IMPACTS OF ETHANOL AND BTX ON MICROBIAL POPULATIONS, PROCESSES AND COMMUNITY COMPOSITION IN A SULFATE-REDUCING CONTAMINATED AQUIFER

Replacement of the fuel oxygenate, methyl tertiary butyl ether (MTBE), by ethanol (EtOH) raises questions regarding potential indirect effects of the labile EtOH on biodegradation processes in groundwater aquifers. Numerous aquifers are still contaminated with MTBE and other petroleum compounds from underground storage tanks. Despite its increasing use as an additive, effects of EtOH releases on aquifer microbial ecology and geochemistry have not been well characterized in situ. EtOH is preferentially consumed over other gasoline constituents (including benzene-B, toluene-T and xylene-X) in anoxic environments, and may delay BTX biodegradation through exerting selective pressures on aquifer microbial communities. Furthermore, release of EtOH at concentrations greater than can be consumed by nitrate, iron, or sulfate reduction can drive the redox of an aquifer into methanogenesis. This redox change would be reflected in the microbial community by an increase in the abundance of fermentative and methanogenic microorganisms (Archaea). We conducted a controlled field release of B, T, and X with and without EtOH at Site 60, Vandenburg Air Force Base. A high-density network of > 200 wells was installed to monitor contaminants, geochemistry and microbes in groundwater. In the presence of EtOH we observed significantly increased B, T, and X plume lengths, depletion of sulfate, induction of methanogenic conditions, and increased alkalinity and dissolved CO2. Bacterial and Archaeal cell densities (based on 16S gene copy numbers), determined by quantitative PCR, were significantly higher in wells influenced by release of EtOH relative to background or wells influenced by BTX alone. Increases in Archaeal cell density were detectable far down gradient (>15m) from where EtOH was consumed. Changes in sulfate-reducing bacteria (SRB) populations were correlated with the depletion of sulfate. Our results indicate that EtOH releases can significantly alter groundwater geochemistry and the spatial and temporal distribution of Archaea, as well as bacterial populations, in this anaerobic aquifer. These results have implications for the use of EtOH as a fuel oxygenate because of its impact on microbial community structure and function and the potential consequences for biodegradation of existing contaminants.