Geochemical and physiological evidence for mixed aerobic and anaerobic field biodegradation of coal tar waste by subsurface microbial communities

We used geochemical analyses of groundwater and laboratory-incubated microcosms to investigate the physiological responses of naturally occurring microorganisms to coal-tar-waste constituents in a contaminated aquifer. Waters were sampled from wells along a natural hydrologic gradient extending from uncontaminated (1 well) into contaminated (3 wells) zones. Groundwater analyses determined the concentrations of carbon and energy sources (pollutants or total organic carbon), final electron acceptors (oxygen, nitrate, sulfate), and metabolic by-products (dissolved inorganic carbon [DIC], alkalinity, methane, ferrous iron, sulfide, Mn²⁺). In the contaminated zone of the study site, concentrations of methane, hydrogen, alkalinity, and DIC were enhanced, while dissolved oxygen and nitrate were depleted. Field-initiated biodegradation assays using headspace-free serum bottle microcosms filled with groundwater examined metabolism of the ambient organic contaminants (naphthalene, 2-methylnaphthalene, benzothiophene, and indene) by the native microbial communities. Unamended microcosms from the contaminated zone demonstrated the simultaneous degradation of several coal-tar-waste constituents at the in situ temperature (10°C). Lag phases prior to the onset of biodegradation indicated the prevalence of both aerobic and anaerobic conditions in situ. Electron acceptor-amended microcosms from the most contaminated well waters demonstrated only aerobic naphthalene degradation. Collectively, the geochemical and microbial evidence show that biodegradation of coal-tar-waste constituents occurs via both aerobic and anaerobic terminal electron accepting processes at this site.