We previously reported on the implementation of a large in-situ bioremediation project at the site of an active chemical plant located in the San Francisco Bay area containing greater than 50 mg/L each of tetrachloroethene (PCE) and carbon tetrachloride (CT), as well as sulfate concentrations up to 800 mg/L. The remediation area is 200 acres, with 3,900 feet of bioactive zone created by 39 wells which inject and circulate amendments. The high concentrations of PCE were found to be readily biodegradaded to ethene by native D.ethenogenes in groundwater from many locations in which PCE was the main contaminant. However, in locations where chlorinated ethenes were mixed with chlorinated methanes, the high concentrations of CT, as well as the product of its reductive dechlorination chloroform (CF), might have presented a challenge for bioremediation, due to potential inhibitory action of these compounds.
The remedial strategy for the site included the use of propylene glycol (1,2-propane diol)(PG), which was shown in microcosm studies with site groundwater to be a highly efficient donor for sulfate reduction, as well as a source of H2 for dechlorination. We show results from field monitoring which demonstrate that in areas where sulfate reduction is supported by added electron donor, high concentrations of CT are degraded, and PCE is reductively dechlorinated to ethene in the presence of chlorinated methanes (CT, CF, DCM). The data to be discussed covers 1.5 years, during which electron donor gradually reached the monitoring well. Since it was known from microcosm studies that PG is converted to acetate during sulfate reduction, as well as to propionate by other microorganisms, these products were also measured in field samples. The field data show a complete reduction of 500 mg/L sulfate concomitant with an increase in products of donor utilization, acetate, propionate and butyrate. Concomitant with sulfate reduction, CT and CF concentrations decreased, followed by an increase in the concentration of DCM. Also, TCE was converted to VC and ethene. Most importantly, the destruction of CT appears to involve only partial convertion to CF and DCM (dichloromethane), and partial conversion to non-chlorinated products, a result that has also been seen in microcosm studies using site groundwater.