GLACIATIONS AND GROUNDWATER FLOW SYSTEMS: INSIGHTS FROM A CONTINENTAL SCALE MODEL

Pleistocene glaciations had a profound influence on the dynamics of groundwater flow systems. Their impact has not been limited to glaciated regions; they also affected parts of the world that were not glaciated because of the related global perturbations in climatic conditions such as changes in precipitation and eustatic sea levels. As a consequence, groundwater recharge patterns were globally modified and affected groundwater flow dynamics at basinal to continental scales in a wide range of geological environments such as coastal aquifers, intercratonic sedimentary basins and crystalline rocks. Several lines of evidence support this paradigm such as groundwater geochemical imprints, regional disequilibrium in subsurface pore pressures and sedimentary structures.

Here, we present results from a numerical model applied to the Wisconsinian glaciation over the Canadian landscape in order to evaluate the possible impacts of the last glacial cycle on Canadian continental groundwater flow systems and better understand the complex interaction of the various processes involved. The numerical model includes key geological and hydraulic processes involved during glacial cycles such as hydro-mechanical deformation due to glacial loading, glacial isostasy, eustatic sea levels variations, proglacial drainage evolution, subglacial recharge and permafrost development.

Our conclusions suggest that (1) subglacial recharge occurs mostly during ice sheet transgression and that groundwater discharge to the surface occurs mostly during ice sheet regression. This implies that the age of most of the glacial meltwater found into glaciated hard rock aquifers should be equals or older than last glacial maximum; (2) Surface permafrost, due to its rapid evolution, has the ability to cap over-pressurized groundwater generated by subglacial recharge; (3) Frozen groundwater within actual permafrost is composed of subglacial meltwater infiltrated during the advance of the ice sheet and the geochemical signature of the permafrost should point toward a glacial origin; (4) Groundwater flow dynamics are not in equilibrium with actual boundary conditions and the flow systems are continuing to evolve from the last glaciation.