THE IMPACT OF QUATERNARY SEDIMENTATION PROCESSES ON HYDROGEOLOGIC SYSTEMS IN THE MAURICIE REGION, QUEBEC, CANADA

A hydrogeological survey is currently being carried out in the Mauricie region, Quebec, Canada, a region that straddles the southern edge of the Canadian Shield as well as the St. Lawrence Lowlands and that includes the St-Narcisse Moraine, a prominent feature emplaced by the Laurentide Ice Sheet during the Younger Dryas cooling. The study is funded by a provincial program aimed at increasing groundwater knowledge at a regional scale. The groundwater assessment includes the collection and integration of existing reliable data, such as hydrogeological reports for groundwater supply, together with the acquisition new field data such as boreholes, 3C high-resolution seismic profiling (MiniVive and Landstreamer), resistivity logging, cone penetrometer testing and water well sampling. Data was integrated and interpreted in order to build a regional hydrostratigraphic 3D model. Understanding of the Quaternary sedimentary architecture allowed us to delineate the subsurface distribution of productive aquifer zones in the foothills and to better understand the main factors controlling the distribution of productive aquifers. These are: 1) the glacial marine setting of the Late Wisconsinan deglaciation, 2) the presence of rugged glaciated bedrock landcapes, 3) the large sediment input provided by subglacial meltwater corridors. These conditions led to the filling of local bedrock depressions by proglacial sand and gravel, which were covered by marine sediments, and then covered again by coarse sediments associated with the St-Narcisse readvance. That sedimentary sequence led to the confinement of the lower aquifer by the fine-grained marine sediments which are in turn covered by a water-table aquifer. This aquifer sequence, particularly the confined aquifer, is being pumped to supply potable water to down-gradient communities located along the Lac-St-Pierre area and the surrounding agricultural plain, where groundwater confined below Champlain Sea sediments are saline. The total retrieved water volume in 10 wells into those aquifers is estimated to be around 3.2 Mm³/year, for a population of 15 000 persons. The current study provides invaluable knowledge to guide future groundwater resource investigations as well as decision-making tools for regional authorities.