REGIONAL GROUNDWATER MONITORING DATA: IMPACTS FROM CBM DEVELOPMENT IN THE POWDER RIVER BASIN, SOUTHEASTERN MONTANA

The Powder River Basin in southeastern Montana is a semi-arid region with an agricultural-based economy that is dependent on the availability of groundwater. Coalbed methane (CBM) production, which began in this area in April 1999, requires the removal and management of large quantities of water from coalbeds. These same coalbeds are commonly used as aquifers in this region, providing a reliable source of domestic and livestock water in these rural areas. In Montana, during 2008, 772 CBM wells discharged at an average rate of 4.0 gallons per minute. Due to the importance of groundwater, and the need to understand the actual impacts to aquifers, a regional monitoring network was established.

The Montana regional CBM groundwater monitoring program has been in operation for 10 years, but was built upon an existing program that has been continually active in the area since 1970. The program includes regular monitoring at 234 wells, 15 springs, and 2 streams. Monitored units include coalbeds, adjacent sandstone units, and alluvium (Meredith et al., 2009). This program provides documentation of the potentiometric surfaces and water level changes and will continue to be active throughout the duration of CBM production and post-production groundwater recovery.

Groundwater levels have been drawn down by over 200 feet in the producing coalbeds. The actual amount of drawdown in some wells cannot be measured due to safety concerns as a result of methane release from monitoring wells. After 10 years of CBM production, the 20 foot drawdown contour extends roughly 1 to 1.5 miles outside the production areas.

Major faults tend to act as barriers to groundwater flow and drawdown does not migrate across fault planes where measured in monitoring wells. However, in cases where faults are not offset by at least 10 feet more than the thickness of the coal, they are less likely to act as a barrier. Vertical migration of drawdown tends to be limited by shale layers; however in some cases minor changes in overburden head have been observed.