Predicting the locations of nitrate removal hotspots at the groundwater-surface water interface in Wisconsin streams

Groundwater nitrate concentrations are elevated in many regions of the world, including central Wisconsin, which can cause human health problems and deleterious ecological impacts. Denitrification, the conversion of nitrate to reduced gases (N2, N20) by bacteria, results in permanent removal of nitrate from groundwater. The reaction rate (denitrification) and groundwater discharge rate will both influence how much nitrate removal will occur at the groundwater-surface water interface. However, conditions that favor the highest reaction rates such as carbon-rich fine sediments would not be expected to be associated with high groundwater discharge rates, which would in turn not result in maximum nitrate removal. We predict groundwater nitrate removal will be optimized at intermediate rates of groundwater discharge, where microbes are exposed to suitable redox conditions for nitrate reduction where groundwater delivers a large supply of nitrate to reaction sites. The main objectives of our research are 1) to determine the roles of groundwater discharge and denitrification reaction rates on nitrate removal rates in shallow groundwater associated with streams in central Wisconsin and 2) to develop and test statistical models that predict nitrate removal rates from groundwater discharge, groundwater nitrate availability and denitrification reaction rates. In Phase I, we will determine how groundwater discharge and nitrate concentration affects nitrate removal rates in shallow groundwater in central Wisconsin. We will measure denitrification, groundwater discharge rates, groundwater nitrate concentration and groundwater nitrate removal rates in shallow sediments associated with seven streams in the Waupaca River watershed. Based on these data, we will develop statistical models that predict groundwater nitrate removal rates from groundwater discharge and nitrate concentration. In Phase II, we will test the statistical models in other major watersheds in central Wisconsin to determine their robustness for predicting nitrate removal rates in shallow groundwater.