Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils

Abstract

Naturally occurring abiotic dechlorination reactions in clayey soils can serve as an important attenuation mechanism for groundwaters impacted by chlorinated solvents such as trichloroethene (TCE). Potential abiotic reactions include both reductive (anoxic) and oxidative (oxic) dechlorination reactions that are facilitated by ferrous minerals. However, tools to provide lines of evidence for such reactions, and ultimately screening-level estimates of dechlorination rate constants that can be incorporated into site fate and transport models, are yet to be widely accepted for these clayey systems. Herein, coupled bench- and field-scale testing at nine locations within the saturated zone showed that measurement of reduced gases in field-collected clayey samples was inconclusive for indicating in situ abiotic reductive dechlorination. However, the use of 1% (v/v) HCl extractions and X-ray diffraction (XRD) for mineral composition provided the information needed to estimate TCE abiotic reductive dechlorination in clays, thereby serving as a potential screening tool for site investigation. While a corresponding screening tool for estimating abiotic oxidative dechlorination in clay was not demonstrated, the rate of hydroxyl radical generation measured for each clay in batch experiments was correlated to in situ hydrogen peroxide concentrations measured in groundwater near the sand-clay interface. Thus, this observation provides (to our knowledge) a first line of evidence that ongoing reactive oxygen species generation is occurring in situ near the sand–clay (oxic–anoxic) interface, potentially serving as a means to facilitate abiotic oxidative dechlorination and mitigate back-diffusion of chlorinated solvents from clay.