GROUND-WATER MONITORING 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 ground water. Coalbed methane (CBM) production is beginning in the area and requires removal and management of large quantities of water from the coal-bed aquifers. Because coal beds in the Fort Union Formation have higher hydraulic conductivity values (average of 1.0 ft2/day), and are more continuous, than the sandstone units (average of 0.06 ft2/day), they are the primary aquifers in this region. The need for a scientific foundation for resource management decisions requires a regional ground-water monitoring program.

The Montana CBM ground-water monitoring program is based on scientific concepts developed over more than 30 years of coal-mine hydrogeology research. The program includes inventories of ground-water resources and regular monitoring at dedicated wells and selected springs. The program is now providing baseline potentiometric and water-quality data, and will continue to be active through the duration of CBM production and post-production ground-water recovery.

In southeastern Montana 158 wells and 29 springs are currently included in the program, and they monitor coal beds, adjacent sandstone units, or alluvium. After 4 years of CBM production, ground-water levels within the producing area have been lowered by as much as 150 feet and by about 20 feet, 1 to 2 miles outside the producing area. CBM production water is dominated by sodium and bicarbonate ions, with sodium adsorption ratios greater than 30 and total dissolved-solids concentrations typically between 1,000 and 2,000 mg/L.

Coordinating the needs of several agencies through a single agency for data collection and reporting improves the economic efficiency and reliability of the program. All data collected as part of this program are made immediately available to the public on the Internet at http://mbmggwic.mtech.edu/. Data and reports are used by industry, landowners, regulators and researchers. Funding for this program is provided by the U. S. Bureau of Land Management, U. S. Forest Service, U. S. Environmental Protection Agency, local Conservation Districts and the State of Montana.