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 ε_C (‰), but starkly contrasting dual element isotope slopes Δδ¹³C/Δδ³⁷Cl for permanganate oxidation (ε_C = − 26‰ ± 6‰, Δδ¹³C/Δδ³⁷Cl ≈ − 125 ± 47) compared to reductive dechlorination (ε_C = − 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 (ε_Cl = + 0.2‰ ± 0.1‰) compared to a large normal isotope effect in reductive dechlorination (ε_Cl = − 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 443 days 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.