IMPACT OF THE WISCONSINIAN GLACIATION ON CANADIAN CONTINENTAL GROUNDWATER FLOW

During the last glacial period, the Canadian landscape was almost entirely covered with ice. The Laurentide ice sheet reached a thickness of about 4 km and the force exerted by its weight on the Earth's crust was sufficient to cause a depression of the surface of as much as 1 km. These dramatic conditions are suspected to have had a large impact on the groundwater flow system over the entire continent. An understanding of the evolution of groundwater flow systems during glacial periods is relevant to a number of modern day problems, such as the long-term stability of a repository located at depth for the safe long-term storage of high-level spent nuclear fuel, and is pertinent to unravelling a number of scientific issues such as quantity and distribution of recharge waters that infiltrated during the late Pleistocene.

A transient, three-dimensional groundwater flow model driven by a thermomechanical ice-sheet model of the last glacial cycle [Tarasov and Peltier, 2004] was constructed to explore the impact of the advance and retreat of the ice sheet over the Canadian landscape during the past 120 kyr. The asynchronously coupled ice-sheet model provides the transient boundary conditions that include the spatio-temporal distribution of the glacial ice, the elevation of the surface topography, subglacial meltwater rates, permafrost thicknesses, temporal changes in sea level along the coastal margins as well as the evolving surface water drainage patterns. The advective-dispersive transport of dense brines that exist at depth over much of the land mass is included in the groundwater model formulation. Simulation results indicate that hydraulic heads at depth below the ice sheet increased by several hundred meters and that groundwater circulation patterns were dramatically different from those observed today. It is also shown that much of the infiltration of subglacial meltwater occurred during ice sheet progression and that during ice sheet regression, groundwater mainly exfiltrated to the land surface, both in the subglacial and periglacial environments. It is estimated that on the order of 5% to 10% of the meltwater infiltrated into the subsurface on average over the last 120 kyr, with values peaking as high as 20% of the meltwater during periods of ice sheet progression.