Delineation of Multiple Chlorinated Ethene Sources in an Industrialized Area—A Forensic Field Study Using Compound-Specific Isotope Analysis

Identification of polluters and the allocation of contaminant sources are key tasks when evaluating complex subsurface contamination with multiple and overlapping plumes, as frequently found in industrial areas. Under certain conditions, compound-specific stable isotope analysis (CSIA) may provide decisive information in such environmental litigation studies. Here, we present an illustrative case study highlighting the potential and limitations of using δ13C values of halogenated hydrocarbons (tetrachloroethene and its transformation products) as a key tracer for discriminating different contaminant sources, even in the presence of biodegradation and a complex hydrogeology. A multiple-line-of-evidence approach, including evaluation of historical, hydrological, geochemical, and isotopic data as well as statistical analysis, was applied to unravel the contamination scenario at the site. A key factor was the coverage of a wide area of the contaminant plumes with highly precise δ13C values of chlorinated ethenes in groundwater at trace concentrations in the low microgram per liter range made possible by the application of online purge-and-trap-GC/IRMS. Our work is the first successful example of a forensic isotope field study on chlorinated ethenes in a fractured bedrock aquifer.