Study of Ground-Water Recharge Rates in the Northern Powder River Basin

Coal-bed methane (CBM) production in the Powder River Basin is growing rapidly. The Bureau of Land Management estimates that by the year 2010 there may be as many as 50,000 producing wells in the Wyoming part of the Basin alone. Development of CBM requires the pumping of water from coal seams. As water pressure in the seam is lowered, methane is released from storage. The pumped water is usually discharged into streams, channels, or impoundments. With a typical well producing about 48 cubic meters of water per day, the rate of discharge for the Basin could exceed 2.4 million cubic meters per day by 2010. The fate of that water and its impact on the environment are topics of some concern. Relevant issues include the rate at which water infiltrates and percolates to the water table, the eventual discharge of infiltrated water to surface water, and chemical changes that occur as water moves through the system. An adequate understanding of these issues requires knowledge of local and regional hydrology. This study is investigating rates of ground-water recharge under natural conditions and as impacted be CBM development. Natural recharge within the Powder River Basin occurs by both diffuse mechanisms (infiltration of precipitation and subsequent travel of water through the unsaturated zone to the water table) and focused mechanisms (infiltration from line and point sources, such as streams and ponds). Under natural conditions, the relative importance of each is difficult to assess. Discharged CBM water should substantially enhance the rate of focused recharge, with less effect expected on diffuse recharge. Objectives of this study are to estimate rates of diffuse recharge, naturally occurring focused recharge, and enhanced focused recharge due to the discharge of CBM water. Multiple approaches are being employed: chloride mass balance; tracer methods based on tritium, stable isotopes, and other compounds; Darcy/unit hydraulic gradient; water budget; water-table fluctuation, and channel loss methods. Preliminary results indicate that natural rates of recharge vary spatially as a function of soil texture and land surface slope and range from less than 1 to more than 200 mm per year.