Ecologically Significant Groundwater Recharge Assessment in the Central Lake Ontario Watersheds
Client: Central Lake Ontario Conservation Authority: Ms. Gail Soo Chan, P.Geo., Director – Groundwater Resources, 100 Whiting Avenue, Oshawa, ON L1H 3T3 (905-579-0411 x139)
Key Personnel: E.J. Wexler
The identification of significant groundwater recharge areas was a key 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 wetlands, coldwater streams or fish spawning areas. To address this limitation, Earthfx updated the existing CLOCA hydrologic and groundwater models and applied particle tracking techniques to delineate the ecologically significant groundwater recharge areas (ESGRAs) supplying flow to wetlands and stream reaches.
Reverse particle-tracking was undertaken with the recalibrated steady-state MODFLOW-NWT model. Particle tracking is widely-used technique for visualizing groundwater flow paths and establishing a link between the ecological feature and recharge area. It is particularly useful in areas with complex, 3-dimensional groundwater flow. Particles were introduced in a dense distribution in wetlands and along stream reaches) and then traced backwards to the point of recharge.
The figure shows show both short and long flow paths tracing back to areas of local and regional recharge, respectively. The longer flow paths track back to the Oak Ridges Moraine while many of the local flow paths track back to the Iroquois Beach and other glaciolacustrine deposits. Of note are pathlines that originate in recharge areas outside the CLOCA watersheds.
Particle endpoints from the backward tracking analysis tend to cluster at areas of groundwater recharge and thereby map the linkage to the ecological features as shown in the second figure. The density of particle endpoints was evaluated using cluster analysis techniques with a bivariate kernel density function. Parameters of the cluster analysis technique were optimized so that the delineated ESGRAs contained the maximum number of endpoints per unit area of ESGRA coverage. Final ESGRA delineation is shown in the third figure.
The fourth figure compares the ESGRAs with HVRAs identified in the Tier 1 study. There is correspondence with HVRAs over 38% of the delineated ESGRAs. Differences can also be seen over the till covered areas where the HVRAs analysis misses the lower-volume local recharge systems. Better correlation is evident in the Iroquois Beach deposits where recharge appears to directly support adjacent local features.