THE INFLUENCE OF THE GASOLINE OXYGENATE ETHANOL ON AEROBIC AND ANAEROBIC BTX BIODEGRADATION

Ethanol is frequently found along with benzene, toluene, and xylenes ( BTX) in groundwater contaminated with gasoline. Yet, little is known a bout its effect on bioremediation of the toxic BTX contaminants. Aquif er microcosms were used to investigate the effect of ethanol on microb ial degradation of representative BTX compounds under electron accepto r conditions commonly found in intrinsic bioremediation projects. Unde r aerobic conditions, ethanol retarded BTX biodegradation and exacerba ted the biochemical oxygen demand (BOD). Anoxic conditions developed q uickly when the BOD exerted by ethanol exceeded the available oxygen. This led to the persistence of benzene, which was also recalcitrant in denitrifying, sulfidogenic, and methanogenic microcosms during 99 day s of incubation. Toluene was degraded under all anaerobic electron acc eptor conditions tested, although the onset of relatively fast degrada tion always commenced after ethanol bad been removed. Toluene degradat ion was not Significantly affected by ethanol in denitrifying microcos ms containing excess nitrate. Yet, no toluene degradation occurred whe n nitrate was limiting because nitrate was depleted while ethanol was being degraded. Ethanol also adversely affected toluene degradation in iron-reducing and methanogenic microcosms. Curiously, ethanol enhance d toluene degradation;in sulfate reducing microcosms. This was hypothe sized to be due to a low initial concentration of toluene degraders an d their incidental growth during ethanol! degradation. Albeit, the pre ferential utilization of ethanol and the accompanying depletion of ele ctron accepters suggest that ethanol would have a negative effect on p assive BTX bioremediation. This is particularly important for the fare of benzene, which is the most toxic of the BTX and the most recalcitr ant under anaerobic conditions. (C) 1998 Elsevier Science Ltd. All rig hts reserved.