Assessment of natural attenuation of chlorinated aliphatics and BTEX in subarctic groundwater

We examined biogeochemistry and microbiology associated with natural attenuation of trichloroethene (TCE), trichloroethane (TCA), and benzene in a subarctic aquifer. Identification of a predominant terminal electron-accepting process (TEAP) and characterization of typical natural attenuation footprints was difficult. Hydrogen and ferrous iron concentrations suggested that iron reduction was the predominant TEAP; calculated in situ Gibbs free energies for iron reduction were energetically feasible at all wells although a source of ferric iron has not been conclusively determined. The presence of dissolved sulfide and favorable free energies for sulfate reduction provided support of concurrent iron and sulfate reduction. Methanogenesis from H2/CO2 was generally not energetically favorable. The presence of TCE and TCA degradation intermediates suggested that biological reductive dechlorination occurred, although proportions of intermediates relative to parent compounds remained stable. By September 2000, contaminant concentrations were within regulatory standards at most sampling points. However, low rates of microbial activity and incomplete degradation imply that intrinsic bioremediation did not likely represent an important contribution to contaminant removal atthis site, where dilution appeared to be the primary attenuation mechanism.