Developing a Turbidity TMDL for a Hydrologically Complex Tributary to the Mississippi River

The Lower Vermillion River (LVR) extends from Hastings, Minnesota, to the confluence of the Vermillion River and the Mississippi River south of Lock and Dam 3. Water quality monitoring of the LVR has shown that turbidity levels exceed the Minnesota Pollution Control Agency's (MPCA) water quality standard approximately 40 percent of the time. As required by the Clean Water Act, MPCA developed a Total Maximum Daily Load (TMDL) report to identify the activities that need to occur to address the turbidity impairment. This paper summarizes the TMDL process and results, including a number of unique issues that had to be addressed regarding water quality standards, modeling, assigning allocations for regulated stormwater, and implementation.The goals of the LVR Watershed Turbidity TMDL Project were to describe the nature and extent of turbidity in the highly complicated setting of the LVR, determine turbidity source load allocations that consider major sources, and produce a final report that expresses the turbidity dynamics in terms of an “allocation” among sources and recommendations for corrective actions. Because of the complexities of the system, the project was implemented in three phases:Phase I: Data Gathering and Conceptual Model DevelopmentPhase II: Sampling and Model SetupPhase III: Model Refinement and TMDL DevelopmentThe LVR system is hydrologically complex with the LVR having a naturally low gradient and occupying the floodplain of the Mississippi River. Flow enters this system from the Upper Vermillion at Hastings, via local tributaries, through movement of groundwater; and by interflow with the Mississippi. Because of the operation of Mississippi Lock and Dam 3 for navigation, normal pool in Mississippi Pool 3 is typically greater than 5 feet above the water surface elevation in the LVR. This creates a tendency for water from the Mississippi to flow into the LVR, seeking steeper gradient to the channel below Lock and Dam 3. Finally, because of its own low channel gradient, flow within the LVR can be affected by the water surface elevation at its confluence with the Mississippi, below Lock and Dam 3, and by flows in the Cannon River.As part of this study the U.S. Army Corps of Engineers CE-QUAL-W2 model was applied to provide a more complete description of the movement of water and sediment in the LVR system and to link sediment sources with turbidity impacts. The CE-QUAL-W2 modeling determined that 72 percent of the sediment to the LVR is from Mississippi River Pool 3 via the various sloughs that connect the two waterbodies. The next most significant source of sediment was found to be the local tributaries draining from the LVR watershed (16%) followed by the Upper Vermillion River (8%). Internal sources of sediment, such as wind- and fish-induced resuspension of fine sediments and the draining of wetlands, were estimated to contribute approximately three percent of the sediment load; however, despite the relatively small load contribution from these sources, they were found to have a significant impact on turbidity during periods when there is little inflow from Pool 3. To develop the TMDL the model was used to identify a scenario that achieved a 30-day average turbidity of less than 20 nephelometric turbidity units (NTUs). Allocations were then made to the facilities and regulated stormwater communities upstream on the Upper Vermillion River, to Pool 3, to the local tributaries, and to the internal sources such as wind and fish-induced re-suspension. A detailed Implementation Plan is now being prepared by local stakeholders to identify activities that should be taken to achieve the desired load reductions.