Water resources of the Fort Union coal region, east-central Montana

The shallow ground-water system in the Fort Union coal region overlies the Upper Cretaceous Bearpaw Shale. It includes the Upper Cretaceous Fox Hills Sandstone and the overlying Hell Creek Formation, Paleocene Fort Union Formation, and Pleistocene and Holocene glacial deposits, terrace deposits, and alluvium. Two general flow patterns are present in aquifers above the Hell Creek Formation and a third may occur in the Fox Hills-lower Hell Creek aquifer. Recharge to the shallow ground-water system from direct infiltration of snowmelt and rainfall is about 50 ,000 acre-ft/yr. Discharge from the system is to perennial streams (about 5,000 acre-ft/yr to the Redwater River), withdrawal by wells (about 2,000 acre-ft/yr for livestock use and 2,500 acre-ft/yr for domestic use), and 34 to 45 in./yr to evapotranspiration. Primary constituents in water above the Hell Creek Formation are sodium, bicarbonate, and sulfate, and dissolved-solids concentrations are about 1,800 mg/L; water below a depth of about 200 feet contains more sodium and bicarbonate. Water in the Fox Hills-lower Hell Creek aquifer has an average dissolved-solids concentration of 1,180 mg/L. Flows in most streams have large seasonal variations, with the largest flows occurring in the spring as a result of snowmelt and rainfall. Dissolved-solids concentrations of streams generally are largest during low flow and smallest during high flow. Concentrations ranged from 160 to 6,960 mg/L in small streams and from 400 to 600 mg/L in the Missouri and Yellowstone Rivers. (USGS)