Tracking chlorinated contaminants in the subsurface usingcompound-specific chlorine isotope analysis: A review of principles,current challenges and applications

Many chlorinated hydrocarbons have gained notoriety as persistent organic pollutants in the environ-ment. Engineered and natural remediation efforts require a monitoring tool to track the progress ofdegradation processes. Compound-specific isotope analysis (CSIA) is a robust method to evaluate theorigin and fate of contaminants in the environment and does not rely on concentration measurements.While carbon CSIA has established itself in the routine assessment of contaminated sites, studiesincorporating chlorine isotopes have only recently become more common. Although some aspects ofchlorine isotope analysis are more challenging than carbon isotope analysis, having additional isotopicdata yields valuable information for contaminated site management. This review provides an overview ofchlorine isotope fractionation of chlorinated contaminants in the subsurface by different processes andpresents analytical techniques and unresolved challenges in chlorine isotope analysis. A summary ofsuccessfulfield applications illustrates the potential of using chlorine isotope data. Finally, approaches inmodelling chlorine isotope fractionation due to degradation, diffusion, and sorption processes arediscussed.