Multi-element isotopic evidence for monochlorobenzene and benzene degradation under anaerobic conditions in contaminated sediments.

Industrial chemicals are frequently detected in sediments due to a legacy of chemical spills. Globally, site remedies for groundwater and sediment decontamination include natural attenuation by in situ abiotic and biotic processes. Compound-specific isotope analysis (CSIA) is a diagnostic tool to identify, quantify, and characterize degradation processes in situ, and in some cases can differentiate between abiotic degradation and biodegradation. This study reports high-resolution carbon, chlorine, and hydrogen stable isotope profiles for monochlorobenzene (MCB), and carbon and hydrogen stable isotope profiles for benzene, coupled with measurements of pore water concentrations in contaminated sediments. Multi-element isotopic analysis of δC and δCl for MCB were used to generate dual-isotope plots, which for 2 locations at the study site resulted in Λ values of 1.42 ± 0.19 and Λ values of 1.70 ± 0.15, consistent with theoretical calculations for carbon-chlorine bond cleavage (Λ = 1.80 ± 0.31) via microbial reductive dechlorination. For benzene, significant δH (122‰) and δC (6‰) depletion trends, followed by enrichment trends in δC (1.6‰) in the upper part of the sediment, were observed at the same location, indicating not only production of benzene due to biodegradation of MCB, but subsequent biotransformation of benzene itself to nontoxic end-products. Degradation rate constants calculated independently using chlorine isotopic data and carbon isotopic data, respectively, agreed within uncertainty thus providing multiple lines of evidence for in situ contaminant degradation via reductive dechlorination and providing the foundation for a novel approach to determine site-specific in situ rate estimates essential for the prediction of remediation outcomes and timelines.