The Oak Ridges Moraine hydrogeology program has built upon an original geological interpretation undertaken during the 1990s by the Geological Survey of Canada (GSC). Since 2001 five key phases of renewed geological interpretation have provided updated digital geological layers across different parts of the study area. In all cases, geological layers were developed using visual interpretation of well records on three dimensional dynamic cross-sections. The five interpretive phases are linked with specific YPDT-CAMC led projects all having contributed to the current geological interpretation and understanding of the Quaternary sediments in the program study area:
• 2001 to 2006 - Regional Model - provided a five layer (strata) geologic model in the area extending from below the Niagara Escarpment in the west to the Trent River Watershed in the east. The geological interpretation was a refinement of the GSC's original geological interpretation.
• 2003 to 2006 - Core Model - provided an eight layer geological model. The "Lower Sediments" found in the original GSC interpretation were divided into three layers: (Thorncliffe Formation (aquifer), Sunnybrook drift/diamict (aquitard) and Scarborough Formation (aquifer). This conceptual model stretched between Lake Ontario in the south and Lake Simcoe in the north, and in an east-west direction, extended from the top of the Humber Watershed in the west to the Duffins Creek watershed in the east.
• 2005 - 2006 - Core Model Extension - the layers developed for the 2003 to 2006 Core Model were extended: i) westward above the escarpment to the west boundary of the Credit River Watershed; and ii) eastward to the east side of the Ganaraska Conservation Authority boundary; and iii) northwards to encompass all of Durham and Peel Regions. In addition the bedrock was subdivided into several Paleozoic Formations.
• 2006 to 2008 - Durham Model - provided a ten layer model for the Regional Municipality of Durham. The Newmarket Till was divided into the Upper Newmarket Till (aquitard), the Inter-Newmarket Sediments (aquifer), and the Lower Newmarket Till (aquitard).
• 2008 - 2010 - East Model - provided a 10 layer geological model (similar to the above) that extended from the York-Durham boundary through to the eastern and northern borders of the Trent Coalition Source Water Protection area.
In addition to these main work phases, geological interpretation has also been continually evolving through more geographically focused work for example, refinements in the city of Toronto along the new subway routes and in the vicinity of recent combined sewer projects. Consultant-led projects commissioned through Source Water Protection initiatives have given rise to differing geological interpretations that must now be reviewed and integrated into an "authoritative" geological interpretation. Program staff are currently exploring the most effective means of incorporating these interpretations back into an updated geological model that will be used moving forward.
For all of the above phases of geological interpretation key aspects of the work included:
• Expert" interpretation: Three-dimensional digital geological contact lines are used to constrain the interpolation of layers between boreholes. Geological contact lines were also used to define layer pinch-outs, subglacial erosion on top of and through confining aquitards (e.g. tunnel channels),and to reflect conditions where a well drilled into a layer provides evidence that the bottom of the layer exists at some depth below the well bottom ("push-down").
• Data integration: The geological interpretation illustrates the importance of integrating all data types in the hydrostratigraphic interpretation process. Effective database querying allows for the identification of complex patterns and correlations between the lithology and other hydrogeologic indicators (e.g., well screen placement) helping to at least partly overcome data quality deficiencies in driller's logs.
A sample north-south cross section through the Oak Ridges Moraine