Hydrogeology of the deep flow system within Quaternary sediments, south-central Ontario

Abstract

Aquifers utilized for private and municipal water supply within south-central Ontario occur within Early to Middle Wisconsinan sediments, specifically the Thorncliffe and Scarborough Formations. These formations, including the Sunnybrook Drift, are often and herein referred to as Lower sediments. These deep sediments are replenished by vertical groundwater flow through overlying aquitards including the Newmarket Till and/or silt-clay rhythmites of the late Thorncliffe Formation. Also, channelization episodes characterized by fining-upward sequences have locally breached the Newmarket Till sediment. Recent work, including detailed geologic and hydrogeologic analysis of cored sediment locations, and isotopic analysis of porewater and groundwater, has led to a refinement of the conceptual hydrogeological model of the Lower sediment package. Based on high-quality seismic, geologic and hydrogeologic data, a revised conceptual model of the 'Yonge Street Aquifer' (YSA) has been postulated. The YSA in the Greater Toronto Area has been utilized since the mid-1900s. The current municipal water supply system consists of nine wellfields installed between 1957 and 1991. Data reveal two generations of roughly north-south channels: older pre-Newmarket Till channels within Lower sediments (termed Thorncliffe channel) and younger post-Newmarket Till ORM-related channels (termed ORM channel) that incise both Newmarket Till and Lower sediments. The YSA is interpreted to occur within a Thorncliffe channel, with possible vertical connection to younger ORM channels and lateral connection to inter-channel Lower sediments. These channel deposits consist of fining upward transitions from coarse gravel, to sand, to rhythmically bedded silt and clay interpreted to be deposited within a channel-esker-subaqueous fan complex. The Thorncliffe Formation contains a wide range of facies assemblages characterized by contrasting permeabilities. The deposits with highest permeability occur within up to 80 m thick gravel and sand sequences at the base of the Thorncliffe channel. The response of piezometric levels to hydraulic stress confirms longitudinal connection along the channel with muted lateral hydraulic response in sediments outside channels. The YSA is considered a semiconfined (leaky) strip aquifer with observed transmissivities between 1000 to 4500 m2/d, in contrast to regional aquifer transmissivities that are generally less than 500 m2/d. Thorncliffe channels are interpreted to be up to 20 km long and approximately 2 km wide. This presentation discusses the development and current state of the hydrogeological conceptual model related to Lower sediments within the study area, and how it can assist in future endeavours. It is anticipated that this refined conceptual model can inform groundwater exploration and development for aquifers to be utilized for water supply or geothermal energy. Also discussed will be efforts to disseminate hydrogeologic knowledge in a format useful to public works, planning and scientific practitioners (e.g. 'groundwater problem area' mapping). A key process in the evolution of the conceptual model is that it is continuously subject to refinement as new data and information becomes available.