C, Cl and H compound-specific isotope analysis to assess natural versus Fe(0) barrier-induced degradation of chlorinated ethenes at a contaminated site

Highlights: • ¹³C to evaluate natural chlorinated ethenes biodegradation. • ¹³C to evaluate the efficiency of a zero-valent iron-permeable reactive barrier. • ¹³C-³⁷Cl to discriminate biotic from abiotic degradation of cis-dichloroethene. • ¹³C-³⁷Cl-²H of cis-DCE and TCE to elucidate different contaminant sources. - Abstract: Compound-specific isotopic analysis of multiple elements (C, Cl, H) was tested to better assess the effect of a zero-valent iron-permeable reactive barrier (ZVI-PRB) installation at a site contaminated with tetrachloroethene (PCE) and trichloroethene (TCE). The focus was on (1) using ¹³C to evaluate natural chlorinated ethene biodegradation and the ZVI-PRB efficiency; (2) using dual element ¹³C-³⁷Cl isotopic analysis to distinguish biotic from abiotic degradation of cis-dichloroethene (cis-DCE); and (3) using ¹³C-³⁷Cl-²H isotopic analysis of cis-DCE and TCE to elucidate different contaminant sources. Both biodegradation and degradation by ZVI-PRB were indicated by the metabolites that were detected and the ¹³C data, with a quantitative estimate of the ZVI-PRB efficiency of less than 10% for PCE. Dual element ¹³C-³⁷Cl isotopic plots confirmed that biodegradation was the main process at the site including the ZVI-PRB area. Based on the carbon isotope data, approximately 45% and 71% of PCE and TCE, respectively, were estimated to be removed by biodegradation. ²H combined with ¹³C and ³⁷Cl seems to have identified two discrete sources contributing to the contaminant plume, indicating the potential of δ²H to discriminate whether a compound is of industrial origin, or whether a compound is formed as a daughter product during degradation.