MONOAROMATIC HYDROCARBON TRANSFORMATION UNDER ANAEROBIC CONDITIONS ATSEAL-BEACH, CALIFORNIA - LABORATORY STUDIES

Anaerobic 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 microco sms. Aquifer sediment and ground water were obtained from the site of a historic gasoline spill at Seal Beach, California. Sulfate is presen t in the site ground water at 80 mg/L, and sulfate-reducing activity a ppears to be the dominant intrinsic BTEX bioremediation process where nitrate is absent. In the laboratory, the microcosms were set up with different electron accepters (sulfate and nitrate) in site ground wate r and various defined anaerobic media to estimate intrinsic biodegrada tion rates and to suggest conditions under which anaerobic bioremediat ion could be enhanced. In unamended microcosms, anaerobic biotransform ation of toluene and mfp-xylene (m-xylene and p-xylene were measured a s a summed parameter) occurred at a rate of 7.2 and 4.1 mu g L(-1) h(- 1), respectively, with sulfate as the apparent electron acceptor. Addi tion of nitrate stimulated nitrate-reducing conditions and increased r ates of toluene and mfp-xylene biotransformation to 30.1 and 5.4 mu g L(-1) h(-1), respectively. The catabolic substrate range was altered t o include ethylbenzene in the nitrate-amended microcosms, suggesting a n apparent preferential use of different BTEX compounds depending on t he electron acceptor available. Under all the conditions studied, more than twice the amount of nitrate or sulfate was used than could be ac counted for by the observed BTEX degradation. Benzene transformation w as not observed under the conditions studied. Although methane was det ected in microcosms prepared with anaerobic media lacking nitrate and sulfate, methanogenic biotransformation of BTEX compounds was not obse rved. The results of these experiments indicate that indigenous microo rganisms from the Seal Beach aquifer have significant capability to de grade BTEX hydrocarbons and that intrinsic processes in the Seal Beach aquifer may remediate a portion of the hydrocarbon contamination in s itu without intervention. However, the data also suggest that interven tion by nitrate addition would enhance the rate and extent of anaerobi c BTEX biotransformation.