Assessing the Effect of Wetland Connectivity on In-Stream Nitrate Concentrations Using an Integrated Wetland-River Network

Abstract

Nitrate degrades water quality from the local to continental scale. Excess nitrate can cause eutrophication and hypoxia. A hypoxic zone exists in the Gulf of Mexico that causes organism death and displacement. One of its primary sources of nitrate is the agriculture region in the Mississippi River Basin. The location for this study is the Minnesota River Basin which is part of the Mississippi River Basin. Excess fertilizer from corn and soybean production runs off the landscape to the waterways. Nitrate is naturally removed from waterbodies through denitrification with wetlands being the most effective due to their long residence time. Wetland restoration has the potential to remove nitrate from small portions of the landscape. The question is how to place wetlands to most effectively remove nitrate across a watershed. There are multiple models and studies looking at this and they need an integrated wetland-river network; however, the common sources of information for wetlands and waterways are the National Wetlands Inventory and the National Hydrography Dataset, respectively. These were not inherently made to be compatible. A method was created to combine these two datasets, which outputs an integrated wetland-river network complex that can be used for further analysis. For this method, floodplain wetlands were first removed and both flowlines and wetlands were assigned unique identifiers and information on how they connect to allow routing through the wetland-river network. A reliable way to predict nitrate is important as sampling and laboratory analysis to determine nitrate concentrations is expensive and time consuming. Two common metrics related to nitrate concentrations are percent wetland area and interception fraction. Interception fraction takes into consideration the spatial placement of wetlands by comparing the area of the watershed intercepted by wetlands to the total watershed area. These metrics were calculated from the integrated wetland-river network and compared to nitrate samples from June 2014 and 2015. Interception fraction was found to be a better predictor of nitrate concentration than percent wetlands and that the percent of non-intercepted cropland and the size of the wetland that intercepts most of the watershed were not useful in understanding the residuals. This work will allow for the more rapid prediction of nitrate watershed exports to inform where wetlands should be restored to reduce nitrate exports into waterways.