Pumping Well Test Analysis: Hell Creek Aquifer, North Cave Hills,

Harding County, South Dakota

Field Test and Report By: Dr. Arden D. Davis, PE

Dr. Larry D. Stetler, CPG

Report # 11-07 Department of Geology and Geological Engineering

South Dakota School of Mines and Technology’ Rapid City., SD 57701

Submitted to US Forest Service Northern Regional Office

Missoula, MT ATT: Mr. Robert Wintergerst

November 27, 2007

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Introduction A pumping test was performed on November 9, 2007, at a well on the Randy Feist property near the North Cave Hills. The well, known as the Feist “old well,” is about 3/4 mile east of Custer National Forest, in the NE ¼ section 14, T. 22 N., R. 5 E., Harding County, South Dakota (Figure 1). The well owner reported that the water level was about 250 ft below the surface in the well, and the total depth was about 360 ft. From a stratigraphic section for the North Cave Hills (Figure 2), the well is believed to be in the Hell Creek aquifer. Depth to water was measured with a sonic water-level indicator. The static water level in the well was 250.6 ft below the top of a temporary riser pipe set up for measuring water levels during the aquifer test. When the pump was started, water-level measurements were taken for 60 minutes (Table 1). The pumping rate was approximately 1.53 gal/min during most of the test. After the pump was shut off, recovery measurements were taken for an additional 15 minutes (Table 1). Water-level measurements during pumping and recovery are shown graphically on Figure 3. Drawdown and recovery data were analyzed with the Theis equation (Freeze and Cherry, 1979) and methods outlined by Driscoll (1986). AQTESOLV® software was used for analysis. During pumping, early drawdown values were influenced by storage of water in the well casing (Figure 4). The Theis method (Figure 4) showed a transmissivity (T) of approximately 8 ft2/day and a storage coefficient of 0.0025. Residual-drawdown analysis of recovery data (Figure 5) also showed a transmissivity of about 8 ft2/day. Hydraulic conductivity (K) was determined with the equation: K = T/b where b = saturated thickness. The value of saturated thickness for the water-bearing part of the Hell Creek aquifer tapped by the Feist “old well” is believed to be about 25 ft, on the basis of well logs in the area (see Appendix I). However, the saturated thickness could reasonably vary from 20 ft to about 30 ft. Hydraulic conductivity, therefore, could range from about 0.27 ft/day to 0.4 ft/day. Ground-water velocity (v) was calculated with the equation: v = (K/θ) (∆h/∆s)

where ∆h/∆s = hydraulic gradient

and θ = effective porosity.

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The hydraulic gradient was assumed to be 0.008, based on the regional dip of rocks and limited water-level data in surrounding wells, but it could vary from about 0.005 to 0.01 in this area. Effective porosity of the Hell Creek aquifer has been estimated to be 0.06 by Rahn (1985), but it could reasonably vary from about 0.05 to 0.1. On the basis of this information, the calculated ground-water velocity could vary from about 0.014 ft/day to about 0.08 ft/day (5 to 29 ft/yr).

The nearest abandoned uranium mines in the North Cave Hills are about 1¾ mile (9200 ft) southwest of the pumped well, in the Riley Pass area, and uranium mining occurred during the 1950s, about 50 years before the writing of this report. If the calculated ground-water velocity is representative of the Hell Creek aquifer, ground water and dissolved contaminants could move about 250 to 1500 ft in 50 years. This estimate does not include the time necessary for contaminants to move downward from the land surface to the aquifer (see Figure 2). The Hell Creek aquifer appears to be confined because the water levels in wells in the aquifer are above the top of the sandy, water-bearing layers (see Appendix I), and because the storage coefficient of 0.0025 is in the range that is typical of a confined aquifer. Thus, it appears likely that the gross alpha measurement of 8 pCi/L in the Feist “old well” is naturally occurring rather than the result of uranium mining in the North Cave Hills.

References Cited Driscoll, F.G., 1986, Groundwater and wells, 2nd ed.: Johnson Division, 1089 p. Freeze, R.A., and Cherry, J.A., 1979, Groundwater: Prentice-Hall, Englewood Cliffs, New Jersey, 604 p. Rahn, P.H., 1985, Ground water stored in the rocks of western South Dakota, in Rich, F.J., ed., Geology of the Black Hills, South Dakota and Wyoming, 2nd ed.: American Geological Institute, Alexandria, Virginia, p. 154-173.

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Figure 1. Location of pumping test (modified from U.S. Geological Survey’s 1:24,000-scale Eagle’s Nest Butte topographic quadrangle).

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Figure 2. Stratigraphic section of North Cave Hills area.

Figure 3. Depth to water vs. time during pumping test and recovery at Feist “old well,” November 9, 2007. Time since pump started = t; time since pump stopped = t’ (see Table 1 for ratio of t/t’ for residual drawdown data).

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0.1 1. 10. 100.1.

10.

100.

Time (min)

Dis

plac

emen

t (ft)

Obs. WellsOld Well

Aquifer ModelConfined

SolutionTheis

ParametersT = 8.043 ft2/dayS = 0.002461Kz/Kr = 1.b = 25. ft

Figure 4. Theis analysis of drawdown vs. time data from pumping test at Feist “old well.” Note effects of storage of water in well casing during first ten minutes of test.

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1. 10. 100. 1000.0.

4.

8.

12.

16.

20.

Time, t/t'

Res

idua

l Dra

wdo

wn

(ft)

Obs. WellsOld Well

Aquifer ModelConfined

SolutionTheis (Recovery)

ParametersT = 8.014 ft2/dayS/S' = 2.592

Figure 5. Residual-drawdown analysis of data from pumping test at Feist “old well.”

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Table 1. Drawdown vs. time data and recovery data from pumping test at Feist “old well.” Static water level was 250.6 ft below the top of the casing. Recovery values determined from Figure 3, by extension of drawdown curve. In the table, t refers to the time since the pump started, and t’ refers to the time since the pump stopped. Time Drawdown Recovery Residual t/t’ Depth to (min) (ft) (ft) drawdown water (ft) Pump on 0.167 1.0 251.6 0.333 1.8 252.4 0.5 1.5 252.1 0.667 1.9 252.5 0.833 2.2 252.8 1 2.4 253.0 2 5.2 255.8 3 6.5 257.1 4 7.9 258.5 5 8.5 259.1 6 9.3 259.9 7 9.5 260.1 8 10.0 260.6 9 10.3 260.9 10 10.4 261.0 12 11.7 262.3 15 12.4 263.0 20 13.8 264.4 25 14.2 264.8 30 14.7 265.3 40 15.3 265.9 50 15.9 266.5 60 16.6 267.2 Pump off 60.167 3.3 13.3 361 263.9 60.333 4.42 12.2 181 262.8 60.5 5.23 11.4 121 262.0 60.667 5.44 11.2 91 261.8 60.833 5.85 10.8 73 261.4 61 6.56 10.1 61 260.7 61.5 7.27 9.4 41 260.0 62 7.98 8.7 31 259.3 63 10.19 6.5 21 257.1 64 11.7 5.0 16 255.6 65 12.92 3.8 13 254.4 66 14.08 2.7 11 253.3 67 14.5 2.3 9.57 252.9 68 14.82 2.0 8.5 252.6 69 15.04 1.8 7.67 252.4 70 15.22 1.7 7 252.3 72 15.4 1.6 6 252.2 75 15.7 1.4 5 252.0

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Appendix I

Well Logs

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