Treatment wetlands are increasingly needed to remove nitrate from agricultural drainage\nwater to protect downstream waters, such as the Gulf of Mexico. This project sought to develop a\nnew edge-of-field treatment wetland, designed to remove nitrate-nitrogen and enhance phosphorus\nremoval by plant harvest and to monitor its effectiveness. A 0.10 ha wetland was designed and\ninstalled to treat subsurface drainage flow from farmland in southwestern Minnesota, USA, in 2013,\nand monitored for three years by recording flow, nitrate-nitrogen, total phosphorus (TP) and soluble\northophosphorus (OP) input to and output from the wetland. Prior to construction, a level-pool\nrouting, mass balance approach with DRAINMOD flow inputs was used to predict nitrate removal\nefficiency. Nitrate load removal averaged 68% over three years, nearly matching model predictions.\nHowever, most denitrification occurred in the sub-soil of the wetland rather than in surface flow as\npredicted. Phosphorus removal was approximately 76% over three years, and phosphorus removed\nby plant uptake exceeded inflow mass in the third year. The edge-of-field design has potential as a\ncost-effective method to treat field outflows because agricultural landowners can adopt this treatment\nsystem with minimal loss of productive farmland. The wet-prairie vegetation and shallow depth also\nprovide the opportunity to remove additional phosphorus via vegetative harvest.
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