Wellhead Protection Area Delineation and Vulnerability Scoring for the Brantford Airport Wellfield

Client: Grand River Conservation Authority (GRCA): 400 Clyde, Cambridge, ON
Contact: Sonja Strynatka, P.Geo, Senior Hydrogeologist
Key Personnel: Michael Takeda, Spencer Malott, E.J. Wexler

The Brantford Airport wellfield, located 6 kilometers west of the Brantford town centre, services a small population of residential and industrial water users. A single well (Airport Well 1) historically supplied the full demand of the service area. A second well (Airport Well 2) was installed in 2014 to meet future demand and provide redundancy to the water distribution system. Earthfx was retained by the GRCA on behalf of the County of Brant to delineate the wellhead protection area (WHPA) and assign vulnerability scores for the Brantford Airport wellfield using a three-dimensional groundwater flow model.
This study took advantage of existing groundwater (MODFLOW-NWT) and hydrologic (PRMS) models, completed by Earthfx for the Whitemans Creek Tier 3 study. The original model area was extended to include the Brantford Airport wellfield within the model areas. The Tier 3 groundwater model was refined in the vicinity of the Brantford Airport wellfield to improve the local calibration. Groundwater recharge was represented as a long-term annual average determined by the PRMS model. This represents a significant improvement on the previous model, which assumed only three zones of recharge, which were assigned according to surficial geology.

The particle tracking analyses were conducted using MODPATH (Version 6.0). Backward particle tracking techniques were applied to delineate the 2, 5, and 25 year time-of-travel WHPAs around the municipal wells. Forward particle tracking techniques were applied to assign vulnerability scores to subzones within each WHPA based on advective travel times from the water table to the well screen (SWAT method). Vulnerability scores were adjusted to account for preferential pathways, including wells of a certain age, and ponds, and gravel pits.

The models used in this study underwent extensive calibration and validation to produce results with a low level of uncertainty. Overall, the findings provide a solid foundation for management decisions to be based.