Three-dimensional hydrostratigraphical modelling of the regional aquifer system of the St. Maurice Delta Complex (St. Lawrence Lowlands, Canada)

Abstract

In the central part of the St. Lawrence Lowlands (Mauricie, Québec), Late Quaternary deglacial events led to the formation of a series of complex granular aquifers, such as those in the (1) Saint-Narcisse morainic complex, (2) paleodelta formed by the Saint-Maurice River, and (3) sandy littoral terraces left during marine regression. The aquifers are an important supply of potable water for most municipalities in the region, including the City of Trois-Rivières, which is a mid-size city where groundwater accounts for 46% of the water supply. The main objectives of this study were to build a three-dimensional (3D) model of the Quaternary deposits to define the main hydrogeological contexts of the Mauricie region and to characterize the regional aquifers. The compilation of existing hydrogeological data led to the selection of 5386 well logs that contained stratigraphic information of variable quality, ranging from only surficial sediment thickness to descriptions of fully cored boreholes. To supplement the existing data, fieldwork was undertaken in areas where few data were available, including 63 km of high-resolution seismic reflection surveys and 34 new boreholes. The final 3D model consists of six layers, from the bedrock surface to the upper littoral and deltaic sands. The total thickness of the deposits ranges from zero, on bedrock outcrops, to 150 m beneath central Trois-Rivières. Taking into account the thickness of the saturated layer and the porosity of the sand and gravel, the upper unconfined aquifer contains an estimated 364 million m3 of water. The 3D model helped refine the understanding of regional aquifers and was used to identify unexploited aquifers, notably around the Saint-Narcisse morainic complex and along the St. Cuthbert Fault. The model clarified the regional stratigraphic architecture, especially topography of the bedrock surface, the lateral extent of Late Quaternary sands and development of Holocene post-glacial sediment sequences.