In Situ Abiotic Detoxification and Immobilizationof Hexavalent Chromium

Detailed site characterization data from the former electroplating shop at the U.S. Coast Guard Air Support Center, Elizabeth City, North Carolina, suggested that the elevated Cr(VI) in the capillary fringe area had contaminated the ground water at the site. Most of the mobile Cr(VI) is present in the capillary fringe zone of the aquifer under an oxidizing environment. Current literature suggests that the reduction of Cr(VI) to Cr(Ill) through in situ redox manipulation in the presence of a reductant is an innovative technique for remediating chromate-contaminated sediments and ground water. The objective of this study was to evaluate the effectiveness of sodium dithionite in creating a reductive environment to remediate Cr(VI) present in soil. Sodium dithionite, a strong reductant, was injected into a small area of the vadose zone where elevated Cr(VI) was identified. Several striking changes observed in the target zone during the post-injection monitoring periods include a significant decrease in Eh(SHE), as much as ~ 700 m V, absence of dissolved oxygen for 48 weeks, and the increase of Fe(II) concentrations. Results indicated that the in situ remedial treatment of Cr(VI) in the capillary fringe area was effective and consequently the concentration of Cr(VI) in ground water dropped below the MCLG level. This research demonstrated the effectiveness of in situ abiotic remediation by reducing Cr(VI) concentrations, mobility, and toxicity in soils and ground water within a short period of time. Therefore, sodium dithionite would be a feasible and cost-effective option for a full-scale remedial approach for the contaminated site at the U.S. Coast Guard Facility. Introduction Chromium is widely used in many industrial applications and has resulted in accidental or improper discharge of Cr(VI) waste into many soil and ground water systems. Chromium is one of the transition metals and may exist in valence state varying from-2 to +6. Under natural environment, Cr(lll) and Cr(VI) are the two most stable oxidation states (Barlett and James 1988). The oxyanions of Cr(VI) are acutely toxic and mutagenic, soluble over a wide pH range, and very mobile in most neutral and alkaline subsurface environment (Barlett 1991; Palmer and Wittbrodt 1991). In contrast, Cr(I1I) has relatively low toxicity and is virtually immobile, largely because it precipitates as chromic or mixed Cr-Fe oxyhydroxide phases (James1996; Palmer and PulsI994). Poor waste management practices of the chrome plating facility at the U.S. Coast Guard (USCG) Support Center, near Elizabeth City, North Carolina, resulted in the release of chromic acid through a hole in the concrete floor into the underlying geologie materials immediately below the plating shop's foundation. A detailed characterization of the underlying geologic materials and ground water of the chrome-plating shop was performed to provide information on the extent of contamination at the site and the potential for off-site migration and environmental impact (Paul et al. 1999). Results indicate that the maximum Cr(VI) concentrations occur between a depth of 1.5 to 3.0 Ground Water Monitoring & Remediation 23. no. l/Winter 2003/ pages 77-84 feet below the ground surface and that these oxyanions are static at these depths but the elevated concentrations of Cr(VI) in the capillary fringe zone (~4.0 to 6.0 feet) are very mobile owing to the fluctuating water table. The ground water table ranges from 5.0 to 7.0 feet below the ground surface. Historical data of MW-12 suggests that the Cr(VI) present in the capillary fringe zone have contaminated the ground water and the concentrations of Cr(VI) were above the National Drinking Water Standards (Khan and PuIs 1999). The release of Cr(VI) from this source area has also created a plume of ground water containing dissolved Cr(VI) which extended from the electroplating shop toward the Pasquotank River. In 1996, a permeable reactive barrier of zero-valent iron was installed to intercept and remediate the ground water plume contaminated with Cr(VI) (PuIs et al. 1999a). This reactive barrier technique is an effective method in reducing and immobilizing Cr(VI) present in the ground water plume, but it has limited effectiveness to reduce contaminants present in the source area as sorbed phases in the vadose zone. An in situ remedial approach for the source area remediation becomes necessary to complement the permeable reactive barrier in achieving a full-scale site cleanup. Use of traditional techniques like pump-and-treat to remediate Cr(VI) in the subsurface environment presents challenges because Cr(VI) can remain either trapped between low permeability zones or as sorbed phases with sesquioxides and