Paper No. 2
Presentation Time: 8:20 AM


VINSON, David S.1, MCINTOSH, Jennifer C.2, DWYER, Gary S.3, WARNER, Nathaniel R.3 and VENGOSH, Avner3, (1)Department of Geography & Earth Sciences, University of North Carolina at Charlotte, 9201 University City Blvd., McEniry 324, Charlotte, NC 28223, (2)Department of Hydrology and Water Resources, University of Arizona, Tucson, AZ 85721, (3)Division of Earth and Ocean Sciences, Nicholas School of the Environment, Duke University, Durham, NC 27708,

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.