Laboratory studies of in-situ redox manipulation for remediation of PCE, TCE and Cr(VI) contaminated groundwater in Atlantic Coastal Plain sediments
Uddin, Md Momin Kamal
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Conventional groundwater treatment methods, such as pump and treat are expensive and generally ineffective, especially for redox sensitive contaminants. An alternative to pump and treat technology is the development of an in-situ permeable, subsurface reactive zone designed to intercept and treat groundwater contaminants. One recent innovation is the subsurface injection of an aqueous chemical reductant to create a treatment zone with low redox potential. This technology is referred to as in-situ redox manipulation (ISRM) and is capable of removing redox sensitive contaminants such as chlorinated solvents (TCE, PCE) and toxic metals (Cr) from groundwater. These contaminants are present in groundwater at the Department of Energy’s Savannah River Site (SRS), located near Aiken, SC. Therefore, SRS subsurface sediments were used as a test matrix in a study evaluating the efficiency of ISRM for treating PCE, TCE and Cr(VI) contaminated groundwater on the SRS. The ISRM method consists of: 1) creation of an Fe(II) reactive zone by the injection of dithionite reagent that reduces Fe(III) naturally present in the aquifer sediment, 2) removal of oxidized groundwater contaminants by reaction within the reduced Fe(II) barrier. The study results indicate that dithionite treatment does not negatively impacts the hydraulic properties of the aquifer if the pH of the injection solution is properly buffered. The reducing capacity attained within the treated sediments was very effective in treating groundwater containing Cr(VI). Based on the measured Cr(VI) treatment capacity, other redox sensitive toxic metals, such as uranium, and plutonium can be successfully remediated in SRS groundwater using the ISRM technology. The slow kinetics of PCE and TCE transformation in contact with dithionite reduced SRS aquifer sediments indicate that the treatment was only partially effective for remediation of these contaminants. Additional studies are necessary to evaluate the relationship between PCE and TCE reaction kinetics and groundwater velocity in order to optimize the design of the ISRM systems for field deployment.