Groundwater modeling of three hydrogeological contexts on St-Lawrence Lowlands and Appalachian foothills (southern Quebec, Canada)

English Abstract:

Regional scale groundwater flow dynamics are influenced by local flow conditions which are rarely characterized thoroughly. In glaciated regions, complex surface geology conditions often create highly variable flow conditions within small distances. The objective of this study is to understand local groundwater flow in different hydrogeological contexts of a regional scale aquifer. The study area is located on the south shore of St. Lawrence River between Montreal and Quebec (4500 km²). In this region, groundwater is the drinking water source for half of the population and is used for agricultural activities. Three different environments are specifically studied: 1) a U-shape valley upstream of watershed filled with glacial sand, till and silt on a fractured rock aquifer, 2) a partially unconfined granular sandy aquifer, linked with the bedrock aquifer with lenses of fine sediments, and 3) a confined/semi-confined bedrock aquifer downstream of the region with till, silt and silty sand. Each context has been simulated in a 2D cross-section having 3 to 8 km in length. The models were developed in Modflow with 1 m X 10 m X 10 m cells over 50 m. The steady-state models were calibrated based on groundwater levels, recharge and field-measured hydraulic conductivities. The transient-state models were further calibrated and run on a daily time step over one year. The three models reproduce well the seasonal variations of groundwater levels. Groundwater recharge is 10 times more important for section one compare to section three. The results show that groundwater dynamics are determined by the hydraulic properties of the different materials on each transect, as well as by the topography variations. The transect located in the granular unconfined sandy aquifer reacts five times faster to hydraulic stresses than the other two transects, and is more sensitive to recharge decreases. The model complexities help to understand discharge to rivers, to detect vulnerable areas and to locate precisely small recharge zones that are imperceptible at the regional scale.