The Yonge Street Aquifer (YSA) in the Greater Toronto Area of south-central Ontario is a prolific municipal supply aquifer. It has been considered to be channelized sand and gravel linked to a bedrock valley. Despite considerable work, the fundamental conceptual model for the YSA is not well developed and documented. Based on high-quality data, a revised conceptual model of the aquifer is presented. Seismic profiles define the geometry of the regional stratigraphy with four distinct units: bedrock, Lower sediments, Newmarket Till, and Oak Ridges Moraine (ORM) sediment. Seismic data reveal two generations of roughly north–south channels: older sub-Newmarket Till channels within Lower sediments (termed Thorncliffe channel) and 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. Thorncliffe channel deposits consist of fining-upward transitions from coarse gravel, to sand, to rhythmically bedded mud interpreted to be deposited within a channel – esker – subaqueous fan complex. Upper Thorncliffe channel mud facies and overlying Newmarket Till provide a capping aquitard. The YSA conceptual model benefits from a strong understanding of facies changes in the Thorncliffe Formation. The deposits with highest permeability occur within up to 80 m thick gravel and sand sequences at the base of the Thorncliffe channel, with transmissivity ranging from 1500 to 4500 m2/day. Groundwater level response to municipal pumping confirms connection along the channel with muted hydraulic response laterally. Thorncliffe channels are interpreted to be up to 20 km long and approximately 2 km wide.

The Great Lakes Basin (GLB) holds vast reserves of groundwater, the great majority of which eventually drains to the lakes. Urban growth significantly affects both the quality and quantity of this groundwater and thereby represents a potential threat to the long-term viability of the Great Lakes hydrologic system. Urban areas import, manufacture, store, transport, and utilise large volumes of chemicals, a proportion of which inevitably finds its way to the shallow sub-surface. In many cases, potentially polluting chemicals are applied directly to urban surfaces (e.g. as road salts, fertilizers and pesticides), are stored in the subsurface (e.g. gasoline tanks) or are released to the subsurface (e.g. septic systems). Because most of the basin's larger urban areas rely almost exclusively on lake-based supplies, very little attention is given to the accumulation of contaminants in shallow urban groundwaters and the serious risks they pose. Assessment of the problem is complicated by the widespread use of urban fill and a complex network of drains, pipes and tunnels that create “urban karst”, a shallow artificial aquifer, unique to urban settings, that exerts a major, yet often unpredictable influence on groundwater flow and contaminant transport. Management of ground water pollution, and its impact on the receiving Great Lakes, will require rigorous audits of all urban sources of contamination together with the development and calibration of groundwater flow and transport models that will enable the fate of urban pollutants to be reliably predicted even when groundwater is not used for supply.

The interest surrounding groundwater protection in Southern Ontario has grown considerably since the Walkerton tragedy of May 2000. Since that time, a number of technical studies have been undertaken by the Province to support the preparation of Source Water Protection plans. Underlying all sound water resource management initiatives in Ontario is the need for a renewed focus on the fundamentals, specifically in this case a focus on groundwater knowledge and its management. Using the Oak Ridges Moraine Hydrogeology Program as a unique example of a groundwater " knowledge management" system, this paper presents some unexplored opportunities that merit further consideration in the application of a " knowledge management" philosophy within Ontario's overall water management framework. Within the Program's study area, the linkage and integration of the Water Well Information System with other borehole datasets and consulting reports, as well as with water use (e. g. municipal pumping) and water quality databases, has created perhaps the most comprehensive, actively managed groundwater " knowledge management" system in Canada. Ongoing Source Water Protection and other work undertaken through consultants, including data and geological/ hydrogeological interpretations, is being re-incorporated, where sound and appropriate, into the program's existing groundwater knowledge infrastructure. The program's groundwater " knowledge management" system has been developed with a long-term (i. e. multi-decade) water management time frame in mind and is made accessible to geoscientists undertaking work in the area.

Recent work in southern Ontario, Canada, demonstrates anomalously high vertical groundwater flow velocities (>1 m/year) through a thick (as much as 60 m), sandy silt till aquitard (Northern till), previously assumed to be of very low permeability (hydraulic conductivity <10–10 m/s). Rapid recharge is attributed to the presence of fractures and sedimentary heterogeneities within the till, but the field-scale flow regime is poorly understood. This study identifies the nature of physical groundwater pathways through the till and provides estimates of the associated groundwater fluxes. The aquitard groundwater flow system is characterized by integrating details of the outcrop and subsurface sedimentary characteristics of the till with field-based hydrogeologic investigation and numerical modeling. Outcrop and subsurface data identify a composite internal aquitard stratigraphy consisting of tabular till beds (till elements) separated by laterally continuous sheet-like sands and gravels (interbeds) and boulder pavements. Individual till elements contain sedimentary heterogeneities, including discontinuous sand and gravel lenses, vertical sand dikes, and zones of horizontal and vertical fractures.

Protection of ground-water resources is an emerging theme in many urban areas. In south-central Ontario, the Oak Ridges Moraine (similar to 1400 km(2)) is a prominent glacial moraine complex on the rapidly urbanizing northern margin of the Greater Toronto Area and constitutes a major regional aquifer system. Given the pre-eminent economic importance of the region, the ground-water resource can be argued to have provincial significance, and there is much current debate regarding the impact of urbanization and other anthropogenic activities on ground-water quality and supply. Particular issues are the degree of acceptable future development and the definition of the most environmentally and hydrogeologically sensitive areas. This paper reviews what is currently understood of the geology and hydrogeology of the Oak Ridges Moraine, and provides an overview of quantitative modelling studies.

In glaciated regions of North America, till is often regarded as the geologic medium of choice when selecting appropriate sites for landfill operations. This is especially true in southern Ontario, where intensive studies are presently being conducted to site municipal landfills to serve the Greater Toronto Area. Throughout this region many potential sites are situated on the Halton Till, a widespread surficial unit of Late Wisconsinan age. This study focuses on the hydraulic characteristics of the Halton Till utilizing two data sets from interim landfill site searches completed in 1990. Small scale methods of estimating hydraulic conductivity within the till such as rising/falling head tests and laboratory triaxial testing typically yield values on the order of 10[sup [minus]7] and 10[sup [minus]9] cm/s, respectively. In contrast, hydrochemical evidence, including the presence of elevated levels of Na, SO[sub 4], Cl and [sup 3]H within the till pore waters to depths of up to 40 meters suggests that vertical groundwater velocities may be in the order of 1.0 to 1.5 m/year. These data imply that contaminants such as farm fertilizers and road salt are migrating through the till at higher velocities than expected and that the bulk hydraulic conductivity may be 1 tomore » 3 orders of magnitude higher than values estimated from field and laboratory methods. Such findings can have serious implications for impact assessment studies required for landfill siting investigations.« less