L.J. Hamilton 1986 Three major glacial-drift aquifers consisting of outwash sand and gravel underlie Hughes County, 784 sq mi of plains incised by the outwash-filled valley of the Missouri River in central South Dakota. Glacial-drift aquifers are recharged by more than 9,000 acre-feet of water annually and store about 1 million acre-feet of water beneath 250 sq mi. Well yields may be as much as 1,000 gal/min. Three major bedrock aquifers of sandstone, shale and limestone store about 100 million acre-feet of geothermal water (22 C to 49 C) beneath the county and yield as much as 1,600 gal/min to flowing wells 800 to 2,600 ft deep. Shut-in pressures in flowing wells increase with depth to more than 400 lbs/sq in. The deepest bedrock aquifer, recharged from the Black Hills of western South Dakota, recharges overlying bedrock aquifers by vertical leakage of 180,000 acre-feet/yr. Dissolved solids concentrations average about 2,000 mg/L in water from both glacial-drift and bedrock aquifers. Average hardness of the water varies from 200 mg/L for the uppermost bedrock aquifer to 900 mg/L for glacial aquifers and 1,400 mg/L for other bedrock aquifers. (USGS) application/pdf 10.3133/wri844195 en U.S. Geological Survey, Water resources of Hughes County, South Dakota reports