CHROMIUM REDOX CHEMISTRY IN A LOWER MISSISSIPPI VALLEY BOTTOMLAND HARDWOOD WETLAND

Experiments were conducted to identify and quantify biogeochemical processes controlling chromium redox chemistry in a seasonally flooded Lower Mississippi Valley forested wetland. Chromium speciation, transformations, and solubility were studied in the overlying floodwater column and in the wetland soil. In the floodwater column, chromium chemistry was regulated by Cr(VI) diffusion into the soil, Cr(III) precipitation, complexation by organic ligands, sorption on suspended solids, and Cr(III) to Cr(VI) oxidation. Wetland soil redox level determined the rate and capacity of the soil to assimilate and retain chromium. Under oxidized and moderately reduced (+500 to +100 mV) soil conditions, chromium behavior was dominated by Cr(VI) sorption and reduction of Cr(VI) to Cr(III). Under more reduced soil redox (<+100 mV) levels, chromium chemistry and solubility was controlled by the chemical reduction of Cr(VI) by soluble ferrous iron. Results obtained suggest that the studied bottomland hardwood wetland or floodplain will serve as a sink for chromium entering the wetland, thereby reducing the contamination of downstream ecosystems.