Simultaneous Abiotic Oxidation and Reduction of Trichloroethylene by Reduced Iron Minerals in Low-Permeability Zones

Back diffusion of trichloroethylene (TCE) from low-permeability zones (LPZs) poses a major challenge to groundwater remediation at many contaminated sites. This study investigated whether abiotic oxidation and reduction of TCE can co-occur at transitions between aerobic aquifers and anaerobic, iron-rich LPZs. Diffusion-vial experiments were conducted using reduced clay exposed to TCE and oxygen. Results showed that oxygen reacts more rapidly with reduced iron minerals (RIMs) than TCE, limiting oxygen penetration and allowing TCE to diffuse deeper. Oxidation products formed near the LPZ surface, while reduced gases were generated at a greater depth. Control experiments revealed that some reduced gases may form from TCE oxidation products via an unknown pathway. A reactive transport model calibrated to experimental data predicted that, at the field scale, TCE reduction dominates over oxidation after several decades due to limited oxygen diffusion into the LPZ. However, this balance depends on site-specific factors such as oxygen availability, RIM content, and LPZ thickness. These findings provide the first direct experimental evidence for the simultaneous abiotic oxidation and reduction of TCE in LPZs and suggest that reduction likely plays a greater role in long-term attenuation. Field validation and rate quantification over different LPZ soils are needed to assess the remediation impact.