GROUNDWATER FLOW AND CHEMISTRY NEAR AN IMPOUNDMENT FOR PRODUCED WATER, POWDER RIVER BASIN, WYOMING

The Powder River Basin of Wyoming and Montana is experiencing large-scale land-use changes as a result of energy-resource development. An important concern in the Basin is the fate of the large amounts of groundwater extracted during production of coal-bed natural gas (CBNG). Most of the produced water is disposed of in unlined surface impoundments. A six-year study of groundwater flow and water chemistry at one impoundment, Skewed Reservoir, has produced the most detailed data set for any impoundment in the Basin. Data were collected from a network of 21 observation wells and 3 suction lysimeters. A groundwater mound formed atop bedrock within initially unsaturated, unconsolidated deposits underlying the reservoir. Heterogeneity in physical and chemical properties of sediments resulted in complex groundwater-flow paths and highly variable groundwater chemistry. Sulfate, bicarbonate, sodium, and magnesium were the dominant ions in all locations, but substantial spatial variability existed in relative concentrations; pH varied from less than 3 to more than 9, and total dissolved solids concentrations ranged from less than 5,000 to greater than 100,000 mg/L. Selenium was a useful tracer of reservoir water; selenium concentrations exceeded 300 mg/L in groundwater samples obtained from 18 of the 24 sampling points. Groundwater travel time from the reservoir to a nearby alluvial aquifer (a linear distance of 177 m) was calculated at 474 days on the basis of selenium concentrations. The CBNG produced water is not the primary source of selenium or other solutes in the groundwater. Naturally occurring salts and minerals within the unsaturated zone, dissolved and mobilized by infiltrating impoundment water, account for most of the solute mass in groundwater. Gypsum dissolution, cation exchange, and pyrite oxidation appear to be important reactions. The complex geochemistry and groundwater flow paths at the study site underscore the difficulty in assessing impacts of surface impoundments on water resources within the Powder River Basin.