Experiments to determine the suitability of sodium dithionite treatment for a Cr(VI) groundwater plume in a deep aquifer of the United States Southwest

Abstract Legacy industrial waste has left groundwater plumes of hexavalent Cr (Cr(VI)) that require treatment, predominantly by reduction to Cr(III) and subsequent precipitation. One promising technology is the injection of a strong reducing agent, sodium dithionite, to create an in-situ redox barrier, but implementation requires a comprehensive understanding of the reactions occurring with dithionite injection. Batch and column experiments were conducted with aquifer sediments to determine both the significant reactions and efficacy of sodium dithionite treatment for a groundwater plume of hexavalent chromium. The batch experiments demonstrate consumption of dithionite over the experiment (43 d) concurrent with leaching of about 1 mM Fe from the sediments. Reactions deduced from batch experiments were incorporated into a 1-D numerical model to simulate reactions occurring during the column injection. The treatment was able to successfully reduce approximately 30 pore volumes of groundwater containing 800 µg kg-1 Cr(VI). The experiments also demonstrate that although mineral forms of Fe are important phases in the reduction of Cr(VI), Fe alone cannot account for the entire reduction capacity imparted to the sediments. Rather, the correlation between Cr and reduced S retained in the columns, combined with Scanning Electron Microscopy (SEM) of treated sediments, suggest that formation of reduced S phases contributes to the prolonged reduction capacity.