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 about its effect on bioremediation of the toxic BTX contaminants. Aquifer microcosms were used to investigate the effect of ethanol on microbial degradation of representative BTX compounds under electron acceptor conditions commonly found in intrinsic bioremediation projects. Under aerobic conditions, ethanol retarded BTX biodegradation and exacerbated the biochemical oxygen demand (BOD). Anoxic conditions developed quickly 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 days of incubation. Toluene was degraded under all anaerobic electron acceptor conditions tested, although the onset of relatively fast degradation always commenced after ethanol had been removed. Toluene degradation was not significantly affected by ethanol in denitrifying microcosms containing excess nitrate. Yet, no toluene degradation occurred when 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 enhanced toluene degradation in sulfate reducing microcosms. This was hypothesized to be due to a low initial concentration of toluene degraders and their incidental growth during ethanol degradation. Albeit, the preferential utilization of ethanol and the accompanying depletion of electron acceptors suggest that ethanol would have a negative effect on passive BTX bioremediation. This is particularly important for the fate of benzene, which is the most toxic of the BTX and the most recalcitrant under anaerobic conditions.