Impact of Abiotic Attenuation Reactions on Chlorinated Solvent Fate in Diffusion-Limited Clay Lenses

Back diffusion of chlorinated solvents from low permeability media remains a challenge to remediation of contaminated groundwater in heterogeneous subsurface environments. Naturally occurring abiotic dechlorination of trichloroethene (TCE) has shown potential as a viable natural attenuation mechanism, particularly at sites where in-situ remediation technologies have generated reducing conditions favorable for the generation of reduced iron minerals. These abiotic processes may occur under anaerobic conditions or aerobic conditions when oxygen is introduced to reduced sediments. Quantification of aerobic/anaerobic abiotic dechlorination rate constants to date has generally been performed using bench-scale batch experiments with low solids to water ratios and well-mixed conditions, confounding extension of the results to the field where mass transfer limitations dominate.  To address this knowledge gap, bench-scale experiments were conducted to evaluate diffusive transport of TCE and coupled aerobic/anaerobic abiotic dechlorination. The natural clays used in this study were collected from several chlorinated solvent impacted sites and characterized for geochemical parameters including ferrous iron content, electron shuttle mediated oxidation-reduction potential (ORP), and mineralogy via X-ray diffraction (XRD). Clays were packed into gas-tight serum bottles under anaerobic conditions to a saturated bed depth of 4 centimeters, and TCE was injected at the top of the clay beds. For the aerobic experiments, a slug of oxygenated water was introduced at the top of the clay immediately prior to spiking with TCE. Headspace and aqueous samples were periodically collected and monitored for reduced gases and organic acids associated with abiotic transformation of TCE in the presence of ferrous minerals. ¹⁴C-radiolabeled TCE was used for select experiments to facilitate detection of dechlorination products at low concentrations indicative of conditions near a dilute solvent plume.   Accumulation of expected TCE dechlorination products under aerobic (organic acids) and anaerobic (primarily acetylene) conditions was observed in the diffusion experiments. Detection of ¹⁴C dechlorination products served to clearly distinguish compounds originating from TCE. A one-dimensional coupled reaction and diffusion model was developed to describe diffusive transport of TCE and dechlorination products into and out of the clay bed. First order abiotic dechlorination rate constants were determined by fitting profiles of reduced gas and organic acid generation over time, using literature values for molecular diffusion coefficients and clay tortuosity. Following conclusion of the experiments, depth-discrete ORP measurements of the packed clay beds from the aerobic diffusion experiments will be conducted, providing further insight into the effect of oxidative TCE transformation on local geochemistry near the groundwater table and the relative contribution of aerobic abiotic degradation. The results of this study elucidate the capacity for abiotic natural attenuation to reduce TCE mass flux out of clays in dilute plumes under a variety of geochemical conditions.