The Groundwater Surface Water Interface (GSI) is a transition zone with bio/geo/chemical properties determined by both the adjacent groundwater and the surface water bodies. Many Superfund sites can cause the subsurface contamination in shallow aquifers and threaten the nearby surface water bodies by transportation of contaminants in the groundwater through GSI. Reduced iron, Fe{sup}(2+), is frequently abundant in a polluted groundwater. Aerobically, Fe{sup}(2+) is stable only under acidic conditions; at neutral pH, it is rapidly oxidized to barely soluble hydrous ferric oxide (HFeO). However, in anoxic and microaerobic habitats where iron autoxidization is retarded, microbes may compete for Fe{sup}(2+) with abiotic reaction. Gallionella ferruginea and Leptothrix species are among the best-known neutrophilic, iron oxidizing bacteria (IOB). HFeO has a large surface area, which supply it a strong affinity for many dissolved metals. Therefore, the formation of iron oxides in the GSI can result in adsorption or immobilization of heavy- and transition- metals like As and Cr species in the groundwater when the contaminated groundwater seeps into the surface water bodies.
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