Wetland 639W Outlet Modification
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This project constructed improvements to Wetland 27-639W, located in the City of Crystal just east of the Crystal Airport, which was found to be exporting a significant amount of phosphorus to Upper Twin Lake. Extensive monitoring and a Feasibility Study concluded that the export is primarily caused by drawdown and drying in the wetland through growing season evapotranspiration by the dense cattail vegetation in the central wetland. This drying speeds the conversion of peat to mineral soil, which then releases phosphorus into water flowing through the wetland.
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A new controlled outlet was added to the wetland, reducing flow-through and increasing storage in the wetland. This keeps the wetland wetter, slowing soil mineralization and reducing the release of phosphorus. A new overflow outlet and channel were constructed on the upstream, west side of the wetland to convey overflow and large events. Followup monitoring was conducted for four seasons following construction, confirming an estimated reduction in total phosphorus export of 130 pounds annually.
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