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Linked Hydrodynamic-Sediment Transport-Water Quality Model for Support of the Upper Mississippi River – Lake Pepin TMDL

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Nationally, nutrients and sediment/siltation rank among the top four impairments reported by states on their 303(d) lists. The State of Minnesota is developing a TMDL for the Upper Mississippi River and its watershed for both turbidity and nutrient enrichment (total phosphorus (TP) and chlorophyll a). In order to support this complex TMDL, the Minnesota Pollution Control Agency (MPCA), through a contract with LimnoTech, undertook the development and application of a linked hydrodynamic-sediment transport-water quality model (the UMR-Lake Pepin model). The model was successfully calibrated and confirmed using a 22-year database of water quality covering the model domain. The calibrated/confirmed model was then applied to develop a suite of solids and phosphorus load reduction scenarios to identify point and non-point source reductions necessary to meet turbidity and chlorophyll a TMDL targets. The results of the model application analysis confirmed that the critical conditions for meeting the turbidity targets was high spring flow while the critical conditions for meeting utrophication targets was low summer flow. Achieving the eutrophication targets in Lake Pepin will require significant phosphorus load reductions in major tributaries, especially the Minnesota River and the Upper Mississippi River at lock & dam no. 1. Phosphorus load reductions on the order of 20–50% for the Upper Mississippi River at lock & dam no. 1, 50–80% for the Minnesota River, and an effluent limit of 0.3–0.5 mg P/L will be required to assure summer average TP concentrations throughout Lake Pepin less than 0.075 mg P/L and chlorophyll a levels less than 30 μg/L during critical low flow conditions. Although the exact metric for the turbidity target has not been determined yet, model results indicate that total suspended solids load reductions should be on the order of 20% for the Upper Mississippi River at lock & dam no. 1 and 50–60% for the Minnesota River in order to meet the proposed total suspended solids target of 32 mg/l at lock & dam no. 3 during critical high flow years.

Document Type: Research Article


Publication date: January 1, 2009

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