How much is enough? The potential use of wetlands to reduce agricultural nitrate losses

Amy Hansen, Research Associate, St. Anthony Falls Laboratory, University of Minnesota

Agriculturally derived nitrogen in Midwestern US streams and rivers causes water quality impairments at the local, regional, and continental scales. Existing and restored wetlands have the potential to substantially reduce nitrate export and are one of the most promising conservation interventions under consideration in the region. However, most research to evaluate wetland performance has focused on nitrate reduction across individual wetlands without considering watershed scale effectiveness of a suite of multiple connected wetlands. I have evaluated watershed scale effectiveness of wetlands for reducing agricultural nitrogen under a range of climate conditions. Together with colleagues, I used field observations in the Minnesota River Basin (MRB) and a spatially explicit, reduced complexity network model of the Le Sueur River basin, a sub-watershed of the MRB. Field nitrate observations were collected during six synoptic sampling campaigns, over three years and 180 locations, including observations at 83 locations during a 0.1% daily exceedance probability streamflow event. Observational data showed that nitrate concentrations decreased exponentially with wetland cover and suggest that during the high streamflow event, wetland margins, floodplains and shallow depressions were actively connected to the network and reducing nitrate. The process based nitrate routing model illustrates the dynamic nature of wetland effectiveness under a range of streamflow conditions and is being used to explore optimal restored wetland placement. Ultimately, these findings can guide wetland restoration by quantifying critical locations and wetland specifications for nitrate containment within agriculturally intensive regions.