Fate and transport of hexavalent chromium in undisturbed heterogeneous soil

The disposal of toxic metals [e.g., Cr(VI)] generated by the Department of Energy during the cold war era has historically involved shallow land burial in unconfined pits and trenches. The objectives of this study were to investigate the impact of coupled hydrologic and geochemical processes on the fate and transport of Cr(VI) in undisturbed soil cores obtained from a fractured, acidic inceptisol that are commonly used in the disposal of waste at the Oak Ridge National Laboratory. The mobility of Cr(VI) was significantly retarded relative to a nonreactive Br^- tracer, and the mobility decreased with increased loading of the solid phase with natural organic matter (NOM). A significant portion of added Cr(VI) did not elute from the columns, and X-ray absorption near-edge structure (XANES) revealed that both Cr(VI) and Cr(III) resided on the soil mineral surfaces. The reduction of Cr(VI) to Cr(III) was dramatically more significant on soils with higher levels of surface-bound NOM. This indicated that NOM was serving as a suitable reductant during Cr(VI) transport even in the presence of potentially competing geochemical oxidation reactions involving Cr. The redox reaction was catalyzed by the presence of soil mineral surfaces, and the reduced product Cr(III) was immobilized as a tightly bound moiety. The effectiveness of surface-bound NOM to reduce toxic Cr(VI) to Cr(III) under acidic conditions has important implications regarding the design and implementation of in situ remedial strategies.