Chromium diffusion and reduction in soil aggregates

The distribution of metal contaminants such as chromium in soils can be str ongly localized by transport limitations and redox gradients within soil ag gregates. Measurements of COO diffusion and reduction to Cr(III) were. obta ined in soil columns representing transacts into soil aggregates in order t o quantify influences of organic carbon (OC) and redox potentials on Cr tra nsport distances and microbial community composition. Shifts in characteris tic redox potentials, and the extent of Cr(VI) reduction to Cr(III) were re lated to OC availability. Depth profiles of Cr(VI, III) obtained with micro X-ray absorption near edge structure (micro-XANES) spectroscopy reflected interdependent effects of diffusion and spatially dependent redox potential s on reduction kinetics and microbial community composition. Shallow diffus ion depths (2-10 mm) and very sharply terminated diffusion fronts in column s amended with OC (80 and 800 ppm) reflected rapid increases in Cr reductio n kinetics over very short (mm) distances. These results suggest that Cr co ntamination in soils can be restricted to the outsides of soil aggregates d ue to localized transport and rapid reduction and that bulk sample characte rization is inadequate for understanding the controlling biogeochemical pro cesses.