| Water and gas samples were collected from coalbed methane wells and surface coal mines in the Powder River Basin and analyzed for solute chemistry, isotopes, and gas composition to determine timing and source of recharge, nutrient influxes, extent of methanogenesis, and redox conditions. delta18O-H 2O values and hydraulic gradients show recharge to coal beds is principally from the southern basin margin with inputs from the western and eastern margins. Detectable 14C in coal waters indicates they were recharged <50,000 BP. Correlation of deltaD-CH4 and deltaD-H2O values suggests that methane has accumulated since the Late Pleistocene. Nutrient concentrations were low and did not correlate to groundwater recharge sources. Coal gases from the northwestern basin are isotopically-depleted suggesting "early stage" methanogenesis, whereas coal gases from the central southeastern basin are isotopically-enriched suggesting "late stage" methanogenesis. Several wells have elevated SO4 and oxygen, which may be due to recent hydrologic changes from groundwater pumping. |
