Gieg_et_al_1999 - Environ. Sci. Technol. 1999, 33,...

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Intrinsic Bioremediation of Petroleum Hydrocarbons in a Gas Condensate-Contaminated Aquifer LISA M. GIEG, ²,‡ RAVINDRA V. KOLHATKAR, §, | MICHAEL J. MCINERNEY, ² RALPH S. TANNER, ² STEVE H. HARRIS, JR., KERRY L. SUBLETTE, § AND JOSEPH M. SUFLITA* ,²,‡ Institute for Energy and the Environment and Department of Botany and Microbiology, The University of Oklahoma, Norman, Oklahoma 73019, and Center for Environmental Research and Technology, The University of Tulsa, Tulsa, Oklahoma 74104 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(II), and methane concentrations in the 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 sulfate reduction and methanogenesis were predominant biological processes, corroborating the geochemical findings. Furthermore, 10 - 1000-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 xylene isomers were biodegraded under sulfate- reducing 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 to the attenuation of hydrocarbons in this aquifer. Introduction Intrinsic bioremediation is a relatively nonobtrusive strategy for reducing the concentration of contaminants in subsurface environments. Despite its natural connotation, intrinsic bioremediation does not imply a “do-nothing” scenario. Rather, it is necessary to carefully document that pollutant disappearance is mediated by natural attenuation processes ( 1 ). This documentation should minimally include (i) mea- suring the mass loss of pollutants over time, (ii) demonstrating that indigenous microorganisms have the potential to biodegrade pollutants under site-specific conditions, and (iii) showing that biodegradation potential is actually realized at the site ( 2, 3 ). The microbial degradation of hydrocarbons is a pre- dominant mechanism for natural attenuation at petroleum- contaminated sites, but contaminant loss may also be attributed to abiotic processes such as sorption, volatilization, dilution, and dispersion ( 4 ). Of these processes, only bio- degradation results in the mass loss of contaminants.
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Gieg_et_al_1999 - Environ. Sci. Technol. 1999, 33,...

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