The investigation was undertaken to define the geohydrology of the Sand Creek-Hanks area and to project probable hydrologic effects of lignite mining on the area.
Aquifers occur in sandstone beds in the Fox Hills Sandstone and the Hell Creek Formation of Cretaceous age and 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 about 1,200 to 2,000 feet below land surface. Yields of wells completed in the aquifer could be as much as 60 gallons per minute. Water in the Fox Hills aquifer is a sodium bicarbonate type and generally
Depths to the top of the Hell Creek aquifer range from about 900 to 1,600 feet. Well yields range from less than 10 to 40 gallons per minute. Water in the aquifer is a sodium bicarbonate type and generally contains between 1,000 and 2,200 milligrams per liter dissolved solids.
Depths to aquifers in the Tongue River and Sentinel Butte Members of Fort Union Formation range from near land surface to about 1,000 feet below land surface. Wells completed in the aquifers may yield as much as 40 gallons per minute of sodium bicarbonate or a sodium sulfate type water that contains about 800 to 4,100 milligrams per liter dissolved solids.
Glacial drift covers most of the study area. The drift thickness ranges from a veneer to about 380 feet. Well yields range from a few gallons per minute to 900 gallons per minute. Dissolved-solids concentrations in water from the glacial drift generally range from 477 to 2,050 milligrams per liter.
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 ground water immediately beneath the mines and possibly will cause some increase in the dissolved solids in low flows in area streams.
