Assessment of Aquifer Contamination near Abandoned Uranium Mines in the North Cave Hills, South Dakota

An aquifer test of a water well located 1.9 miles NNE of the abandoned Riley Pass uranium mine in the North Cave Hills was conducted to assess potential for aquifer contamination from past mining activity.  The well was drilled in 1979 and open hole completed in a 23 foot sand lens at a depth of 363 feet.  The static water level was measured using a sonic water-level indicator at 250.6 feet below the top of the casing.  The well was pumped for 60 minutes resulting in a drawdown of 16.6 feet followed by a 15 minute recovery period in which the water level rebounded to within 0.4 feet of the initial static level.  Flow rates during pumping varied for the first 9 minutes increasing to 1.53 gal/min. for the duration of the test.  Drawdown and recovery data were analyzed using the Theis method and AQTESOLV software.  Transmissivity (T) calculated from both the drawdown and recovery data was 8 ft²/day.  Hydraulic conductivity was determined to be 0.35 ft/day.  Effective porosity and hydraulic gradient for the Hell Creek aquifer were determined to be ~0.06 and 0.008 (<1° to the NNE), respectively.  The resulting ground-water velocity was determined to be ~0.047 ft/day.  In the 50 years since the mines have been active, contamination having reached this aquifer would have been able to travel about 860 feet, or <10 % of the distance from the mines to the well.  This value does not include the additional time required for the contamination to travel vertically downward from the surface to the aquifer.  The aquifer storage value of 0.0025 indicated aquifer confinement, restricting potential for downward contaminant migration through fractures in the overlying shale layers.   Therefore, it appears unlikely that uranium mining in the North Cave Hills has affected groundwater wells in the regional aquifers.