Hydrogeology of the Oak Ridges Moraine aquifer system: implications for protection and management from the Duffins Creek watershed

Abstract

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.

Understanding the Factors That Influence Headwater Stream Flows in Response to Storm Events1

“…That stream responses in the well drained soils tended to be low regardless of both rainfall and land use/land cover was expected as these areas tend to deliver water to the channel as shallow groundwater (Ragan 1967;Hewlett et al 1969) than flatter and less porous areas on the Halton till (Gerber and Howard 2002). However, I expected that the event discharges from catchments located on the Halton till, where much of the intensive agriculture is located, to be higher than was observed in this study.…”

“…Finally, glaciolacustrine deposits of silts and clays, from glacial Lake Iroquois, are generally found in the areas closest to present-day Lake Ontario (Figure 3). This complex geology produces a tiered groundwater aquifer system which, at least in one of the study watersheds (Duffins Creek), results in 75-80% of the average watershed baseflow discharge, originating from the uppermost aquifer and another 20-25% from the much deeper aquifers underlying the extensive Northern-Newmarket till aquitard (Gerber and Howard 2002). This water generally enters the rivers down-slope of the Lake Iroquois shoreline.…”

“…That I observed similar patterns when the response data were standardized to remove the effect of baseflow, gives further confidence that the observations are valid. Both geology and land use/land cover have been shown in many cases to be independent predictors of flows (for geology see Gerber and Howard (2002); for base flows see Moore and Wondzell (2005); for land use/land cover see Dunne and Leopold (1978); Cook and Dickinson (1986); and see Dinicola (1990) for peak flows). Many simulation models consider the combined influence of forest cover and geology (Ng and Marsalek 1989;Beckers and Frind 2001;Li et al 2008), but my study supports the inclusion of land use/land cover as a major modifying factor for headwater systems and provides direction for how the two factors interact, in different geologic settings.…”

Simulating the multi-seasonal response of a large-scale watershed with a 3D physically-based hydrologic model

“…The remainder of the hydrostratigraphic units were assumed to act as aquitards. The three aquifers have been extensively characterized for water supply purposes, with hydraulic conductivity values reported by (Gerber and Howard (2002)) as well as Earthfx Inc (2004). Each of the hydrostratigraphic units were assumed to be homogenous throughout the study area in terms of their hydrogeologic properties, with the assigned hydraulic conductivity values reported in Table 1.…”

“…This framework of units is based on the initial work of the Geological Survey of Canada by Sharpe et al (1999). Following Gerber and Howard (2002), Earthfx Inc (2004) further refined the interpretation of the Quaternary succession into eight hydrostratigraphic units. This latest interpretation forms the foundation of the work used herein.…”