THE NIAGARA PENINSULA PROJECT: AN INTEGRATED APPROACH TO 3-D MAPPING FOR GROUNDWATER CHARACTERIZATION

The Niagara Peninsula project, encompassing 5000 km² between lakes Ontario and Erie, is a collaboration between the Ontario Geological Survey, the Geological Survey of Canada, and the Niagara Peninsula Conservation Authority. The project is one in a series of regional scale 3-D sediment mapping projects designed to provide geoscience information for the identification, protection and sustainable use of the provincial groundwater resource.

The area is characterised by a relatively thin sediment cover; however, two major escarpments and an extensive network of buried bedrock valleys have confounded previous groundwater flow mapping efforts. Results from a ground gravity survey suggest valley morphologies vary from small and narrow (extending south from the Onondaga Escarpment) to broad and shallow (Erigan and Chippawa valleys) to deeply incised (Dundas and St. David’s valleys). Two shallow, high-resolution seismic reflection lines have been run to determine whether multiple thalwegs exist within the Erigan valley, and to define the geometry of lower drift gravel beds. A third line was acquired to define the extent and shape of the Chippawa valley. These surveys guided the search for drilling and monitoring well targets.

To date, 77 continuously cored boreholes have been drilled, 28 of which have been converted into monitoring wells. A lower drift package, representing the oldest Quaternary sediments in the area, consists of stony silt to sand till with local deposits of gravel. As ice retreated from the Late Wisconsinan maximum, glacial lakes Whittlesey and Warren inundated the peninsula, depositing a lower glaciolacustrine unit. A late glacial lobate ice advance out of the Lake Ontario basin deposited clay-rich Halton drift and an ice contact–delta complex, important for groundwater recharge, in a re-entrant along the ice margin. Following ice retreat north of the Niagara Escarpment, glaciolacustrine sedimentation once again dominated the region.

Groundwater elevations in the upper bedrock aquifer show an overall decreasing trend from Lake Erie to Lake Ontario along the Erigan channel. Downhole geophysical logs were collected in 7 wells to characterise lithological units based on variations in their physical and chemical properties, and to verify the conversion of the seismic sections into depth sections.