The sources and cycles of iron and manganese in surface water supplies

Abstract

Evaluation of the sources and cycles of water quality contaminants in watersheds is critical for effective surface water resource management. In particular, iron (Fe) and manganese (Mn), commonly found in rocks and sediments, have adverse impacts on water quality. However, controlling Fe and Mn in surface water systems is often complex and requires careful consideration of the hydrologic and biogeochemical factors that influence the speciation and mobility of these metals. This dissertation investigates the sources and cycles of Fe and Mn in surface waters designated for human use. Here, I present the findings from three field- and laboratory-based studies conducted at sites in western Virginia, United States. The first study examines the impacts of reservoir-derived and watershed-derived metals on water quality along the 180 km reach of the Roanoke River downgradient from Leesville Dam. The results from this study showed strong temporal influences on river water quality immediately downgradient of the dam, resulting from seasonal reservoir dynamics. Further downgradient in the Roanoke River, water quality was strongly tied to hydrologic conditions resulting from influences generated in the watershed. The second study investigated the effects of increasing dissolved oxygen (DO) concentrations in the hypolimnion of stratified drinking water reservoir on Fe and Mn oxidation and removal. Results from a whole-ecosystem experiment showed that increasing DO concentrations through hypolimnetic oxygenation was effective for preventing the accumulation of soluble Fe in the water column. Although Mn oxidation increased under well-oxygenated conditions, soluble Mn still accumulated in the hypolimnion. Results from a laboratory experiment demonstrated that the oxidation of Mn was strongly tied to the activity of Mn oxidizing microbes. The third study examined the relative contribution of external and internal metal loadings to the exchange of metals between sediments and the water column and the source/sink behavior of a seasonally stratified reservoir under varying hydrologic conditions in the inflows and outflows and redox conditions in the reservoir hypolimnion. Results from this study showed that redox conditions strongly influenced the exchange of metals between the sediment and aqueous phase, but had little effect on the source/sink behavior of the reservoir, while external tributary loadings had little effect on internal redox cycles, but was a strong indicator for whether the reservoir behaved as a net metal source or sink. Overall, the findings from these studies exemplify the value of characterizing the hydrologic and biogeochemical drivers of Fe and Mn cycles for managing the water quality effects of these metals in surface water supplies.