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Bedrock Aquifers in Southern Ontario


Three groundwater regimes have been identified in the Paleozoic bedrock terrain of southern Ontario based on geological, geochemical and isotopic criteria: a shallow fresh water regime at depths of up to 250 metres, an intermediate depth brackish to saline sulphur water (dissolved H2S) regime at depths of up to 450 metres, and a deep brine regime. The shallow fresh water regime is an active, topography-driven flow system. Fresh water occurs in unconsolidated glacial and post-glacial sediments overlying Paleozoic bedrock, as a regional contact aquifer at the interface between the sediments and the bedrock, and in karstic Paleozoic limestones and dolostones in areas of thin drift or outcrop. Individual aquifers in glacial drift are generally unconfined to partly confined, forming complex, local systems with limited lateral extent. The contact aquifer extends beneath most of southern Ontario, including the uppermost few metres of bedrock, and is the most extensive continuous fresh water aquifer in southern Ontario. The karst aquifer system occurs in carbonate rocks beneath thin drift or in outcrop areas with reported occurrence of fresh water up to 130 metres below the bedrock surface. Aquifers in the intermediate to deep regimes are contained in several discrete regional paleokarst horizons developed in limestones and dolostones at regional unconformities. Aquifers are generally thin, and are confined between thick aquitards of shale, evaporites and non-karstic carbonates. All of these confined aquifers display downdip geochemical zonation from fresh water at the subcrop edge to brines in the deep subsurface, separated by an intermediate zone of brackish to saline sulphur water. The brine aquifers contain evaporatively concentrated Paleozoic seawater, modified by Previous HitrockNext Hit-Previous HitwaterTop interaction, with unique isotopic signatures in individual aquifer and reservoir systems. The intermediate regime displays isotopic signatures characteristic of meteoric water influenced by microbial activity, with isotopic evidence of deep penetration of cold-climate water inferred to be Pleistocene in age.