Three dimensional modelling to assess contributions of major tributaries to fecal microbial pollution of lake St. Clair and Sandpoint Beach

The presence of high levels of E. coli in Lake St. Clair is a significant concern for millions of people, in Canada and the United States, who rely on that water source for drinking, fishing and recreational purposes. A combination of mathematical modelling and monitoring techniques in the lake can provide an efficient and cost-effective framework for management of microbial pollution of beach waters, as well as serving as a timely reporting tool to communicate associated human health risks from recreational use. Here, a high resolution 3D hydrodynamic model is developed and applied to assess the flow and microbial contribution of major Lake St. Clair tributaries during the Summer of 2010. The model skill in reproducing water temperature is in good agreement with the observed data (γ2, NRMSE, R2 and WS values of 0.12, 0.37, 0.88 and 0.96, respectively). Assuming E. coli input estimates to be conservative, the model results show that the maximum predicted fecal concentrations from the combined input of the major tributaries to be <100 CFU/100 mL for most of the lake. High dominant flow with low E. coli input from the St. Clair River and microbial decay due to residence time in the lake are largely responsible for the results obtained. The results indicate that the four major tributaries are unlikely to be responsible for the observed E. coli exceedances of the Ontario safety guidelines for recreational activity at the highly visited Sandpoint Beach.