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Chlorinated Solvent Source Treatment with Zero-Valent Iron

The Environmental Security Technology Certification Program (ESTCP) funded project number ER-0719 to demonstrate the benefits of combining low-energy electrical resistance heating (ERH) and zero-valent iron (ZVI) for treatment of trichloroethene (TCE) present in an aquifer as dense non-aqueous phase liquid (DNAPL).  The project included assessment of the extent to which contaminant degradation was enhanced during heating (compared to ambient temperatures), the relative contribution of biotic and abiotic contaminant degradation mechanisms at different temperatures, and the cost/benefit analysis of applying low-energy heating with in situ treatments.

Approximately 190 kg of two-micron-diameter ZVI particles were injected into a test zone in the top two meters of an unconfined aquifer within a trichloroethene source zone.  A shear-thinning fluid was used to deliver the micron-scale ZVI to enhance the delivery of ZVI in the subsurface to a radial distance of up to four meters.  The ZVI particles were mixed in-line with the injection water, shear-thinning fluid, and a low concentration of surfactant.  Seven monitoring wells within the targeted radius of influence were used to assess performance.  ZVI concentrations at the monitoring wells showed a good radial distribution of particles at the end of the injection period.  TCE dechlorination was monitored at the ambient groundwater temperature of 10°C over a period of two months.  The starting concentration of TCE in the groundwater was nominally 2 mg/L.  Only dechlorination products were present by the end of two months, including a significant molar percentage of ethene and ethane.  A mixture of abiotic reactions and biological dechlorination reactions were occurring at the ambient groundwater temperature, as indicated by the mixture of daughter products.  Electrical resistance heating was applied to raise the temperature in the test zone uniformly to 50°C.  The elevated temperature increased the dissolution of contaminant into the groundwater and increased the rate and extent of dechlorination.

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