Forecasting receiving water response to alternative control levels for combined sewer overflows discharging to Toronto’s Inner Harbour

This article evaluates the role that different levels of control for combined sewer overflows have in addressing the recreational water quality objectives of the Toronto Inner Harbour of the Toronto and Region Remedial Action Plan. Three models are used to establish the predictive methodology: the Infoworks model for the combined sewer service area, the Hydrologic Simulation-F model for the remainder of the watershed, and the MIKE 3 computer code to evaluate Lake Ontario response to control. Each model was calibrated with E. coli densities observed respectively in sewer discharges, instream, and in the Inner Harbour. Two indices are used to evaluate the response of water quality in the Inner Harbour – fraction of the surface area achieving Blue Flag status, and portion of the swimming season (June to August) above recreation objectives. Analyses of control options led to the recommendation that virtual elimination of combined sewer overflows (one overflow per season control strategy) should be pursued, rather than the lower level of control of 90% volumetric control, which is the minimum provincial environmental requirement. Implementation of priority projects for improving water quality along the Toronto waterfront, including the Don River and Central Waterfront project, are integral to delisting Toronto as a Great Lakes Area of Concern.