A stable isotope approach for source apportionment of chlorinated ethene plumes at a complex multi-contamination events urban site

• Stable carbon isotope of chlorinated compounds apportion production lines of solvents
• Groundwater samples of 7 sites (31 boreholes) in Ferrara were compared.
• 2 major sources were distinguished, one strongly depleted in 13C (<−60‰).
• Production lines which have used depleted methane for synthesis are well defined.
• The presented case could be useful as a general forensic approach to identify polluters.

The stable carbon isotope composition of chlorinated aliphatic compounds such as chlorinated methanes, ethanes and ethenes was examined as an intrinsic fingerprint for apportionment of sources. A complex field site located in Ferrara (Italy), with more than 50 years history of use of chlorinated aliphatic compounds, was investigated in order to assess contamination sources. Several contamination plumes were found in a complex alluvial sandy multi-aquifer system close to the river Po; sources are represented by uncontained former industrial and municipal dump sites as well as by spills at industrial areas.The carbon stable isotope signature allowed distinguishing 2 major sources of contaminants. One source of chlorinated aliphatic contaminants was strongly depleted in 13C (<−60‰) suggesting production lines which have used depleted methane for synthesis. The other source had typical carbon isotope compositions of >−40‰ which is commonly observed in recent production of chlorinated solvents. The degradation processes in the plumes could be traced interpreting the isotope enrichment and depletion of parent and daughter compounds, respectively. We demonstrate that, under specific production conditions, namely when highly chlorinated ethenes are produced as by-product during chloromethanes production, 13C depleted fingerprinting of contaminants can be obtained and this can be used to track sources and address the responsible party of the pollution in urban areas.