Groundwater in unconsolidated aquifers is the most reliable and accessible source of potable water in Big Horn County, Wyoming. Well yields generally ranged from 25 to 200 gal/min; however, yields of 1600 gal/min are reported from wells in the gravel, pediment, and fan deposits. Bedrock aquifers that yield the most abundant water supplies are the Tensleep Sandstone, Madison Limestone, Bighorn Dolomite, and Flathead Sandstone. The Madison Limestone, the Darby Formation, and the Bighorn Dolomite form the Madison/Bighorn aquifer. Shut-in pressure from flowing wells in bedrock indicate declines, from the time the wells were completed to 1988, by as much as 390 ft. Water samples from wells completed,in unconsolidated aquifers had concentration of dissolved solids less than 2000 mg/L. Water samples from wells in aquifers in Paleozoic and Precambrian rocks had median concentrations of dissolved solids ranging from 111 to 275 mg/L. Perennial streams originate in the mountains and ephemeral streams originate in the Bighorn Basin. The predominant dissolved constituents are calcium or sodium and bicarbonate or sulfate. Concentrations of pesticides detected in surface-water samples were less than the U.S. Environmental Protection Agency (USEPA) maximum contaminant levels. Pesticides detected in groundwater samples included dicamba and picloram at a concentration of 0.40 microg/L, atrazines (0.40 microg/L), aldicarb sulfoxide (1.44 microg/L), aldicarb sulfoxide (0.52 microg/L), and malathion (0.02 microg/L). Analyses of groundwater samples for radionuclides indicated that concentrations from four municipal wells exceeded the maximum contaminant levels established by the USEPA. Surface water accounts for 96 percent and groundwater accounts for 4 percent of total off-stream water use in Big Horn County. Irrigation is the largest off-stream use of both surface and groundwater. Groundwater supplies 89 percent of water used for domestic purposes and about 16 percent of water used for public supplies, which shows that groundwater is a primary domestic water supply in rural areas where public supplies are not available.
Additional publication details
USGS Numbered Series
Water resources of Big Horn County, Wyoming
Water-Resources Investigations Report
District Chief, U.S. Geological Survey, Water Resources Division ;
U.S. Geological Survey, Earth Science Information Center, Open-File Reports Section [distributor],