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INJECTION OF A FERROUS SULFATE/SODIUM DITHIONITE REDUCTANT FOR IN-SITU TREATMENT OF HEXAVALENT CHROMIUM
Citation:
Ludwig*, R AND C. Su. INJECTION OF A FERROUS SULFATE/SODIUM DITHIONITE REDUCTANT FOR IN-SITU TREATMENT OF HEXAVALENT CHROMIUM. Presented at 7th ICOBTE Conf, Uppsala, SWEDEN, June 15 - 29, 2003.
Impact/Purpose:
To inform the public.
Description:
An in situ pilot study was conducted to evaluate the performance of a ferrous iron-based reductant solution in treating hexavalent chromium within a saturated zone source area at a former industrial site in Charleston, South Carolina (USA). The hexavalent source area, consisting of slag and other solid wastes generated in the production of ferrochrome alloy, is located near a saltwater tidal marsh. A dissolved-phase hexavalent chromium plume, originating from the source area, is migrating toward the tidal marsh. Groundwater and aquifer solids in the source area are highly alkaline with groundwater pH values measured as high as 11.5 and hexavalent chromium concentrations measured as high as 57 mg/L. Laboratory tests were initially conducted to evaluate candidate reductants potentially capable of converting hexavalent chromium in the groundwater and solid aquifer matrix to the less mobile and less toxic trivalent form. These tests showed that sodium dithionite was ineffective while ferrous sulfate was highly effective. The injection of ferrous sulfate into the highly alkaline source area aquifer, however, posed a significant concern due to the anticipated rapid precipitation of iron that would be expected to occur around the injection well. To address this concern, sodium dithionite was added to the ferrous sulfate to act as a carrier fluid to help maintain the ferrous iron in solution and thereby maximize the travel distance of the ferrous iron in the subsurface.
A total of 18,000 liters of a 0.2M ferrous sulfate/0.2M sodium dithionite solution were injected into the source area through a single 5-cm diameter PVC injection well screened from 75 cm to 300 cm below the water table. Injection in the source area occurred at a rate of approximately 60 liters per minute and average injection pressure of 70 KPa. Groundwater samples were collected before and after injection from a system of monitoring wells. Groundwater samples were monitored in the field for pH, oxidation-reduction potential (ORP) conductivity, temperature, dissolved oxygen, ferrous iron, and sulfide. Samples were analysed in the laboratory for cations, anions, alkalinity, TOC/DOC, and TIC/DIC. Due to observed interferences in the measurement of hexavalent chromium in the presence of sodium dithionite, total chromium measurements were used as a surrogate measurement for hexavalent chromium in groundwater samples collected from monitoring wells following injection.
Post-injection analyses of groundwater samples indicated complete removal/treatment of hexavalent chromium out to a distance of at least 2 m from the injection point and significant reductions in total chromium concentrations out to a distance of at least 3 m. Injection of the reductant resulted in increases in alkalinity, Ca, Mg, Na, Ba, Mn, Sr, and SO42- concentrations and decreases in K, B, Sb, Se, Cl, TIC, and DIC concentrations in the treated aquifer groundwater. Significant decreases in ORP and pH values were also observed in the groundwater following treatment. Results from the source area before and after treatment are summarized in Table 1.