In southern Ontario, vertical leakage through a regionally extensive till is the primary source of recharge to underlying aquifers used for domestic and municipal water supply. Since leakage is largely controlled by the bulk hydraulic conductivity (K) of the aquitard, accurate estimates of K are necessary to quantify the resource. Considerable controversy exists regarding estimates of K for this aquitard, which vary according to the scale of the test method. For the till matrix, estimates from core samples and slug tests consistently range from 10−11 to 10−10 m/s. Isotopic evidence (3H), on the other hand, indicates that nonmatrix structures such as sand lenses, erosional surfaces, joints, and fractures significantly enhance till permeability. This is confirmed by slug test, pump test, recharge, and water balance studies, which show that K varies over seven orders of magnitude (10−12 to 10−5 m/s). To provide a regional estimate of bulk K and a reliable estimate of vertical recharge through the Northern Till, a numerical ground water flow model was constructed for the Duffins and Petticoat Creek drainage hasin. The model was calibrated to measurements of hydraulic head and estimates and measurements of base flow throughout the basin. This model demonstrates that the vertical hydraulic conductivity (Kv) for the Northern Till ranges from 5 × 10−10 to 5 × 10−9 m/s, values that are up to 2.5 orders of magnitude greater than matrix K estimates. Regional recharge through the Northern Till is estimated to range from 30 to 35 mm/a.

The Oak Ridges Moraine aquifer feeds the headwaters of major rivers in the Greater Toronto Area and is an important source of domestic water supply. Recognizing the rapid rate of urban growth in the region, there is a concern that changing land use along the moraine must be strictly controlled if groundwater is to be adequately protected. To date, efforts to incorporate groundwater protection into the land use planning process have been hampered by an inadequate quantitative hydrogeological understanding of the aquifer system. Focusing on the Duffins Creek watershed, comprehensive hydrogeological studies including a numerical flow model now provide a quantitative insight into the hydrogeologic function of the moraine. These studies demonstrate that 60% of the entire basin groundwater discharge to streams occurs along the south flank of the moraine, and 60% of this headwater discharge occurs below the 275 m above sea level contour, one of the commonly accepted planning boundaries of the moraine. The remaining discharge is contributed by aquifers within and underlying deposits that extend to the south of the moraine. While 75–80% of the watershed discharge to streams is received from the uppermost aquifer, 20–25% is contributed by deeper aquifers underlying the extensive Northern–Newmarket till aquitard. This work shows that the moraine sediments represent just one component of a regional flow system that extends beyond the morphological boundary of the moraine. This has important implications for groundwater protection, as it demonstrates the need for management strategies that incorporate the regional groundwater flow system and not the moraine in isolation.

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