RAPID IN-SITU DECHLORINATION OF SOLVENTS BY ABIOTIC AND BIOTIC MECHANISMS

A combined abiotic/biotic field pilot study was performed at an industrial site with high concentrations of chlorinated solvents in a fractured bedrock groundwater system. The primary groundwater contaminants consist of 1,1,1-trichloroethane (TCA) and related degradation products, including 1,1-dichloroethane (DCA), 1,1-dichloroethene (DCE), chloroethane (CA), and vinyl chloride (VC). The pilot study consisted of a co-injection of submicron zero-valent iron (ZVI) powder and a biological enhancement amendment into the source area within the bedrock formation. The ZVI catalyzes a rapid abiotic reductive dechlorination reaction, resulting in complete degradation of the chlorinated compounds to ethane and ethene. The biological enhancement amendment consists of emulsified soybean oil and sodium lactate that provides a combination of fast and slow-release electron donors to enhance and support biological degradation of the chlorinated solvents for long periods of time. There is clear evidence of biological degradation by the bacteria Dehalococcides ethenogenes (D. ethenogenes) and TCA-degrading microorganisms, with no inhibition of D. ethenogenes activity by the presence of TCA. Groundwater monitoring was performed for 13 months subsequent to the injection, and results indicate rapid dechlorination of all of the chlorinated compounds both by abiotic and biotic mechanisms. Specifically, TCA concentrations decreased from 10,000 µg/L to below detection limits within seven months and continue to be below detection limits more than one year after the pilot study.