Groundwater Nitrate Processing in Deep Stream Sediments

Groundwater in Wisconsin and in many regions of the world has elevated nitrate concentration that can compromise drinking water quality and harm ecosystems. Greater understanding of nitrate retention processes is important for predicting the effects of elevated groundwater nitrate on downstream ecosystems. It is well known that processes in riparian zones, in hyporheic zones, and in the surface water of lotic ecosystems can retain substantial amounts of available nitrogen. Much less is known about nitrogen processing in groundwater associated with deep sediments beneath the stream channel (below the hyporheic zone). We plan to address this gap in knowledge by answering the following questions: 1) Is nitrogen processing in groundwater associated with deep stream sediments widespread throughout a river network? 2) Does variation in groundwater nitrate concentration, organic carbon, and dissolved oxygen drive variation in denitrification in deep sediments? 3) Does nitrate concentration in groundwater saturate denitrification in stream sediments? We will identify eight study sites on streams and rivers in the Waupaca River Network in Central Wisconsin, where groundwater nitrate concentration reaches 40 mg NO3-N L-1. We will measure denitrification on sections from five sediment cores collected from each stream to determine how denitrification rates vary by depth, among cores, and among streams. Regression models will be used to determine if high groundwater nitrate concentration saturates denitrification rates and to predict denitrification rates from sediment properties and groundwater nutrient concentrations. Peeper samplers and piezometer nests will be deployed in each stream to determine fine-scale vertical profiles of nitrate concentration in the groundwater to a sediment depth of 70 cm. This combined approach will provide some of the most high-resolution estimates of groundwater nitrate processing in stream sediments and will fill an important gap in understanding of nitrogen biogeochemistry at the groundwater-surface water interface.