Groundwater beneath an active chemical plant located in the San Francisco Bay area contains elevated concentrations (>50 mg/L) of tetrachloroethene (PCE) and carbon tetrachloride (CT). The remediation area is 200 acres, with 3,900 feet of bioactive zone created by 39 wells which inject and circulate amendments. Evidence for prior natural attenuation was provided by the presence of daughter products, as well as by microcosm studies and PCR analysis which documented the presence of D. ethenogenes. The microcosm studies also showed that the addition of electron donor greatly accelerated the rate of contaminant destruction. In several site locations, the monitoring data has shown a decrease in PCE and trichloroethene (TCE), with a concomitant increase in daughter products, including ethene, the product of D. ethenogenes dechlorination. In addition, site data shows a decrease in CT with an increase in chloroform (CF) and dichloromethane (DCM). Microcosm studies have been undertaken to elucidate the mechanism of CT destruction, and thereby to optimize the effectiveness of the treatment process.
Experiments by others with cultures of anaerobic bacteria have shown that CT can be degraded by reductive dechlorination to CF and DCM, as well as by an abiotic substitutive process accelerated by cellular constituents which converts CT and CF to CO2. Only DCM can be readily used by bacteria as a sole carbon source. Destruction of CT, at high concentration (20 mg/L) in soil/groundwater samples from three site areas have been studied in microcosm tests. Sulfate-reducing bacteria were important since the site has sulfate ranging from 300 to 900 mg/L. Groundwater samples collected from one area (BW-004) contain only traces of the original contaminant but do contain ethenogens as well as sulfate-reducers. When CT is added during sulfate reduction with added donor, CT was converted to non-chlorinated products at 1.1 mM/day, and to CF at 0.6 mM/day. Also, during dechlorination of PCE, added CT was converted to non-chlorinated products at 2.7 mM/day and to CF at 0.4 mM/day. A culture of sulfate-reducing bacteria from this well was able to convert CT completely to 54% non-chlorinated products and 46% DCM. Groundwater collected from another area (BW-619) also contain only traces of the original contaminant, but show significant concentrations of reduced compounds, ethenes and light alkanes, suggestive of abiotic processes. When a donor which produces high molecular H2 concentrations was added to microcosms, sulfate was quickly reduced, after which several additions of CT were degraded, at the rate of 3 mM/day to non-chlorinated products and 0.6 mM/day to CF. The same soil/groundwater, when autoclaved and given sulfide reducing agent, briefly converted CT to non-chlorinated products at 2.6 mM/day. However, in an acid-killed control, the CT was stable for 72 days. Groundwater samples collected from a third location (BW-607) still exhibit CT, CF, and TCE at about 20 mg/L each. Soil/groundwater microcosms from this area bioaugmented with site Dehalococcoides ethenogenes culture, and given donor, destroy CT during DCE dechlorination at the rate of 3.8 mM/day to non-chlorinated products and 0.4 mM/day to CF. It is concluded that reductive processes stimulated by donor addition at this site are accelerating the rate of abiotic and biologically-mediated destruction of CT.