Influence of Wetland Hydrodynamics on Subsurface Microbial Redox Transformations of Nitrate and Iron

Understanding controls on the transport and fate of N in groundwater is critical to predicting the impact of human activity on the ecological health and economic viability of aquatic resources. Wetlands and riparian zones are major recipients of nitrate-polluted groundwater in Wisconsin and many other landscapes in the U.S. Denitrification in such environments is a well recognized mechanism that can limit transfer of agriculturally-derived groundwater nitrate to surface waters. However, the impact of competing microbial metabolic pathways and redox interactions are not well understood in relation to hydrological dynamics. In particular, interaction between N and Fe redox cycles represents a potentially important but poorly constrained driver of aquifer N dynamics. This project will examine the fate of groundwater nitrate in a hydrologically dynamic wetland system (Dorn Creek) in Dane County, WI. This aquifer system is representative of a many environments in Wisconsin that are strongly influenced by surface water-groundwater hydrologic exchange. Spatial and temporal variations in groundwater-surface water flux and a variety of geochemical parameters will be determined in multilevel samplers across a transect perpendicular to the creek. These data, in parallel with microbiological analyses, will be used to assess the significance of denitrification vs. reduction of nitrate to ammonium in relation to two key microbial Fe redox processes: dissimilatory iron oxide reduction (DIR) and subsequent nitrate-driven oxidation of the ferrous iron produced by DIR. The combination of hydrologic, geochemical, and microbiological data will allow us to assemble a robust conceptual model of how wetland hydrodynamics affect microbial redox metabolism and the fate of groundwater nitrate. The conceptual model will serve as the basis for development of a two-dimensional reactive transport model that will be useful for predicting the influence of groundwater-surface water interactions on the fate of N in a broad range of shallow aquifer systems in Wisconsin.