9624 ‎(2)‎

• UCREFRP Catalog Number: 9624
• Author: Bauch, N. J. and J. B. Bails.
• Title: Water-Quality Characteristics and Ground-Water Quantity of the Fraser River Watershed, Grand County, Colorado 1998-2001.
• USFW Year: 2004.
• USFW - Doc Type: \
• Copyright Material: NO

Ground-Water Quantity 27
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Figure 15. Ground-water level and temperature in the alluvial aquifer
(site 9) in the upper Fraser River watershed, 1998-2001.
refinement of the well-log selection in sections with a large
number of available well logs included preference to wells of at
least 200 ft in depth and wells that were drilled through the
Troublesome Formation. Tabulated measurements then were
summarized to give average percentages of water-bearing units
compared to nonwater-bearing units per section. Total average
percentages of water-bearing compared to nonwater-bearing
units then were calculated from the section averages. This
method was used to give an equally weighted spatial represen-
tation of the Troublesome Formation aquifer properties. To esti-
mate the available water in the Troublesome Formation, a total
Troublesome Formation aquifer thickness of 500 ft was
assumed. Though the aquifer is thicker than 500 ft in some parts
of the area, 500 ft was selected as a reasonable maximum depth
for a water-production well. Wells drilled in the Troublesome
Formation where aquifer testing was done were completed to
depths less than 300 ft, except for one well completed to 535 ft,
and had sufficient yield for community-supply wells (Resource
Engineering, Incorporated, 1999 and 2000).
Results of tabulation of well data indicate that the average
percentage of water-bearing units in the upper 500 ft of the
Troublesome Formation aquifer is 11 percent, with average
values for each section ranging from 2 to 35 percent. For a
water-storage calculation of the upper 500 ft of the Trouble-
some Formation, this equates to an average of 55 ft of saturated
thickness. Because the Troublesome Formation is a confined
aquifer, water does not drain from the aquifer as in unconfined
aquifers but is released due to compression of pore spaces and
the elasticity of the aquifer itself. The total volume of water
stored in the Troublesome Formation aquifer is the sum of the
water released from elastic compression of the aquifer (water
under pressure) and the amount that would be released from the
pore spaces of the saturated thickness of the aquifer under
unconfined conditions as the potentiometric surface drops
below the upper limit of the confined aquifer (that is, aquifer
dewatering). Because the Troublesome Formation is a series of
siltstone and interbedded sandstone and conglomerate and little
reliable information is available on the depth to the water-bear-
ing units as well as the aquifer's response to dewatering, the
amount of water released from elastic compression was not cal-
culated. An average specific yield for unconfined sedimentary
deposits similar to the water-producing zones of the Trouble-
some Formation give an approximate value of 0.2 (Fetter,
1994). The volume of the saturated thickness of the Trouble-
some Formation is calculated as the areal extent of the aquifer
times the saturated thickness. This calculation yields a saturated
aquifer volume of 7.98 x 1010 ft3. This volume multiplied by the
specific yield of 0.2 results in an amount of stored water equal
to approximately 370,000 acre-ft, which would be available if
the water table dropped below the top of the aquifer and the
aquifer became unconfined. The total amount of available water
in the Troublesome Formation might be greater than 370,000
acre-ft because the amount of water under pressure has not been
accounted for in the estimate. However, if the specific yield is
only 0.1 rather than 0.2, then the amount of available ground
water would be one-half of the calculated estimate.
Results from five aquifer tests completed in the Trouble-
some Formation (Resource Engineering, Incorporated, 1999
and 2000) indicate that the average value for coefficient of
transmissivity for the Troublesome Formation is approximately
5,000 gpd/ft. When combined with the information on the aver-
age saturated thickness of the Troublesome Formation, hydro-
logic conductivity of the water-bearing units is estimated to be
about of 12 ft/d, which is in the range of aquifers consisting of
silty to fine sands (Fetter, 1994). Because the testing of the
Troublesome Formation aquifer was completed without collect-
ing drawdown data from nearby reference wells, the storage
coefficient could not be determined.
In order to better estimate maximum potential withdrawals
of water from the Troublesome Formation (that would not cause