A study was designed to determine if the intrinsic bioremediation of gas condensate hydrocarbons represented an important fate process in a shallow aquifer underlying a natural gas production site. For over 4 yr, changes in the groundwater,sediment, and vadose zone chemistry in the contaminated portion of the aquifer were interpreted relative to a background zone. Changes included decreased dissolved oxygen and sulfate levels and increased alkalinity, Fe(I I), and methane concentrations inthe contaminated groundwater, suggesting that aerobic heterotrophic respiration depleted oxygen reserves leaving anaerobic conditions in the hydrocarbon-impacted subsurface. Dissolved hydrogen levels in the contaminated groundwater indicated that sulfatereduction and methanogenesis were predominant biological processes, corroborating the geochemical findings. Furthermore, 101000-fold higher numbers of sulfate reducers and methanogens were enumerated in the contaminated sediment relative to background.Putative metabolites were also detected in the contaminated groundwater, including methylbenzylsuccinic acid, a signature intermediate of anaerobic xylene decay. Laboratory incubations showed that benzene, toluene, ethylbenzene, and each of the xyleneisomers were biodegraded under sulfatereducing conditions as was toluene under methanogenic conditions. These results coupled with a decrease in hydrocarbon concentrations in the contaminated sediment confirm that intrinsic bioremediation contributes tothe attenuation of hydrocarbons in this aquifer.
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