In situ treatment of a complex mix of chlorinated solvents: anaerobic reductive dehalogenation after chemical oxidation
Treatment of chlorinated organic compounds has been ongoing at this New England facility (Site) for over 15 years. The extraction and above ground treatment system for groundwater containing tetrachloroethene (PCE), trichloroethene (TCE), 1,1,1-trichloroethane (1,1,1-TCA) and other chlorinated organic compounds has been running since September 1992. The full-scale remedy of in-situ chemical (ISCO) oxidation, which consisted of periodic injections of potassium permanganate, was implemented in February 2004 – June 2005. A total of approximately 16,000 gallons of two to four percent potassium permanganate solution has been added to the saturated source zone area that has remained mildly reducing and anaerobic. In an area hydraulically isolated from the ISCO injection area, an enhanced in-situ bioremediation (EISB) pilot study was completed in 2004 using emulsified edible oil (EEO, Newman Zone) as the electron donor. The yearlong pilot study provided data in support of enhanced degradation and complete sequential dehalogenation of 1,1,1-TCA to chloroethane and of PCE and TCE to ethene. Subsequently, an alternate full-scale remedy using EISB was approved for this facility. The full-scale remedy consists of the injection of EEO in a grid pattern within the source zone area and the construction of a biobarrier to treat the downgradient edge of the plume. The bio-barrier has not yet been completed as of the date of this paper. EEO comprised 2.5 percent (%) of the total volume of amendment in the source zone. The remaining 97.5% is comprised of groundwater from a deeper aquifer with similar geochemical characteristics. Recent monitoring data show a 99% conversion of TCE and 1,1,1-TCA to ethene, ethane and chloroethane in monitoring wells located along the periphery of the source area. Data from monitoring wells located within the source area show up to a 67% increase in the ratio of daughter to parent compounds six-months after the initial EEO injection.