BORON DESORPTION AND BORON ISOTOPES IN SODIUM BICARBONATE COALBED METHANE WATERS
To investigate the behavior of B during the evolution of Na-bicarbonate microbial CBM waters, boron concentrations and isotope ratios (δ11B as ‰ NBS951) were determined in groundwater from the Powder River Basin (PRB) in Montana and Wyoming. Preliminary results from CBM production wells indicate generally low B concentrations (42-178 µg/L) and variable δ11B (24.4-35.2‰). Sodium (358-752 mg/L) and HCO3- (930-2010 mg/L) concentrations negatively correlate with δ11B, implying that δ11B may record the extent of B desorption during the evolution of CBM waters. Given that the higher-TDS Na-HCO3- waters are associated with lower δ11B, we suggest that preferential B leaching from desorption sites is associated with the base-exchange reactions and microbial respiration that characterize CBM waters. The lack of apparent relationship between B concentration and δ11B may imply that the quantities of solid-phase B vary within the aquifer, and that δ11B records the extent to which desorption occurs. These isotope trends indicating boron desorption are consistent with trends documented in Na-bicarbonate groundwater in the Atlantic Coastal Plain exhibiting high concentrations of marine-derived boron. Overall, B isotopes could be a valuable tracer for evaluating the evolution and distribution of CBM waters.