Contrasting dual (C, Cl) isotope fractionation offers potential to distinguish reductive chloroethene transformation from breakdown by permanganate.

cis-1,2-Dichloroethene (cis-DCE) and trichloroethene (TCE) are persistent, toxic and mobile pollutants in groundwater systems. They are both conducive to reductive dehalogenation and to oxidation by permanganate. In this study, the potential of dual element (C, Cl) compound specific isotope analyses (CSIA) for distinguishing between chemical oxidation and anaerobic reductive dechlorination of cis-DCE and TCE was investigated. Well-controlled cis-DCE degradation batch tests gave similar carbon isotope enrichment factors εC (‰), but starkly contrasting dual element isotope slopes Δδ13C/Δδ37Cl for permanganate oxidation (εC=−26‰±6‰, Δδ13C/Δδ37Cl≈−125±47) compared to reductive dechlorination (εC=−18‰±4‰, Δδ13C/Δδ37Cl≈4.5±3.4). The difference can be tracked down to distinctly different chlorine isotope fractionation: an inverse isotope effect during chemical oxidation (εCl=+0.2‰±0.1‰) compared to a large normal isotope effect in reductive dechlorination (εCl=−3.3‰±0.9‰) (p≪0.05). A similar trend was observed for TCE. The dual isotope approach was evaluated in the field before and up to 443days after a pilot scale permanganate injection in the subsurface. Our study indicates, for the first time, the potential of the dual element isotope approach for distinguishing cis-DCE (and TCE) concentration drops caused by dilution, oxidation by permanganate and reductive dechlorination both at laboratory and field scale.