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 δ