BORON DESORPTION AND BORON ISOTOPES IN SODIUM BICARBONATE COALBED METHANE WATERS

Coalbed methane (CBM) waters commonly exhibit Na-bicarbonate composition when the methane is of a microbial origin. The evolution of anoxic, Na-bicarbonate groundwater is characterized by Ca-Na cation exchange, a process linked in many aquifers to microbial CO₂ production and carbonate dissolution. In general, the development of Na-HCO₃^- waters can coincide locally with naturally-occurring water quality problems, including sodium, arsenic, fluoride, and/or boron. Boron isotope ratios are connected to mineral surface charge conditions by the fractionation between boron species, uncharged B(OH)₃ (enriched in ¹¹B) and B(OH)₄^- (enriched in ¹⁰B). Boron desorption typically mobilizes more ¹⁰B from the mineral surface, causing lower ¹¹B/¹⁰B ratios in water.

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.