Modeling of Phosphorus Transport in Swan Lake

As part of the work plan for a Swan Lake nutrient study, St. Anthony Falls Laboratory was asked to assess phosphorus transport through the lake water column assuming the Minnesota Steel Industries (MSI) mining operations would increase internal loading of phosphorus to the lake and would therefore reduce water clarity of the lake. To assess phosphorus concentration in the surface mixed layer of Swan Lake due to internal loading, a two step water quality modeling was approach was selected. In the first step the lake water quality model MINLAKE was used to simulate the depth of the surface mixed layer, the turbulent diffusion coefficients below the surface mixed layer, and the dissolved oxygen (DO) concentration through the water column to determine the timing of anoxic conditions. In the second step, the above information was input to a onedimensional diffusion equation to simulate the transport of phosphorus as a conservative material through the lake water column, i.e. no phosphorus uptake by algae or settling was simulated.