COUPLED GEOCHEMICAL AND HYDROLOGIC PROCESSES GOVERNING CR(VI) TRANSPORT IN HANFORD FORMATION SEDIMENTS

Accelerated migration of Cr(VI) in the vadose zone has been observed beneath the tank farms at the U.S. Department of Energy's Hanford Reservation. This paper focuses on quantifying the coupled hydrologic and geochemical processes controlling the transport of Cr(VI) in the unsaturated sediments beneath the Hanford tank farms. Our approach involves the use of bench-scale sorption and transport experiments as well as field-relevant, long-term unsaturated transport experiments in undisturbed sediments from the Hanford Formation. Kinetic batch sorption experiments and repacked saturated columns were used to quantify geochemical processes controlling the rates and mechanisms of Cr(VI) interactions with a variety of Hanford sediments. Results indicate that Cr(VI) is reactive in the Hanford media, as evidenced by retardation and mass loss of Cr(VI) during transport. It is hypothesized that magnetite and biotite are probable sources of Fe(II) that promote the reduction of Cr(VI) to Cr(III) and hence the precipitation of Cr(III) / iron oxide. Unsaturated flow experiments were conducted in the undisturbed cores using multiple non-reactive tracers and Cr(VI) to investigate coupled geochemical and hydrologic processes. Results suggest that physical non-equilibrium processes such as preferential finger-flow coupled with immobile water as well as sediment mineralogy may control Cr(VI) transport in the Hanford Formation. This study shows that investigative approaches using a combination of batch and transport experiments will contribute to the conceptual and quantitative understanding of contaminant mobility in the Hanford vadose zone.