CHLORINE ISOTOPE INVESTIGATION OF NATURAL ATTENUATION OF TRICHLOROETHENE IN AN AEROBIC AQUIFER

Natural attenuation of chlorinated aliphatic hydrocarbons (CAHs) can b e an important mechanism for groundwater remediation. it is difficult to determine the effectiveness of natural CAH attenuation from chemica l analyses of groundwater samples because mixing, dispersion, and seco ndary reactions can mask the chemical evidence of attenuation. In this paper, we explore the application of stable chlorine isotope ratio me asurements as a new tool for for evaluating natural attenuation of CAH s. We report stable isotope ratios of chlorine in both trichloroethene (ICE) and inorganic chloride in groundwater from an aerobic aquifer b eneath an extensively contaminated industrial site, the Paducah Gaseou s Diffusion Plant in western Kentucky. Variations in the concentration s and chlorine isotope ratios of TCE and chloride in the groundwater a re consistent with those expected from natural attenuation. These dat a support a model in which partial TCE degradation occurred in: relati vely impermeable, clay-rich sediments above the aquifer, and little or no further degradation of TCE occurred within the aquifer. A record o f 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.