| Iron reduction was explored in two iron-reducing hydrogeologic environments: an uncontaminated surficial riparian aquifer at Reedy Creek, Virginia, and the U.S. Geological Survey Toxic Substances Hydrology site near Bemidji, Minnesota, which is contaminated with crude oil. The two differ in sulfate concentrations and sediment characteristics, including iron content. Specific objectives were to determine the mechanism of iron reduction in the presence of sulfate reduction, to evaluate changes in sediment geochemistry due to iron reduction, and to test the effect of sediment iron content on the extent of iron reduction. The research approach involved field and laboratory studies of groundwater and sediments as well as microcosm experimentation.; Concentrations of iron, sulfide, and dissolved oxygen in groundwater from the surficial aquifer at Reedy Creek, Virginia, indicated a redox gradient with concurrent iron and sulfate reduction in the riparian areas (approximately 20-107 {dollar}mu{dollar}M dissolved iron and approximately 5-35 {dollar}mu{dollar}M dissolved sulfide) and little or no iron reduction in the oxygenated upgradient area. Generation of dissolved iron in microcosm experiments demonstrated that the mechanism of iron reduction in the Reedy Creek system is by direct microbial activity; there was no conclusive evidence for significant reduction of iron by reaction with sulfide.; Microcosms also compared iron reduction in sediments from the two sites. Bemidji sediments have much more extractable iron than Reedy Creek sediments. Nevertheless, the microcosm experiment demonstrated that more iron was reduced in incubations with the sediments from Reedy Creek (up to 300 {dollar}mu{dollar}M) than in the incubations using sediments from Bemidji (up to 70 {dollar}mu{dollar}M). This is hypothesized to be due to greater available surface area of iron-bearing minerals, or to the iron being present in a more labile form.; Sediments at both sites show measurable depletion of sediment ferric iron in the iron-reducing areas relative to background. The Bemidji sediments from the most contaminated part of the aquifer had 30% less ferric iron than the background sediments. Iron reducing sediments at Reedy Creek have an order of magnitude less ferric iron than oxidized sediments. This is consistent with removal of ferric iron due to iron reduction coupled to the degradation of organic compounds. Storage of reduced iron in the solid phase was apparent in both systems; up to a fourfold increase in sediment ferrous iron was found in the iron reducing sediments at Bemidji and an order of magnitude increase was found at Reedy Creek. At Bemidji, a ferroan calcite was observed and the formation of magnetite is likely. At Reedy Creek, formation of pyrite has occurred in the riparian areas. At both sites, there is evidence for the formation of organically bound iron.; This research provides some understanding of the sources and products of iron reduction and provides evidence that microbial reduction can be significant in the presence of sulfate reduction. Because iron reduction is coupled to organic carbon oxidation, understanding of the redox biogeochemistry of iron has implications for the fate of natural organic matter in an uncontaminated system and for the fate of contaminants in a polluted setting. |
