The East Gate Disposal Yard at Fort Lewis contains DNAPL material that is a source to a large plume of trichloroethene (TCE) in the groundwater. The DNAPL material contains TCE, petroleum hydrocarbons, and small amounts of other contaminants. A selected high-saturation portion of the DNAPL was remediated using electrical resistance heating. Prior to and during heating, groundwater samples were collected periodically at eight locations within and adjacent to the heated area and analyzed for constituents related to potential TCE transformation reactions. Measured constituents included TCE and its dechlorination products (e.g., dichloroethene isomers, vinyl chloride, chloride, ethene, ethane, and acetylene) geochemical indicators of selected reactions related to dechlorination (e.g., carbon dioxide, methane, ferrous iron, sulfide, dissolved oxygen, and oxidation/reduction potential), and temperature. The subsurface temperatures during the monitored portion of heating varied from pre-test conditions of approximately 15 to 20°C up to around 80°C. Indications of dechlorination activity during heating, though not uniform across the heated area, included a significant increase in chloride concentration and increases in concentrations of vinyl chloride, ethene, and acetylene compared to baseline concentrations. The highest concentrations of these constituents were typically measured at temperatures between 30 and 50°C. Methane and carbon dioxide concentrations also increased during heating. Geochemical conditions were generally reducing at locations showing more significant dechlorination, but progressed toward more oxidizing conditions as the temperature approached the boiling point of the DNAPL/water mixture (about 80°C).
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