Hydrogeochemical and microbiological studies for enhanced groundwater bioremediation

Contamination of shallow aquifers by dissolved aromatic hydrocarbons resulting from leaking underground storage tanks is a widespread problem.  Of particular concern are the BTEX (benzene, toluene, ethylbenzene, and xylenes) compounds because they are both toxic and highly mobile in groundwater.  Bioremediation is often considered for treatment of hydrocarbon-contaminated groundwater.  Because electron acceptor demands typically created in aquifers following fuel contamination greatly exceed dissolved oxygen supplies (Chapelle et al., 1995), contaminated aquifers naturally tend to become anaerobic.  Biodegradation processes operative under anaerobic conditions are thus important for supporting intrinsic bioremediation.  Anaerobic processes have also been considered for potential use in enhanced bioremediation to avert difficulties associated with introducing, dispersing, and maintaining oxygen at levels adequate to sustain aerobic biodegradation (National Research Council, 1994).  For these applications, electron acceptor supplementation with nitrate has attracted particular attention because of its high water solubility and general lack of noxious by-products (Hutchins et al., 1991; Batterman, 1986; Reinhad et al., 1997).

Elucidation of the microbiological and hydrogeochemical influences on anaerobic biodegradation of hydrocarbons in aquifers is needed to gain an improved understanding of both intrinsic and enhanced bioremediation processes (National Research Council, 1994).  The main objectives of this study were to examine processes involved in anaerobic biodegradation through field and laboratory experiments and to incorporate results into numerical modeling to more accurately predict biodegradation.  Specific tasks associatedw ith these were to 1) evaluate the extend of biodegradation that occurs naturally in the plume (intrinsic bioremediation) through field monitoring; 2) conduct tracer tests to evaluate if addition of electron acceptors, specifically nitrate and sulfate, enhances biodegradation and, if so, to quantify rates; 3) construct microcosm experiments to test specific processes and to examine degradation under sequential electron-accepting processes; 4) use computer models to simulate the current and possible future configurations of the BTEX plume.