ing that the flow of the Cache la Poudre River at the gaging station at
the mouth of the canyon is a good qualitative index to water availability
figures taken from
to the ditches, the water -year annual runoff were pub-
lished records as follows:
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TEST BORING LOCATION PLAN
Fig. 1. Exploration
Program of February 1,
1925 (reproduced from
Empire Laboratories, Inc.
report)
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TEST BORING LOCATION PUN
IV
f �= moo
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Fig. 2. Exploration Program
of March 8 & 10, 1933 (repro-
duced from Empire Laboratories,
Inc. report)
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Table 1
Summary of Water Level Measurements
(taken from reports of Empire Laboratories, Inc.)
(all units in feet)
Measurements made on February 2. 1985
Hole No. Depth to Water Depth to Bedrock
1 8.4 20.0
2 5.5 18.0
3 4.7 17.5
4 4.5 18.0
5 6.4 20.5
6 4.6 17.0
7 5.7 21.0
8 5.5 17.0
9 6.0 17.5
1988 Measurements
Hole No.
1
2
3
4
5
6
7
8
9
Depth to Bedrock
16.0
15.5
18.0
17.0
17.5
16.5
16.5
17.0
17.0
March 14
Depth to Water
6.3
6.5
7.0
9.0
7.7
9.2
7.8
8.8
6.0
5
March 28
Depth to Water
6.0
6.25
7.0
9.1
7.25
9.0
8.4
8.7
6.75
REMURCE CONSULTANTS INC
Water Year Water -Year Runoff
' (ac-ft)
1983 644,800
' 1984 386,100
1985 204,100
1986 318500�
1987 174:400'
Obtained from the Water Commissioner since publication is
not yet available.
' The water year for a given year begins on October 1 of the previous
year and ends on September 30 of the subject year. Though some ditches do
' carry water during the first part of October, the irrigation year is practi-
cally ended with September of the year, and any contribution to ground-
water recharge is represented within the water year. From the above runoff
tabulation, water year 1984 is representative for the 1985 water level
' measurements. Considering that the long-term average runoff of the river
at the mouth of the canyon is about 280,000 ac-ft/year, the 1984 runoff
• exceeded this by over 100,000 ac-ft, showing about a 38 percent higher
runoff than in an average year.
' Ground -water elevations measured in March 1988 similarly reflect water
availability in the preceding water year of 1987. This runoff was below
the average by over 100,000 ac-ft, just the opposite of the runoff
reflected by the 1985 water level measurements. The differences in the
runoff of 106,000 ac-ft above the average in 1984 and the same amount below
' the average in 1987 are significantly large, which could account for the
differences in the water table.
Another factor is the time of the year the two sets of measurements
were taken. The annual fluctuation of the ground -water table under agri-
cultural land with primary surface -water irrigation is such that highest
water levels are observed at the end of the irrigation season with lowest
' levels occurring at the beginning of the irrigation season. After irriga-
tion has been completed for the season, ground water gradually drains into
the nearest river throughout the non -irrigation part of the year. Since no
' importation of water takes place at the upper regions of the aquifer, water
tables decline through this period.
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In 1985 the water level measurements were made on or about February 2.
The 1988 measurements were made on or about March 14. This difference is
' about 42 days. In light of the above explained behavior of the water
table, a later measurement would be expected to find lower water levels
than one made earlier. A quantification of this water level decline would
' be possible only with longer observation of the local water table, which
data are not available for analysis.
The 1988 water levels also show the effect of the dewatering operation
at the latest mining site in part of the NE 1/4 of the SE 1/4 of Section
' 33. Water -table lowering at this site commenced in April 1987 and has been
ongoing ever since. To estimate water level decline attributable to pit
dewatering, two cross sections were analyzed. One, starting at the north-
east corner of the pit, runs north through holes 4, 3, 2, and 1. The
second runs west to east beginning at the same corner of the pit and pro-
ceeding through holes 6, 7, 8, and 9. After the nearly 11 months of
dewatering operation, the water -table lowering at hole number 4 of the 1988
iexploration appears to be about 2 feet and at hole number 6 of the same set
of drilling it is about 1 foot. Beyond these holes no evidence of water -
table decline was found that could be attributed to the pumping.
Finally, one more factor contributes to the position of the ground-
water table in the project site. The "Callahan Drain and Armstrong Irri-
gating Ditch" constructed in April 1901 begins in the NE 1/4, NE 1/4 of
Section 32 as an underground drain tile, delivering its water into the Gill
and Taylor Ditch in the NW 1/4, NE 1/4 of Section 33. The water was then
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diverted from said Gill and Taylor Ditch at the NW corner of the SW 1/4,
1/4
NW of Section 34 for irrigation of lands in the SW 1/4, NW 1/4, and in
the NW 1/4, SW 1/4 of said Section 34.
rSometime
in 1987 it drain became
was reported that the above tile
damaged and stopped draining the lands it was intended to drain, and, conse-
quently, did not contribute, at least through part of the year, to the
water supply of Gill and Taylor Ditch which imports water directly to the
north of the project site. The magnitude of this contribution and the
effect of the lack of contribution to the ground water of the area is not
Known at this time.
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Depth -to -Water Estimates
Based upon the more conservative 1985 water level measurements, when
' the water table was higher than normal below the land, it can be concluded
that no benefit is evident to agricultural crops due to the water table
naturally approaching the root zone. The ground water in these measure-
ments appeared at a depth of 5.7 feet below the land, on the average, and
1 in most cases not even reaching the top soil. In the 1988 measurements,
the water table at each location was well into the gravels, implying no
significant contribution by capillary rise to the top soil and the root
' zone.
1 In order for subirrigation to become a significant component in satis-
fying the irrigation requirement of agricultural crops generally grown in
the area, water must approach the surface to a depth of 2-3 feet. Excep-
tions are well -established alfalfa and orchards which plants and trees have
root systems sufficient to go after the water into depths exceeding even 10
1 feet. The quoted water level measurements do not give any indication that
subirrigated conditions were ever prevalent on or around the project area.
1 Other Water Rights
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The State
Engineer's
Tabulation
of Water Rights (July
1, 1978) show
four wells in
the vicinity
of the
proposed
gravel mining.
Two small domes-
tic wells are
shown in the
North
1/2
of the NE 1/4, NE 1/4
of Section 33
and one in the
NE 1/4, NE
1/4, NW
1/4
of the same section.
One small
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domestic well
is shown in
the NW
1/4,
NW 1/4, NW 1/4 of Section 34. The
yields of these wells range from 4.49 gpm to 14.81 gpm.
1 Well registration records show one 10 gpm domestic well in the NW 1/4,
NW 1/4 of Section 34 (registration number 02460) and a similar domestic
1 well in the SW 1/4, NW 1/4 of Section 34 (registration number 18808) at 2412
North Taft Hill Road. On the west side of North Taft Hill Road, a 12 gpm
domestic well, registered to William Seaworth is shown in the NE 1/4, SE
' 1/4 of Section 33. According to Mr. Seaworth, there were a total of three
wells on the property in the East 1/2 of Section 33 but use of these wells
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has been discontinued and they are not intended to be used in the future.
Ground -water use was also reported at 2320 North Taft Hill Road in the
SW 1/4, NW 1/4 of Section 34, and at 2129 Highway 287 N in the NE 1/4, NE
1/4 of Section 33. So far no registration records have been found for
these wells and they do not appear in the water -right tabulations either.
' Of the above listed wells, two may require special attention during
the mining operation. These are both on the east side of North Taft Hill
' Road at properties at street numbers 2320 and 2412. They will be slightly
over 100 feet from the proposed pit and may experience excessive water
' level declines during the mining period. The mining plan for the 58-acre
tract in the South 1/2, NE 1/4 of Section 33 is a phased dewatering and
gravel mining operation starting on the west side of the property. Multi-
ple dewatering trenches running north to south will be constructed as min-
ing progresses from the west, with the last to the east along North Taft
Hill Road being put in only for the last quarter of the duration of the
entire mining operation of the 58-acre tract. Assuming this mining period
will be 15 years, the adverse effect of the dewatering on the above two
wells will be of concern over a period of about 45 months.
' Summary and Recommendations
' Ground -water investigations conducted thus far relative to Sterling
Companies' proposed mining operations in the East 1/2 of Section 33, TBN,
R69W of the 6th P.M. in Larimer County revealed no prohibitive negative
' effects on existing ground -water users in the area. Only two shallow
ground -water wells are sufficiently close to the mining pit that they may
' require mitigative measures through a part of the mining period. The
ground -water resource of the site is greatly dependent upon surface -water
' import and application on lands lying generally to the north and northwest.
Water level measurement data are available only for the two times
exploratory drillings were made in early 1985 and 1988. The second set of
measurements made in 1988 are not sufficiently different from the first and
• do not provide distinctive new information. The seasonal behavior of the
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water table is not known at the present, therefore it is recommended that
water levels be measured monthly in the observation holes completed in
March 1988 and in any other wells that could be included in the program.
' These measurements should continue through 1988 and possibly thereafter.
Based upon the results of the above water level observations, the
construction of other observation wells may be recommended to document the
water -table behavior through the entire mining operation. The need for
' mitigation and the effectiveness of various mitigative measures will be
evaluated with the results of this ground -water monitoring program.
' Mitigative measures that could be considered are directed toward
either reducing the amount and extent of the water -table lowering resulting
' from the pit dewatering, or delivering water directly to the location where
a shortage was created. The selection of the proper mitigative measure or
a combination of same will be based on economic evaluations throughout the
period of mining with strong reliance on the water -table monitoring pro-
gram.
Water -table related mitigations that may be considered are:
1. Artificial recharge from
a. Gill and Taylor Ditch
b. Artificial recharge ditch along North Taft Hill Road
C. Existing or new recharge ponds
2. Reducing permeability of the pit sides
a. Packing top soil as part of the restoration
b. Slurry trench cutoff
' C. Chemical or cement injection cutoff
Mitigation of injury to ground -water users to the east of the site
' could be by:
1. Providing surface water in the Gill and Taylor Ditch
2. Delivering ground water to the Gill and Taylor Ditch
3. Delivering surface or ground water to user's pond
' 4. Pressure delivery of ground water to user
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Prior to considering any of these mitigation possibilities, the
existence and magnitude of the injury should be determined and matched with
the most feasible mitigation measure. At this time it appears that, if any
injury is recognized at all, a direct delivery of surface water to the
injured party would be the most desirable solution.
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