Source Water Protection in Quebec City: Using an integrated 3D hydrological model to investigate surface water - groundwater interactions

In the province of Quebec, Canada, the role of groundwater and its contribution to baseflow are rarely included to assess the vulnerability of surface water sources. However, in the case of Quebec City (560,000 inhabitants), stakeholders prefer that an integrated surface water and groundwater analysis be carried out to meet the highest standards of sustainable management. That approach goes beyond legislation, which does not require a fully integrated study.

A research project has been initiated to develop a set of stakeholder-oriented tools to assess both quantitative and qualitative vulnerability of the city's drinking water sources. The project focuses on the 350 km² catchment of the city’s main drinking water intake, which is in the Saint-Charles River. Due to intensive low flow periods, stakeholders are currently facing quantitative problems, with up to 95% of the river's flow being pumped. It is therefore crucial to characterise the water cycle in the area, including the identification of the main hydrological processes and the estimation of transient water availability. This requires a better understanding of the interactions between surface water and groundwater.

For that purpose, and to assist stakeholders, we developed a 3D integrated surface and subsurface flow model for the catchment with the HydroGeoSphere platform. The model is calibrated to observed times series of water table elevations and stream discharges from a network of monitoring wells and steam gauging stations. We then assess seasonal variations in water balance, resurgence and infiltration rates. Using a hydraulic mixing-cell postprocessing tool, we determine the different fractions of each streamflow component. This highlights the predominance of groundwater at the surface water intake, in agreement with isotopic analyses.

Finally, we also simulate the spatiotemporal vulnerability of the water intake by integrating climate change and urban development scenarios. Our study demonstrates that integrated surface and subsurface hydrological models are valuable tools to assist in designing water source protection plans, paving the way to new resource management policies.