A GEOLOGICAL INVESTIGATION OF THE DUNDAS BURIED VALLEY, SOUTHERN ONTARIO, CANADA

A partnership agreement between the Grand River Conservation Authority and the Ontario Geological Survey to study the Dundas buried valley (DBV) in southern Ontario was initiated in 2007. The study was undertaken to refine current understanding of the origin, location and geometry, sediment infill and hydrogeology of the valley. This information will assist in better understanding groundwater flowpaths at a watershed scale and in assessing the feature for its potential to host significant aquifers that could augment current municipal water supplies.

The DBV has been traced from the west end of Lake Ontario, where geotechnical records on the Burlington bar suggest valley incision to depths well below sea level (60 m bsl) westward towards a significant re-entrant along the Niagara Escarpment at the town of Dundas where overburden depths exceeding 200 m have been documented. Beyond this point, the valley is concealed beneath variable thicknesses of Quaternary sediments that prevent a clear depiction of the valley thalweg location. Previous interpretations suggest the DBV extends northwestward through the Regional Municipality of Waterloo toward Lake Huron; a distance of over 185 km.

To better characterize the DBV, a multi-phased investigation consisting of: 1) data compilation and preliminary conceptualization; 2) geophysical surveying; 3) drilling and hydrogeological testing and 4) development of a conceptual 3-D geologic model was implemented. Drill targets were chosen following an assessment of existing records of both high and low quality drillholes coupled with the results of a regional, ground-based gravity survey. Ten continuously-cored boreholes with single level monitors and several multi-level, mud rotary holes in the vicinity of 3 of these borings were constructed to better understand the geologic framework and to further characterize the aquifers.

A highly variable and complex geologic record characterizes the valley fill with significant highly conductive aquifers present in selected segments of the valley. Despite being concealed beneath tens of meters of impermeable sediments, several of the aquifers show influence of recent recharge suggesting a complex groundwater flow path. Water quality issues also exist in a number of the aquifers presenting challenges for municipal water supply.