Aquatic nitrate removal depends on interactions throughout an interconnected network of lakes, wetlands, and river channels. The main question motivating this work was: How do we optimize the placement and characteristics of wetland restorations within a watershed to reduce nitrate loading from a basin (e.g., to improve drinking water or reduce hypoxia) and/or nitrate concentrations throughout the basin (e.g., to improve aquatic life)? Optimality here is the minimum cost of wetland restorations to achieve a given level of nitrate reduction. In answering this question, we present a network-based model that quantifies nitrate-nitrogen and organic carbon concentrations through a wetland-river network and estimates nitrate export from the watershed. This model dynamically accounts for multiple competing limitations on nitrate removal, explicitly incorporates wetlands in the network, and captures hierarchical network effects and spatial interactions. We applied the model to the Le Sueur Basin, a 2,880 km2 agricultural landscape in southern Minnesota, and validated the model using synoptic field measurements during June for years 2013-2015. We varied wetland location, type (e.g., with emergent vegetation or an open water pond), and size, and optimized for reducing nitrate concentration at the basin outlet or the average nitrate concentration throughout the basin.
Results/Conclusions
Model results show that local nitrate removal via denitrification can be limited by nitrate availability, organic carbon availability, or residence time depending on discharge, characteristics of the waterbody, and location in the network. These results demonstrate that reduction of watershed-scale nitrate concentrations and downstream loads in the Le Sueur Basin can be more effectively achieved by increasing water residence time (by slowing the flow) than by increasing organic carbon concentrations (which may limit denitrification). In the optimization analysis, the wetlands selected to minimize nitrate concentration at the basin outlet were generally larger, fewer, and located closer to the outlet than those selected to minimize the average nitrate concentration throughout the basin, which were generally smaller, more numerous, and farther upstream in the river network. This difference meant that optimizing for nitrate concentration reductions at the basin outlet did not substantially reduce average nitrate concentrations throughout the basin. Thus, outcomes of optimized wetland restoration strategies are substantially different for downstream vs. spatially distributed water quality improvement goals. Achieving comprehensive reductions in nitrate throughout watersheds requires compromise strategies that incorporate upstream and downstream restoration implementation.