Simulation of groundwater age evolution during the Wisconsinian glaciation over the Canadian landscape

The simulation of groundwater age (residence time) is used to study the impact of the Wisconsinian glaciation on the Canadian continental groundwater flow system. Key processes related to coupled groundwater flow and glaciation modeling are included in the model such as density-dependent flow, hydromechanical loading, subglacial infiltration, glacial isostasy, and permafrost development. It is found that mean groundwater ages span over a large range in values, between zero and 42 Myr; exceedingly old groundwater is found at large depths where there is little groundwater flow because of low permeabilities and because of the presence of very dense brines. During the glacial cycle, old, deep groundwater below the ice sheet mixes with the young subglacial meltwater that infiltrates into the subsurface; the water displacement due to subglacial recharge reaches depths up to 3 km. The depth of penetration of the meltwater is, however, strongly dependent on the permeability of the subsurface rocks, the presence of dense brines and the presence or absence on deep fractures or conductive faults. At the end of the simulation period, it was found that the mean groundwater age in regions affected by the ice sheet advance and retreat is younger than it was at the last interglacial period. This is also true for frozen groundwater in the permafrost area and suggests that significant parts of this water is of glacial origin. Finally, the simulation of groundwater age offers an alternative and pragmatic framework to understand groundwater flow during the Pleistocene and for paleo-hydrogeological studies because it records the history of the groundwater flow paths.