Influence of saline water and heavy rain on the fate of chlorinated ethenes in groundwater characterized by compound-specific isotope and microbial data

Abstract

Chlorinated ethenes are prevalent contaminants in industrial wastewater that detrimentally affect human health. As elevated tetrachloroethene (PCE) concentrations (18.0–18.7 mg/L) have been observed in the groundwater of an industrial area near the ocean in Incheon, South Korea, effective remedies are required to degrade these contaminants. However, the effects of saline–fresh water mixing on microbial biodegradation and contaminant attenuation are not well known. To characterize the groundwater contamination and in situ biodegradation during salt water intrusion mixing dynamics, compound-specific isotope analysis (CSIA) data combined with hydrogeochemical and microbial data were employed to delineate the contaminant sources and the degradation processes of chlorinated ethenes in groundwater near the sea. The presence of degradation by-products indicated the occurrence of biodegradation. The highest PCE concentration (18.7 mg/L) and the most depleted δ¹³C (–28.7‰) and δ³⁷Cl (–1.2‰) in PCE were observed in groundwater at well NDMW-13, revealing a potential source of contamination. Slightly enriched δ¹³C (–26.2 to –25.0‰) in PCE compared with that of source zone (–28.7 to –26.0‰) and organohalide-respiring bacteria (OHRB), as well as chlorinated ethene degraders, were observed in other wells (NDMW-9, NDMW-10, NDMW-11, and NDMW-12), providing confirmatory evidence for the biodegradation of chlorinated ethenes. The CSIA and microbial data results indicate only a minor amount of natural attenuation in the source zone area. Chlorinated ethenes in the plume boundary zone are close to the sea and affected by saline–fresh water mixing; therefore, the microbial community structures slightly differ from those in the source zone. This study suggests that CSIA and microbial and groundwater age data are useful for evaluating the effects of the mixing dynamics of saline water intrusion and heavy rainfall infiltration on the dechlorination of chlorinated ethenes in groundwater.