Field and laboratory studies of sequential anaerobic-aerobic chlorinated solvent biodegradation

Recent investigations of intrinsic biodegradation of chlorinated solvents at three different sites indicate that while reductive dechlorination is occurring at each site, anaerobic processes alone cannot fully account for the observed attenuation. Aerobic processes, in sequential combination with anaerobic processes, appear to be impacting, and in some cases controlling, the distribution of the solvents in the groundwater at each site. At the first site, vinyl chloride (VC) and dichloromethane (DCM) produced in an anaerobic source area appear to be mineralized in a downgradient aerobic zone. At the second site, cis-1,2-dichloroethene (cis-1,2-DCE) produced in an anaerobic source area appears to be cometabolically biodegraded in the presence of toluene and methane in a downgradient aerobic zone. At the third site, cometabolic biodegradation of cis-1,2-DCE and VC in the presence of toluene (spilled at the site) and direct VC biodegradation appear to impact the distribution of the solvents in the site groundwater. The anaerobic and aerobic processes believed to be occurring at each site were confirmed in microcosm studies. The results of these three investigations suggest that intrinsic sequential anaerobic-aerobic processes may occur more frequently than previously thought, and that natural sequential redox environments play a significant role in the attenuation of chlorinated solvents in groundwater.