The identification of significant groundwater recharge areas (SGRAs) was an important element of recent Source Water Protection initiatives in Ontario. Under SWP, the term “significant recharge” has been taken to be synonymous with high volume recharge. However, simply mapping areas of high recharge (HVRAs) may not be enough to protect the function of key wetlands, coldwater streams or fish spawning areas. To address this limitation, Lake Simcoe Conservation Authority retained Earthfx to develop a consistent and scientifically sound methodology for delineating ecologically significant groundwater recharge areas (ESGRAs) across the Lake Simcoe watershed.
A new methodology was developed to assess significance using a combination of numerical modelling and particle-tracking techniques to identify the hydrologic connection between recharge areas and ecologically important surface features. This methodology was first applied in a pilot study for the Barrie Creeks, Lovers and Hewitt subwatersheds and utilized an existing FEFLOW groundwater model. The model was created for the Tier 2 Water Budget study for the South Georgian Bay – Lake Simcoe (SGBLS) SWP area. The FEFLOW model was primarily developed to assess regional groundwater flow patterns at a watershed-scale for water budget analysis and assessment of wellfield response. The model was locally refined in the study area by Earthfx to better represent the shallow groundwater system for the ESGRA analysis.
To link the surface features with the groundwater system, a particle-tracking exercise was undertaken with the recalibrated steady-state FEFLOW model. Particle tracking is an excellent technique for visualizing and understanding groundwater flow paths. It is particularly useful in areas with complex, three-dimensional groundwater flow. For the ESGRA analysis, particles were introduced in a dense distribution at the point of known groundwater discharge and around ecologically significant discharge features (i.e., wetlands and coldwater stream reaches) and then traced backwards to the point of recharge. A benefit of the reverse tracking technique is that attention can be focused on specific ecological features.
Particle endpoints from the backward tracking analysis tend to cluster at areas of groundwater recharge and thereby map the linkage to the ecological features. An un-biased cluster analysis technique, using a bivariate kernel density function, was applied to identify areas of higher particle density. These areas were delineated as ESGRAs. Optimization of the technique was achieved by adjusting the kernel’s smoothing and threshold parameters until the delineated ESGRAs contained the maximum number of endpoints per unit area of ESGRA coverage.The ESGRA delineation results for the Barrie area clearly identified the highland areas supplying the streams in the Hewitts and Lovers Creek subwatersheds and the local recharge feeding Barrie Creeks. There was some correlation between SGRA and ESGRA zones, particularly in the west-central portion of Lovers Creek.