ABSTRACT
The proposed replacement of the gasoline oxygenate MTBE with ethanol represents potential economic and environmental quality benefits. However, these benefits may be offset to some extent by potential detrimental effects on groundwater quality and natural attenuation of released petroleum products. The objectives of this literature review are to bound the extent to which these impacts may occur, summarize the available information on the biodegradation of ethanol in the environment, assess the potential effect that biodegradation processes may have on the fate and transport of BTEX compounds, and provide recommendations for research to enhance related risk assessment and management decisions. Ethanol that reaches groundwater aquifers is likely to be degraded at much faster rates than other gasoline constituents. If the carbon source is not limiting, a preferential degradation of ethanol over BTEX may be observed under both aerobic and anaerobic conditions. Depending on the extent of the release, ethanol may exert a high biochemical oxygen demand that would contribute to the rapid depletion of dissolved oxygen in the groundwater. Thus, ethanol will likely be degraded predominantly under anaerobic conditions. None of the potential ethanol metabolites that could accumulate in groundwater are toxic, although some potential biodegradation by-products such as butyrate could adversely affect the taste and odor of drinking water sources. In addition, acetate and other volatile fatty acids could accumulate at high concentrations, causing a pH decrease in poorly buffered systems. It is unknown, however, whether the pH would decrease to a point that inhibits natural degradative processes. Inhibition of microbial, activity near the source is likely to occur as a result of exposure to high alcohol concentrations, and bactericidal effects are likely to occur when cells are exposed to ethanol concentrations exceeding 10,000 mg/L. However, the maximum allowable ethanol content in gasoline is 10% by volume in the United States. Thus, such high ethanol concentrations are unlikely to be encountered at sites contaminated with ethanol-gasoline blends, except near the fuel/water interfaces or in the case of neat ethanol releases. Downgradient of the source area, biodegradation is unlikely to be inhibited by alcohol toxicity as concentrations decrease exponentially with distance. The preferential degradation of fuel alcohols by indigenous microorganisms and the accompanying depletion of oxygen and other electron acceptors suggest that ethanol could hinder BTEX bioremediation. This is particularly important for the fate of benzene, which is the most toxic BTEX compound and the most recalcitrant under anaerobic conditions. Alternatively, ethanol represents a carbon and energy source that is likely to stimulate the growth of a variety of aerobic and anaerobic microbial populations, including those that can degrade BTEX compounds. A higher concentration of BTEX degraders would be conducive to faster BTEX degradation rates under carbon-limiting conditions. Nevertheless, controlled studies that assess the overall effect of ethanol on BTEX bioremediation are lacking. In theory, ethanol could also contribute to longer BTEX plumes by enhancing BTEX solubilization from the fuel phase and by decreasing sorption-related retardation during transport. The overall effect of ethanol on BTEX plume length and treatment end points is likely to be system specific, and will depend largely on the release scenario and on the buffering and dilution capacity of the aquifer. Additional research is needed to understand the effect of ethanol on the stability and dimensions of co-occurring and pre-existing BTEX plumes. Future laboratory and field studies should also address response variability as a function of release scenario and site specificity, to facilitate risk assessment and remedial action decisions.