The persistence of a large stagnation zone in a developed regional aquifer, southwestern Ontario

Abstract

An extensive groundwater zone of exceptionally negative δ18O (–17.5‰ to –16.0‰) exists in a thin, regional, freshwater aquifer between Lake St. Clair and the southern shore of Lake Huron in southwestern Ontario. The zone occurs at the interface between the overlying thick Quaternary clay aquitard of glaciolacustrine origin and the underlying bedrock shale. This isotope signature, which is 7‰ more negative (lighter) than modern water, indicates a Late Pleistocene origin of the aquifer water. This zone occurs only where the Quaternary aquitard is greater than 35 m thick. In the Quaternary aquitard, the Pleistocene isotope signature extends upward from the aquifer and then has a gradational transition to signature resembling modern water (–10‰) near the ground surface. This regional-scale study of the aquifer–aquitard system indicates that the pattern of flow of the aquifer has allowed the persistence of the Pleistocene groundwater since the aquitard was deposited approximately 10 000 years before present. As part of the regional study, a two-dimensional groundwater flow model was used to assess the origin and persistence of the Pleistocene zone under natural conditions. The persistence of the Pleistocene zone was also assessed based on water budgets prepared from aquifer use history and aquifer–aquitard parameters. Our study finds that the low yield and poor quality of groundwater in this zone have caused minimal use, resulting in the persistence of the Pleistocene groundwater since the beginning of major aquifer use in the 1940s.Key words: regional aquifer, aquitard, Pleistocene water, stagnation, aquifer use.