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IN-SITU CR(VI) SOURCE AND PLUME TREATMENT USING A FERROUS IRON-BASED REDUCTANT
Citation:
Ludwig*, R, C. Su, S Acree*, R Ross*, F P. Beck*, P J. Clark*, AND P K. Jones*. IN-SITU CR(VI) SOURCE AND PLUME TREATMENT USING A FERROUS IRON-BASED REDUCTANT. Presented at RTDF Meeting, Niagara Falls, NY, 10/15-16/2003.
Description:
A large volume of chromite ore processing residue (COPR) generated from ferrochrome production operations is present at the Macalloy Corporation Superfund site in Charleston, S.C. Groundwater hexavalent chromium (Cr(VI)) concentrations in the approximately 20 acre-foot COPR saturated zone source area measure as high as 57 mg/L and pH as high as 11.5. Laboratory tests indicated that sodium hydrosulfite (dithionite) was ineffective in treating the solid phase and dissolved phase Cr(VI) while ferrous iron in the form of ferrous sulfate was highly effective. However, it is known that the delivery of ferrous iron into the subsurface is normally impractical due to the tendency of iron to rapidly precipitate out of solution particularly under high pH conditions. This generally results in injection well and aquifer formation clogging. In the presence of a reducing agent such as sodium hydrosulfite, however, it was hypothesized that the ferrous iron could be stabilized in solution for an extended period of time to allow for its effective dissemination within the subsurface.
Two in situ field pilot studies were initiated at the Macalloy Corporation site in August 2002. One was conducted in the source area and involved injection of 4800 gallons of a 0.2M ferrous sulfate/0.2M sodium hydrosulfite solution directly into the saturated zone source material. The objective of this study was to determine whether solid phase Cr(VI) in the COPR source zone could be effectively treated. The second pilot study was conducted outside the source area in the path of a dissolved phase Cr(VI) plume and involved injection of 1300 gallons of the same reductant blend into each of two injection wells in the saturated zone. The objective of this study was to determine whether a ferrous iron based treatment zone could be established for passive treatment of the dissolved phase Cr(VI) plume. Groundwater and solid phase samples were collected before and after injection from both pilot test areas. Groundwater samples were monitored in the field for Cr(VI), pH, oxidation-reduction potential (ORP), conductivity, temperature, dissolved oxygen, ferrous iron, and sulfide. Samples were analyzed in the laboratory for cations and anions.
Preliminary post-injection analyses in the source area pilot study has indicated effective treatment of dissolved phase and solid phase Cr(VI) based on groundwater sampling results and analyses of aqueous and phosphate solution extracts. (The results of alkaline digestion extracts on core samples are still pending.) Post-injection analyses in the dissolved phase plume pilot study area has indicated the establishment of an effective groundwater treatment zone to date with Cr(VI) being consistently treated to non-detectable levels (< 0.003 mg/L) after five pore volumes of treatment. Post-injection analysis also indicated no measurable decrease in formation hydraulic conductivity following injection of the ferrous iron into the aquifer formation. Injection of the reductant solution did result in mobilization of metals in the treated aquifer formation, however, metal mobilization is expected to be limited to the boundaries of the treatment zone.