INVESTIGATION OF THE GEOCHEMICAL EVOLUTION OF COALBED NATURAL GAS WATER AS IT INFILTRATES THROUGH THE UNSATURATED ZONE OF THE TONGUE RIVER MEMBER OF THE FORT UNION FORMATION – POWDER RIVER BASIN, MT

Coalbed Natural Gas (CBNG) represents a significant new domestic energy source; however there are environmental challenges associated with its development. CBNG in the Powder River Basin is produced by reducing the hydrostatic pressure within the coal seam by removing coalbed water, causing the CH4 to desorb from the coal surface and flow to the pumping well. Environmentally sound management of produced water is a major environmental issue associated with the development of CBNG.

One proposed method of CBNG water management is to place the water in unlined basins, allowing the water to infiltrate and recharge underlying aquifers. One of the potential problems associated with this method of water management is that the water will partake in geochemical reactions as it infiltrates which may degrade the underlying groundwater. The basic geochemical evolution of the water as it infiltrates can be anticipated based upon known processes which have been extensively investigated in relation to coal mining over the past 30 years. This study uses this geochemical framework, water chemistry and unsaturated soil and shallow bedrock mineralogy, to develop models of the chemical evolution of the CBNG water as it travels along its flow path.

Data from one well in an active CBNG pond shows a total dissolved solids (TDS) increase from 1,433 mg/L to 12,000 mg/L, due mainly to increases in sulfate concentration and near equal increases in cations. Saturated paste extract data indicate the reactions are occurring in the uppermost 10 feet of material beneath the pond. Possible sources of sulfate include gypsum dissolution and pyrite oxidation. At another site TDS has decreased in one well from 2,500 mg/L to 925 mg/L, due primarily to loss of sulfate and near equal losses of all cations. In the first case, infiltration is occurring through a previously unsaturated zone where soluble salts may have built up over time. At the second site the pond is located in an ephemeral channel with alluvial ground water that may have flushed salts, allowing the CBNG water to dilute the alluvial water.

Data and models generated during this research will allow for the development of scientifically sound siting and monitoring criteria for infiltration basins. These criteria will aid in the responsible use of infiltration basins, and avoid the siting of infiltration basins in unsuitable locations.