CONSIDERATIONS FOR EVALUATING COALBED METHANE INFILTRATION POND SITES BASED ON SITE STUDIES IN THE POWDER RIVER BASIN OF MONTANA AND WYOMING

摘要

Significant volumes of ground water are produced in association with coalbed-methane (CBM) production in the Powder River Basin in Montana and Wyoming. This water must be managed in a manner that is both economical and sensitive to the semi-arid agricultural area of southeastern Montana and northeastern Wyoming. Infiltration ponds are one of the primary methods of handling production water and have been in use in Montana and Wyoming for several years. A solid conceptual framework of the parameters that control water quality and flow allows for the selection of infiltration pond sites which maximize impoundment life and minimize impacts. The ponds have several advantages in that they require a low initial investment and can help recharge the shallow ground-water system, which makes the production water available for future uses. However, as the infiltrated water moves through the shallow weathered bedrock, a series of chemical reactions typically take place (primarily dissolution and oxidation) which temporarily increase the total dissolved solids (TDS) due primarily to increases in Mg, Na, and SO_4. As the available salts are removed along the ground-water flow path through the bedrock, the concentrations of dissolved constituents in the ground water tend to decrease. Preliminary interpretations of data suggest that saturated paste extract (SPE) analyses and lithologic investigations may be used to predict the types of changes in water quality that can occur. The fate and transport of the dissolved salts is controlled to a great extent by the rate of infiltration and the duration of saturated flow from the ponds. The rate of infiltration can be severely reduced as the clays in the pond floor and underlying material are exposed to the high SAR produced water, which causes dispersion and reduced vertical hydraulic conductivity. Order-of-magnitude decreases in vertical hydraulic conductivity have been observed, which represent a trade-off. First, the changes will effectively decrease the volume of water that can be managed via an individual pond. Secondly, the mobilized salts may be effectively sequestered by reduced ground-water flow; substantially reducing the temporal and geographic extent of impacts.