Natural Attenuation Of Chlorinated Solvents In Subarctic Ground Water

Abstract

Little is known about natural attenuation of chlorinated solvents in subarctic ground water. This study aimed to better understand the biogeochemistry and microbiology associated with naturally occurring processes of contaminant removal at two hydrologically diverse sites near Fairbanks, Alaska. Six Mile Village, located several km north of the Tanana River, is hydrologically stable, experiencing minor fluctuations in ground-water levels. Fort Wainwright is located adjacent to the Chena River and is hydrologically dynamic, experiencing seasonal flow reversals and substantial fluctuations in water-table elevations. By comparing data collected seasonally and with data collected at the two sites, I determined how ground-water/surface-water interactions affected in situ redox conditions and, hence, natural attenuation processes. A portion of the aquifer at Fort Wainwright was undergoing active treatment so I was also able to compare differences in chlorinated solvent transformations in treated and untreated ground water. Although ground water at Fort Wainwright was generally more oxidized than ground water at Six Mile Village, hydrogen concentrations at both sites were almost uniformly within ranges suggestive of iron or manganese reduction. However, aquifer sediments in the Tanana/Chena Alluvium are composed of mafic (containing reduced iron and manganese) minerals; suspended ferric iron appeared to result from oxidation of ferrous iron as ground water rose through the unsaturated zone. Sulfate concentrations were substantial and dissolved sulfide in most samples suggested that sulfate reduction might have been an important process. Calculated in situ Gibbs free energies for iron and sulfate reduction were energetically favorable at both sites; given other geochemical data, it seems likely these two processes co-occurred. Although methane was present in most samples, methanogenesis from H2 /CO2 was generally not energetically feasible at either site. Methane likely diffused from underlying permafrost or peat. The presence of less chlorinated intermediates of solvent degradation suggested that biological reductive dechlorination occurred, providing further support that sulfate-reducing conditions existed. However, low rates of microbial activity, incomplete degradation and persistence of contaminants imply that biologically mediated mechanisms did not likely represent an important contribution to natural attenuation of contaminants at either site where dilution appeared to be a major attenuation mechanism.