ASSESSING THE CUMULATIVE IMPACTS OF SURFACE MINING AND COAL BED METifANE DEVELOPMENT ON SHALLOW AQUIFERS IN THE POWDER RIVER BASIN, WYOMING

Abstract

Large scale surface coal mining taken place along the cropline of the Wyodak-Anderson coal seam since approximately 1977. Groundwater impacts due to surface mining of coal and other energy-related development is a primary regulatory concern and an identified Office of Surface Mining deficiency in the Wyoming coal program. The modeled aquifers are the upper unit (coal) of the Paleocene Fort Union Formation and the overlying Eocene Wasatch Formation. A regional groundwater model covering 790 square miles was constructed using MODFLOW, to simulate the impacts from three surface coal mines and coal bed methane development occurring downdip. Assessing anisotropy of the coal aquifer, quality checking of in situ aquifer tests and database quality control were precursors to modelling. Geologic data was kriged to develop the structural model of the aquifers. A Geographic Information System (GIS) was utilized to facilitate storage, analysis, display, development of input modelling arrays and assessment of hydro1ogic boundaries. Model output presents the predicted impacts of likely development scenarios, including impacts from coal bed methane development and surface coal mining through anticipated life of mining, and surface mining impacts independent of gas development.

Conceptual model of the uppermost principal aquifer systems in the Williston and Powder River structural basins, United States and Canada

“…Koch and others (1982) and Peacock (1997) simulated shallow regional groundwater flow for the Powder River structural basin to assess possible effects of surface coal mining and coalbed-methane extraction. Hotchkiss and Levings (1986) simulated groundwater flow in the lower Tertiary and Upper Cretaceous aquifer systems in the Powder River structural basin and estimated transmissivity for these aquifers on the basis of geophysical borehole logs.…”