AN OVERVIEW OF CONCEPTUAL AND QUANTITATIVE MODELS OF GROUNDWATER IMPACTS FROM CBM DEVELOPMENT IN THE POWDER RIVER BASIN, SOUTHEASTERN MONTANA, AND COMPARISON TO MONITORING DATA

Coalbed methane (CBM) production began in southeastern Montana in April 1999. Production requires the removal and management of large quantities of water from coalbeds that are commonly used aquifers in this region. The Powder River Basin is a semi-arid region with an agricultural-based economy that is dependent on the availability of groundwater. Due to this conflict, a variety of conceptual and quantitative models have been developed to evaluate the potential impacts to water availability (e.g. Wheaton and Metesh, 2002 and Myers, 2009). The differences in these models make it clear that long-term monitoring of actual impacts is required to determine which modeling assumptions are valid and to verify the magnitude of actual impacts as well as the lack of impacts.

In Montana, a regional CBM groundwater monitoring program is operated that includes inventories of groundwater resources and regular monitoring at 234 wells, 15 springs, and 2 streams. Data are also collected from CBM wells, domestic wells and livestock wells. Monitored units include coalbeds, adjacent sandstone units, and alluvium (Meredith et al., 2009). This monitoring shows that after 10 years of CBM production, the 20 foot drawdown contour extends roughly 1 to 1.5 miles outside the production areas; that major faults tend to act as barriers to ground-water flow; and that vertical migration of drawdown tends to be limited by shale layers.

The hydrogeologic assessment in the BLM’s Montana CBM EIS was based in part on computer modeling. It was projected that after 10 years of development the 20 foot drawdown contour would reach approximately 2 miles from the edge of development. It was also anticipated that faults would act as barriers to drawdown, and that that impacts to overlying or underlying aquifers would be minor due to the confined nature of the coals. As such, observed impacts appear to be in line with, or somewhat less than, the projections contained in the Montana CBM EIS. Other models which predict greater impacts do not appear to be supported by the monitoring data.