Chlorine isotope investigation of natural attenuation of trichloroethene in an aerobic aquifer

Natural attenuation of chlorinated aliphatic hydrocarbons (CAHs) can be an important mechanism for groundwater remediation. it is difficult to determine the effectiveness of natural CAH attenuation from chemical analyses of groundwater samples because mixing, dispersion, and secondary reactions can mask the chemical evidence of attenuation. In this paper, we explore the application of stable chlorine isotope ratio measurements as a new tool for for evaluating natural attenuation of CAHs. We report stable isotope ratios of chlorine in both trichloroethene (ICE) and inorganic chloride in groundwater from an aerobic aquifer beneath an extensively contaminated industrial site, the Paducah Gaseous Diffusion Plant in western Kentucky. Variations in the concentrations and chlorine isotope ratios of TCE and chloride in the groundwater a re consistent with those expected from natural attenuation. These data support a model in which partial TCE degradation occurred in: relatively impermeable, clay-rich sediments above the aquifer, and little or no further degradation of TCE occurred within the aquifer. A record of changing conditions within the TCE source area can be inferred from the spatial variation of chlorine isotope ratios for TCE and chloride within the plume.