Challenges in Scaling Subsurface Microbial Processes: Linking Contaminants, Geochemistry and Microorganisms.

Microorganisms unquestionably play central roles in bioremediation of groundwater contaminants. Development and deployment of molecular tools targeting specific microbial taxa and functional genes can provide information about potential, rates and regulation of biodegradation reactions. New discoveries and commercialization of molecular tools create opportunities to incorporate microbial ecology into site characterization and monitoring plans. Though attributing biodegradation to specific microbial populations is relatively easy in the laboratory, making these linkages to in situ bioremediation is challenging, especially at the field scale where management decisions are made. Ethanol (EtOH) is a common gasoline additive and increasingly popular new fuel. Because leakage of EtOH from underground fuel storage tanks is inevitable, understanding and predicting impacts of these releases on co-contaminants, aquifer microbial ecology and geochemistry is needed. In a field study, we measured the effect of a controlled EtOH release on biodegradation of petroleum constituents and MTBE in a coastal aquifer underlying a former fuel station. We measured contaminant, geochemical and microbial parameters in multiple wells and over one year. Plumes of benzene, toluene, and o-xylene extended further down gradient in the presence compared to absence of ethanol. MTBE was transformed to tert-butyl alcohol (TBA), an undesirable by-product, only in the presence of ethanol. Bacterial and sulfate-reducing bacteria (SRB) densities increased with and without ethanol amendment. Significant increases in Archaeal species richness and cell densities, depletion of sulfate, and induction of methanogenic conditions were only observed with ethanol. Some of the challenges in this study help define future research directions. These include more fundamental studies to define relationships between microbial data (e.g., densities, diversity) and bioremediation progress; development of inexpensive, broad, high throughput microbial methods to permit large-scale spatial and temporal comparisons; and application of spatial statistics and models to integrate microbial, contaminant and geochemical information.