An analysis of intrinsic vulnerability to contamination at 37 active municipal groundwater supply wells operated by the Regional Municipality of York was completed as part of a Source Water Protection initiative. The Schomberg, Aurora, Newmarket, Ansnorveldt, Holland Landing, Queensville, Ballantrae, and Mt. Albert wellfields are located north of the regional groundwater divide that runs parallel to the east-west axis of the Oak Ridges Moraine (ORM). The Nobleton, Kleinburg, King City, and Stouffville wellfields are located south of the regional groundwater divide. Most of the wells are screened in the deeper, confined aquifers (i.e., the Thorncliffe Aquifer Complex or Scarborough Aquifer Complex).
Previously defined wellhead protection areas (WHPA) based on conservative time-of-travel (TOT) analyses did not account for the true time of travel from ground surface to the well screen. A more realistic assessment of the vulnerability of York Region’s municipal wells to contamination from surface and near-surface sources was made through (1) refinement of the earlier TOT analyses; (2) use of forward particle-tracking techniques to determine the water-table-to-well advective times (WWAT); and (3) assignment of intrinsic vulnerability scores (IVS) to areas within the TOT zones. Additional objectives of this study were to (1) collect, organize and analyze data required to conduct the TOT and WWAT analyses; (2) identify data gaps and data uncertainty; (3) assess the relative uncertainty of the IVS scoring due to data gaps and uncertainty; and (4) provide assistance to York Region staff in interpreting study results and provide recommendations on how to reduce uncertainty. Results of this study provided the foundation for water quality risk assessment and analysis of threats to municipal drinking water sources and supplies.