Design and Hydrologic Performance of a Tile Drainage Treatment Wetland in Minnesota, USA

• Main Authors: Christian Lenhart, Brad Gordon, Joshua Gamble, Dean Current, Nikol Ross, Lydia Herring, John Nieber, Heidi Peterson
• Format: Article
• Language: English
• Published: MDPI AG 2016-11-01
• Series: Water
• Subjects: treatment wetland; nitrate; subsurface drainage water; plant harvest
• Online Access: http://www.mdpi.com/2073-4441/8/12/549

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