Monoaromatic hydrocarbon transformation under anaerobic conditions at seal beach, California: Laboratory studies

Abstract

AbstractAnaerobic biotransformation of several aromatic hydrocarbons found in gasoline including benzene, toluene, ethylbenzene, m‐xylene, p‐xylene, and o‐xylene (BTEX) was studied in batch anaerobic laboratory microcosms. Aquifer sediment and ground water were obtained from the site of a historic gasoline spill at Seal Beach, California. Sulfate is present in the site ground water at 80 mg/ L, and sulfate‐reducing activity appears to be the dominant intrinsic BTEX bioremediation process where nitrate is absent. In the laboratory, the microcosms were set up with different electron acceptors (sulfate and nitrate) in site ground water and various defined anaerobic media to estimate intrinsic biodegradation rates and to suggest conditions under which anaerobic bioremediation could be enhanced. In unamended microcosms, anaerobic biotransformation of toluene and m+p‐xylene (m‐xylene and p‐xylene were measured as a summed parameter) occurred at a rate of 7.2 and 4.1 μg L−1 h−1, respectively, with sulfate as the apparent electron acceptor. Addition of nitrate stimulated nitrate‐reducing conditions and increased rates of toluene and m+p‐xylene biotransformation to 30.1 and 5.4 μg L−1 h−1, respectively. The catabolic substrate range was altered to include ethylbenzene in the nitrate‐amended microcosms, suggesting an apparent preferential use of different BTEX compounds depending on the electron acceptor available. Under all the conditions studied, more than twice the amount of nitrate or sulfate was used than could be accounted for by the observed BTEX degradation. Benzene transformation was not observed under the conditions studied. Although methane was detected in microcosms prepared with anaerobic media lacking nitrate and sulfate, methanogenic biotransformation of BTEX compounds was not observed. The results of these experiments indicate that indigenous microorganisms from the Seal Beach aquifer have significant capability to degrade BTEX hydrocarbons and that intrinsic processes in the Seal Beach aquifer may remediate a portion of the hydrocarbon contamination in situ without intervention. However, the data also suggest that intervention by nitrate addition would enhance the rate and extent of anaerobic BTEX biotransformation.