Characterization of a site contaminated by chlorinated ethenes and ethanes using multi-analysis

Abstract

Chlorinated ethenes and ethanes are the most commonly detected groundwater contaminants in industrial areas. Compound-specific stable isotope analysis combined with hydrogeological, and geochemical analyses was applied to identify contaminant sources and to monitor the natural degradation processes of chlorinated ethenes and ethanes in an industrial area of Asan, Korea. The concentrations of the detected trichloroethene (TCE) and 1,1,1-trichloroethane (1,1,1-TCA) ranged from 0.004 to 5.8 mg/L and from non-detected to 1.8 mg/L in August 2013. TCE concentrations correspond to 65–87 % of the molar fraction of the volatile organic compounds. Most of the flow system in the study area is aerobic with dissolved oxygen (DO) ranging 0.1–8.3 mg/L. The highest concentration of TCE (5.8 mg/L) and the most depleted δ13C and δ37Cl values (−27.1 and −1.7 ‰, respectively) were observed in the well near an electrical parts company, indicating a source area. The slight enrichments in both the carbon and chlorine isotopic values of contaminants in some wells can be explained by isotopic shifts caused by degradation processes in the study area. 1,1-dichloroethene (1,1-DCE) was present in higher concentrations than cis-DCE and trans-DCE. The concentrations of 1,1-DCA, the biotic degradation product of 1,1,1-TCA, were much lower than those of 1,1-DCE due to the limited anaerobic conditions of the study area. High correlations of 1,1-DCE and 1,1,1-TCA in the study area indicated that some 1,1-DCE might be the dehydrohalogenation product of 1,1,1-TCA. Further isotopic analysis of 1,1,1-TCA are recommended to identify the degradation pathway of 1,1,1-TCA. Hydrogeochemical, VOCs, and dual isotope data showed that dilution with an insignificant rate of biodegradation is the main process attenuating VOCs in the groundwater system. Thus, an active remediation plan is necessary to reduce contaminant concentration in the study area.