A methodology for identifying ecologically significant groundwater recharge areas

Abstract

A methodology has been developed to delineate recharge areas that support groundwater-dependent ecological features such as wetlands and coldwater streams. Numerical groundwater models are employed together with particle tracking techniques to link recharge areas to specific ecological features. Kernel density estimation methods are used to identify areas that contain the maximum number of particle endpoints per unit area. These high-density endpoint clusters are subsequently mapped as ecologically significant groundwater recharge areas (ESGRAs). The technique can be utilized with either finite-element or finite-difference groundwater models which are loosely coupled or fully integrated with a distributed hydrologic model. A good representation of both the surface water and the shallow groundwater system is required and the models should be based on a rigorous understanding of the regional, watershed and local-scale geologic, hydrogeologic and hydrologic setting. This manuscript describes how the methodology was employed to delineate ESGRAs for the Oro North, Oro South and Hawkestone Creeks subwatersheds, located in the northwest portion of the Lake Simcoe watershed in southern Ontario, Canada. The three study subwatersheds are connected to the Oro Moraine complex, a high-recharge feature that supports multiple watersheds and the regional groundwater flow system. A transient, integrated surface water/groundwater model was developed for the watersheds flanking the moraine and was calibrated to daily streamflow and groundwater-level observations. Particle tracking was then used to define the groundwater flow system between all streams and wetlands and the areas of recharge, and provided insight into the hydrologic function of the Oro Moraine. After optimization of the cluster analysis procedure, 24% of the 125-km² study area was mapped as having recharge areas that support significant ecological features. As required by the Lake Simcoe Protection Plan, the ESGRAs will be protected from future development to ensure the maintenance of the groundwater-fed ecosystems they support. The methodology described in this paper provides a consistent, objective and technically sound means of identifying and delineating ESGRAs, and has recently been applied to other watersheds in southern Ontario.