USGS Publications Warehouse

Abstract

Aquifers occur in sandstone beds in the Fox Hills Sandstone and the Hell Creek Formation of Cretaceous age in sandstone lenses and lignite beds in the Tongue River and Sentinel Butte Members of the Fort Union Formation of Tertiary age. The top of the Fox Hills aquifer ranges from 1,200 to 2,000 ft below land surface. Yields of wells completed in the aquifer could be as much as 60 gal/min. Water in the Fox Hills aquifer is a sodium bicarbonate type, containing between 1,300 and 2,100 mg/L dissolved solids. Depths to the top of the Hell Creek aquifer range from about 900 to 1,600 ft. Well yields range from < 10 to 40 gal/min. Water in the aquifer is a sodium bicarbonate type and between 1,000 and 2 ,200 mg/L dissolved solids. Depths to aquifers in the Tongue River and Sentinel Butte Members of the Fort Union Formation range from near land surface to about 1,000 ft below land surface. Wells completed in the aquifers may yield as much as 40 gal/min of sodium bicarbonate or a sodium sulfate type water that contains about 800 to 4,100 mg/L dissolved solids. Glacial drift covers most of the study area. The drift thickness ranges from a veneer to about 380 ft. Well yields range from a few gal/min to 900 gal/min. Dissolved solids concentrations in water from the glacial drift generally range from 477 to 2,050 mg/L. Mining of lignite will destroy all aquifers in and above the mined lignite and will expose overburden to oxidation. Leaching will cause an increase in dissolved solids in groundwater immediately beneath the mines and possibly will cause some increase in the dissolved solids in low flows in area streams. (Author 's abstract)