GROUND-WATER QUALITY IMPACTS RELATED TO AN IMPOUNDMENT FOR COALBED METHANE WATER, POWDER RIVER BASIN, WYOMING

The U.S. Bureau of Land Management estimates that by the year 2010 as many as 50,000 wells will be producing methane from coalbeds in the Wyoming portion of the Powder River Basin. The fate of water produced along with the natural gas and its impact on the environment are topics of some concern. The federal Environmental Impact Statement cites use of surface impoundments as a preferred alternative for containment of the water. Over 3,000 such impoundments had been permitted by the Wyoming Department of Environmental Quality in the Basin as of June 2005. A study is being conducted to describe flow paths and chemical evolution of water infiltrating from Skewed Reservoir, an impoundment that was in operation for approximately one year beginning in August 2003. Sediment core samples were obtained from beneath the reservoir prior to filling and after the cessation of operations, when the reservoir was drained. Sediment samples were analyzed for physical properties and leachable ions. Subsurface water samples were obtained from a network of observation wells and a series of suction lysimeters installed at depths of 3, 4.9, and 6.7 m beneath the base of the impoundment. Borehole and surface geophysics were used to assess the extent of the ground-water solute plume. Coalbed methane (CBM) water discharged to the reservoir was typically a sodium-bicarbonate type having total dissolved solids (TDS) concentration of about 2300 mg/l. As this water moves through the subsurface it acquires chemical characteristics (sodium-magnesium-sulfate type and TDS concentration sometimes in excess of 100,000 mg/l) quite unlike those of the CBM water or the native shallow ground water. Dissolution and cation exchange appear to be important reactions in the evolution of the water chemistry in this complex hydrogeologic setting. The eventual fate of the ground-water solute plume and the significance of our findings to CBM development in the Powder River Basin as a whole will be discussed.