REDOX PROCESSES IN SURFACE WATERS: DYNAMIC INTERACTIONS INVOLVING PHOTOCHEMISTRY, MICROBIAL PROCESSES AND HYDROLOGIC TRANSPORT

In stream ecosystems, important photochemical processes include production of hydrogen peroxide from dissolved humic substances and photoreduction of iron and manganese in both dissolved and solid phases. We have found that in streams enriched with metals by acid mine drainage, interactions between these photochemical processes can control iron concentrations and the downstream transport of metals sorbed by iron oxides. In addition to photochemical processes, the chemical form of dissolved humic substances and trace metals can also be changed by microbial processes occurring in anoxic zones in the hyporheic zone of streams or riparian wetlands. Specifically, some microorganisms in soils and sediments can use quinone moieties in humic substances as electron acceptors or as electron shuttles in the microbial reduction of ferric iron, and gradients in humic redox state may therefore occur as stream water is exchanged with water in stream hyporheic zones. In turn, ferrous iron can reduce nitrate, facilitating the formation of organic nitrogen moieties. We conducted a conservative tracer injection experiment in an alpine stream-wetland system located in the Front Range of the Colorado Rocky Mountains and determined the redox state of dissolved humic substances using fluorescence spectroscopy. In this system, concentrations of nitrate and dissolved organic carbon both increase with the onset of snowmelt as nitrate deposited in the snow pack from sources in the Colorado Front Range is mobilized and DOC is flushed from upper soil horizons. By sampling wells adjacent to the stream during the tracer experiment, we found that lower nitrate concentrations occurred in wells with slower hyporheic exchange and more reduced dissolved humic substances. These studies show that dynamic redox processes are coupled to hydrologic transport processes in streams and that this coupling can control the fate and ecological effects of trace metal and nutrient contaminants in surface waters.