Tier 3 Water Budget and Water Quantity Risk Assessment, Regional Municipality of York, Ontario, Canada

Client: Regional Municipality of York, Water and Wastewater Branch, Environmental Services: Mr. Don Goodyear P.Geo., Risk Management Official, (1.877.GO4.YORK ext. 5050)
Key Personnel: D. Kassenaar and E.J. Wexler

Under the 2006 Clean Water Act, watershed-based Source Water Protection studies are conducted to assess risks to municipal water supplies from a water quantity and water quality perspective. Assessing risks due to land use change, drought, and interference from other water takings requires an understanding of the water budget in the area surrounding the municipal wells. York Region operates 12 municipal wellfields. The risk to the wellfields, as well as the impact of the wellfields on other users, cold water streams, and Provincially Significant Wetlands was evaluated.The study focussed on the development, calibration, and application of an integrated surface water/groundwater model to assess the water budget in the study area, delineate wellhead protection areas, and assess the risk to the municipal supply wells.

The USGS GSFLOW code, which integrates the MODFLOW groundwater model and the PRMS hydrologic model, was selected for this study. Work started with a comprehensive data compilation and an update of the geological and hydrostratigraphic models. These formed the basis of the groundwater submodel which was developed on a uniform 200 m grid. The PRMS submodel incorporated climate data from multiple stations as well as detailed soil and land use information to calculate soil water balances on a 50 m grid. The integrated model was calibrated to match observed flows at 16 gauged streams, observed regional groundwater levels and groundwater flow patterns, and to continuous water levels from York Region monitoring wells and PGNM wells. The model was peer reviewed and then applied to evaluate water quantity risk scenarios under current and drought conditions. Model results were used to identify areas of high and moderate risk to the water supply.