PERMFILE49399

• Permit No: m2004044
• IBM Index Class Name: Permit File
• Doc Date: 8/4/2004
• Doc Name: Groundwater Model
• From: Tetra Tech RMC
• To: DMG
• Media Type: D
• Archive: No

Table 1: Aquifer Tests Conducted Near Tucson South Mine (Wilson, 1965)
Well Depth Pumping Rate Hydraulic
Well Location Geologic Unit Ut) (8Pm) Conductivity
da )
T1S, R67W, Sea 35 Va1leyFill 39 360 307
4 miles SW of site De osits
T1S, R66W, Sec. 30 Valley Fill 35 140 334
3 miles SE of site De osits ( )
T1N, R66W, Sec. 8 Valley Fill 36 540 561
6 miles NE of site De osits
T1N, R66W, Sec. 18 Valley Fill 50 1020 602
5 miles NE of site De osits
T1N, R66W, Sec. 19 Valley Fill 23 450 668
4 miles NE of site De osits ( )
T1N, R66W, Sec. 30 Valley Fill 40 610 695
3 miles NE of site De osits ( )
T1N, R67W, Sec. 13 Valley Fill 38 575 762
5 miles N of site Deposits (Qp)
A pump test was conducted at the east end of the site, near the South Platte River, in 2004 by Bishop-
Brogden Associates. Reportedly, the hydraulic conductivity in the clean sand and gravel was 1700 feet
per day at a pumping rate of 565 gpm. The well was approximately 30 feet deep. Note that the
hydraulic conductivity was quite high because was tested only in the sand and gravel zone. The
hydraulic conductivity used in the model reflects an average value for the entire alluvial cross section
(overburden, mud lens, sand and gravel) at every location.
No information was available in Wilson (1965) for the hydraulic conductivity of terrace deposits (Qb)
and the wind-blown deposits (Qes) west of the site. However, other literature sources report that
hydraulic conductivity values of alluvium in northeastern Colorado may range from 20 to 750 feet per
day (Hershey and Schneider, 1972). Therefore, the range of hydraulic conductivities for the three
geologic units in the area of the reservoir are expected to be from 20 to 1700 feet per day. The hydraulic
conductivity of the terrace deposits is expected to be similar to the valley fill deposits, as the percentage
of gravel, sand and silt are similar. The wind-blown deposits are expected to have a lower hydraulic
conductivity because they consist primarily of fine to medium sand.
In the western part of the model, the bedrock rises, reducing the saturated thickness of the alluvium. To
simulate this, the hydraulic conductivity was adjusted based on the ratio of alluvium thickness to the
height of the cell. For example, a cell that was vertically half alluvium and half bedrock would have a
hydraulic conductivity of 200 feet per day instead of the 400 feet per day used in the cells that consisted
entirely of alluvium.
Based on experience and literature values, the Specific Yield of the alluvial aquifer was assumed to be
0.2 throughout the domain.
2.4 Recharge
Recharge was applied in the model to account for rainfall and snow melt that infiltrates into the ground
and reaches the water table. Precipitation recharge in north-central Colorado has been reported to be
about five percent of the total precipitation (RMC, 1991). This translates into an annual recharge rate of
- 4 - August 2004
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