Fate and Transport of Hexavalent Chromium in Undisturbed Heterogeneous Soil

The disposal of toxic metals such as Cr(VI) by the Department of Energy has historically involved shallow land burial in unconfined pits and trenches. This study aimed to investigate the impact of coupled hydrologic and geochemical processes on the fate and transport of Cr(VI) in undisturbed soil cores from an acidic inceptisol used for waste disposal at the Oak Ridge National Laboratory. Results showed that the mobility of Cr(VI) was significantly retarded compared to a nonreactive Br- tracer, and decreased with increased loading of the solid phase with natural organic matter (NOM). Notably, a significant portion of added Cr(VI) did not elute from the columns, and X-ray absorption near-edge structure (XANES) analysis revealed that both Cr(VI) and Cr(III) were associated with soil mineral surfaces. The reduction of Cr(VI) to Cr(III) was notably more effective in soils with higher levels of surface-bound NOM, which acted as a reductant during Cr(VI) transport despite competing geochemical oxidation reactions. The catalysis of the redox reaction by soil mineral surfaces resulted in Cr(III) being tightly bound and immobilized. These findings have important implications for the design and implementation of in situ remedial strategies in contaminated environments.