Chromium speciation and mobility in contaminated soils, Sault Sainte Marie, Michigan

The fate and mobility of chromium in a wetland area was studied using both field and laboratory techniques. Wetlands have been used as sinks for chromium because in reducing wetland environments chromium will exist as Cr(III) which is a less toxic form of chromium and inorganic Cr(III) solids are very insoluble. However, there is very little information concerning the mobility of chromium in organic rich environments like wetlands. Chromium speciation in the soils was investigated using sequential chemical extractions. Chromium speciation in the aqueous samples was investigated using solid phase extraction resins that removed the cationic, anionic, and hydrophobic organic species from solution. The mobility of chromium in these soils was assessed using intact soil core microcosms. The microcosms were treated with solutions to simulate acid rain deposition and the influx of nitrate and potassium.; The speciation work indicated that chromium was associated with both inorganic and organic components of the system. The results from the soil speciation studies showed that the solid phase chromium was primarily extracted by the acidic moderately reducible (MR) and basic oxidizable extractions. These results indicate that the solid forms of chromium in these environments will be either a chromium hydroxide or associated with the soil organic matter, with chromium hydroxide becoming more dominant at higher total concentrations of chromium in the soils. The aqueous phase chromium concentrations in the surface and pore waters at this site are higher than would be predicted by inorganic thermodynamic calculations. No Cr(VI) was observed in these samples. The aqueous chromium in the field samples was found to be slightly correlated with dissolved organic carbon (R2 = 0.66). The results of the solid phase extraction performed on the aqueous field samples show that aqueous chromium in field samples exists primarily as an anion in these waters (96%). It is concluded that the solubility and mobility of chromium is controlled, at least in part, by complexation with dissolved organic carbon and that this may be a thermodynamically driven process.; Intact soil core microcosms were used to investigate the mobility of chromium in laboratory studies. The data from the microcosm experiments also indicated that the aqueous chromium existed as an organically complexed anion. The results of the microcosm experiments indicate that the solubility of chromium may also be increased if the soils experience periods of cyclic saturation and unsaturation. There also may be an increased solubility of chromium if the degradation of soil organic matter is increased.