WHPA Vulnerability Analysis for the Region of Peel Wellfields, Ontario

Client: Region of Peel Infrastructure Planning, Engineering and Construction Group
A. Korniluk, M.Sc., P.Geo. Project Manager Hydrogeologist
10 Peel Centre Drive, Brampton, ON L6T 4B9

Project Description:

Earthfx was hired by the Region of Peel to develop wellhead protection areas and contaminant vulnerability assessments for the Caledon East and Palgrave communities. Project work began with the extension and refinement of a model originally developed for the Oak Ridges Moraine. The model layers were expanded to include the main units of the Niagara Escarpment.

The numerical groundwater flow model was built using the US Geological Survey MODFLOW code. A model grid was designed with square cells, each 100 m on a side. Recharge to the model was based on annual average rates determined with a PRMS hydrology model developed for the Humber Watershed. The model was calibrated to match water levels at 1985 overburden wells and 6472 bedrock wells. Baseflow at 10 gauges was also matched as part of the calibration.

Well Head Protection Areas (WHPAs) were first delineated using a lateral Time of Travel (ToT) approach. Two-year, five-year, and 25-year ToT zones were identified. Aquifer vulnerability was determined based on advective travel times from land surface to the water table (unsaturated zone) and travel time from the point on the water table to the well screen. The surface to well advective time (SWAT) was used to delineate zones of low, medium and high vulnerability. The vulnerability zones were overlaid on the ToT WHPA zones and assigned a numerical vulnerability score (VS) that ranged from 2 to 10. Finally, the relative uncertainty of the WHPA delineation and vulnerability scoring was assessed based on the hydrogeologic setting, data gaps, data uncertainty, and uncertainty in the evaluation methodology.

The WHPAs and vulnerability scores determined in this study provided the basis for future work including listing of drinking water quality issues, listing of drinking water quality threats (both activities and conditions) and the identification of areas of low, moderate, and significant drinking water threats within the WHPAs.