Appraisal of the water resources of the Skunk Creek Aquifer in Minnehaha County, South Dakota

The Skunk Creek aquifer, a major glacial outwash deposit in the Skunk Creek drainage basin, consists of a 30-sq-mi shallow stream connected sand and gravel aquifer in southeastern South Dakota. The aquifer thickness ranges from 1 to 74 ft. Average annual fluctuation of the water table is 2.5 ft. The water has an average dissolved-solids content of 620 mg/L and is very hard , averaging 403 mg/L calcium carbonate hardness. A numerical model was developed and calibrated under steady-state and transient conditions. The model contained 484 active nodes each representing 0.0625 sq mi. Hydraulic conductivities of the aquifer used in the model range from 10 to 400 ft/d, and average specific yield is 20%. Recharge from infiltration of precipitation was estimated to be 6 inches/yr or 24% of average annual precipitation. Maximum evapotranspiration rate was 32 inches/yr and the evapotranspiration extinction depth for the model was 5 ft. The steady-state hydrologic budget was about 11 ,000 acre-ft/yr. Recharge by precipitation was about 9,500 acre-ft and recharge from streams was about 1,100 acre-ft. Discharge by evapotranspiration was about 5,000 acre-ft and discharge to streams was about 5,700 acre-ft. A hypothetical simulation to determine maximum withdrawal under steady-state conditions resulted in a groundwater withdrawal of about 15,700 acre-ft/yr from 19 hypothetical wells pumping at a rate of 500 gal/min and 13 existing wells pumping at a combined average rate of 24 gal/min. (USGS)