Chapter 16 - In Situ Reduction of Chromium-Contaminated Groundwater, Soils, and Sediments by Sodium Dithionite

Abstract

This chapter focuses on the results of the various laboratory studies conducted to characterize the extent of chromium (Cr) contamination in the groundwater and underlying soils and sediments of a chrome-plating shop at the United States Coast Guard (USCG) Support Center near Elizabeth City, North Carolina. Most of the mobile hexavalent Cr [Cr(VI)] was present in the vadose zone just above the capillary fringe and in the upper zone of the aquifer under oxidizing conditions. Three liquid reductants, which include sodium dithionite, l-ascorbic acid, and free hydroxylamine were evaluated in laboratory studies to determine their ability to detoxify and immobilize Cr(VI) in situ. Sodium dithionite proved to be the most effective at reducing Cr(VI) to Cr(III) with the least adverse side effects. Postinjection monitoring results show aqueous Cr(VI) concentrations decreased from up to 5 to <0.01 mg/liter in all wells within the 1-m cylindrical treatment zone. Continued Cr(VI) reduction was seen 48 weeks postinjection, indicating that long-term reduction was occurring. Analyses of postexperimental sediment cores indicate that nonmobile, reduced structural iron (Fe) within the silicate minerals present in the treated sediments might be contributing to the long-term remediation of Cr(VI).