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In this 10 year study, Brazilian gasoline (100 L, containing 24% ethanol by volume) was released to a sandy aquifer to evaluate the natural attenuation of benzene, toluene, ethylbenzene, and total xylenes (BTEX) in the presence of ethanol. Groundwater concentrations of BTEX, ethanol, and degradation products (e.g., acetate and methane) were measured over the entire plume using an array of monitoring well clusters, to quantify changes in plume mass and region of influence. Ethanol biodegradation coincided with the development of methanogenic conditions while acetate (a common anaerobic metabolite) accumulated. The benzene plume expanded beyond the 30 m long monitored area and began to recede after 2.7 years, when ethanol had disappeared. Theoretical calculations suggest that the transient accumulation of acetate (up to 166 mg L(-1)) may have hindered the thermodynamic feasibility of benzene degradation under methanogenic conditions. Yet, benzene removal proceeded relatively fast compared to literature values (and faster than the alkylbenzenes present at this site) after acetate concentrations had decreased below inhibitory levels. Thus, site investigations of ethanol blend releases should consider monitoring acetate concentrations. Overall, this study shows that inhibitory effects of ethanol and acetate are relatively short-lived, and demonstrates that monitored natural attenuation can be a viable option to deal with ethanol blend releases.

The rates of biodegradation of benzene, toluene and xylene by indigenous microorganisms in a natural aquifer sand were investigated. Despite highly favorable nutrient and electron acceptor conditions, the onset of measurable microbial oxidation of these readily degraded compounds was observed to be delayed in systems containing small populations of microbes. The delays varied inversely with the initial number of microbes capable of degrading a particular compound, suggesting the need for development of some critical population of microorganisms. In terms of the onset of oxygen consumption, xylene involved the longest critical population development period, consistent with the low comparative numbers of indigenous microbes found capable of degrading this compound.