Natural and enhanced bioattenuation potential of an aquifer contaminated by mixed wastes.

Abstract

Intrinsic bioremediation is scientifically valid for controlling fuel-contaminated aquifers, but its potential to remediate chlorinated solvents is less understood. This study used field and laboratory methods to assess bioattenuation potential in a shallow unconfined aquifer contaminated by mixed wastes from fire training activities at Wurtsmith Air Force Base in Michigan. Biogeochemical characterization of a cross-sectional plume transect revealed spatial and temporal heterogeneities in contaminant composition, microbial community structure, and terminal electron accepting processes (TEAPs). Over one year, TEAPs evolved from methanogenesis to sulfate reduction, with corresponding shifts in microbial phylogeny. Chlorinated solvent and fuel biodegradation evidence included methane, carbon dioxide, aromatic acids, <italic>cis</italic>-dichloroethylene, and vinyl chloride accumulation in groundwater. However, cell numbers were low outside the highly-contaminated capillary fringe, and cells were small and appeared nutritionally stressed. Furthermore, concentrations of alkylbenzenes (570 mg/kg), chloroalkenes, and chlorobenzenes (3000 m g/kg) have remained essentially constant during the past ten years, despite dynamic redox zonation throughout the plume. Laboratory biodegradation assays for sediments from discrete redox zones supported field observations. Toluene and trichloroethylene mineralization did not exceed 5%, regardless of sediment origin. Dechlorination intermediates accumulated only in methanogenic-zone soil. Methane and carbon dioxide were produced from mineralization of contaminants other than toluene and trichloroethylene in all anaerobic microcosms after 200 days. Neither purged nor contaminated groundwater was toxic to pure toluene-oxidizing or methanotrophic cultures, nor was xylene at field concentrations. Nitrogen and phosphorus amendments had little effect on microbial activity, but inoculum density influenced toluene degradation by <italic>Pseudomonas putida</italic>. At low initial densities (10<super>3</super>--10<super>7</super> CFU/mL), groundwater impeded toluene degradation compared to media, but this behavior was reversed above 10<super>8</super> CFU/mL, possibly from culture history or nutrient depletion during growth, rather than from variable enzyme induction. This study demonstrated that chlorinated solvent bioattenuation in mixed waste environments may be overestimated without laboratory-measured biodegradation kinetics. Adequate biomass and catabolic capacity may be absent even when electron donors and acceptors are plentiful, and in such cases bioaugmentation or other engineered strategies should be considered for subsurface remediation.