Source differentiation for benzene and chlorobenzene groundwater contamination: A field application of stable carbon and hydrogen isotope analyses

This article investigates the combined use of compound-specific isotope analyses, hydrogeologic data, contaminant concentrations, and site history to investigate sources of benzene and chlorobenzene contamination in a shallow and deep aquifer. For benzene, delta C-13 and delta H-2 values suggested two source areas in the northeastern and southeastern portion of the shallow aquifer and possible contaminant migration from the shallow to the deep aquifer consistent with local hydraulic gradients. Conversely, uniform delta C-13 chlorobenzene values on the eastern portion of the shallow aquifer suggested a potential single source for this compound in this area of the site. Isotopically C-13-depleted values for benzene and chlorobenzene in wells downgradient of the northeastern source zone were inconsistent with the effects of biodegradation, suggesting other sources of contamination in the western portion of the shallow aquifer. Only one well showed an enrichment in C-13 in the downgradient wells relative to proposed source areas, which was observed only for chlorobenzene, suggesting that biodegradation is not a major influence on isotopic signatures at this site.