Intrinsic biodegradation of trichloroethene and trichloroethane in a sequential anaerobic-aerobic aquifer

Trichloroethene (TCE) and 1,1,1-trichloroethane (1,1,1-TCA) are being intrinsically biodegraded in the groundwater near a former unlined septic-waste lagoon (former lagoon) at a site in Sacramento, California. Although the groundwater at the site is predominantly aerobic, groundwater in the immediate vicinity of the former lagoon is anaerobic. In the anaerobic zone, TCE and 1,1,1-TCA are being sequentially dechlorinated to ethene and ethane. However, the rate of dechlorination is not sufficient to prevent transport of TCE, 1,2-dichloroethene (1,2-DCE), vinyl chloride (VC), 1,1-DCA, and chloroethane (CA) from the anaerobic zone. In the aerobic zone, the lateral distributions of VC and CA are considerably smaller than the lateral distributions of the higher chlorinated volatile organic compounds (VOCs). Because VC and CA are more mobile in groundwater than the higher chlorinated VOCs, and therefore should have greater lateral distributions in the groundwater, the authors conclude that VC and CA are being biodegraded in the aerobic zone. These results provide field evidence for the intrinsic biodegradation of TCE and 1,1,1-TCA to nonchlorinated end products via two processes: (1) complete reductive dechlorination to ethene and ethane under anaerobic conditions; and (2) reductive dechlorination to VC or CA under anaerobic conditions, followed by biodegradation of VC and CAmore » under aerobic conditions.« less