Integrated surface and subsurface hydrological modelling to support the assessment of the vulnerability of surface water supplies.

The protection of water resources is a challenging task and requires a detailed understanding of surface water and groundwater dynamics and interactions. Integrated groundwater and surface water models represent useful tools to assess how groundwater affects the quantity and quality of surface water, which is often a source of drinking water. It is the case for the City of Quebec, Canada, where surface water is the only source of drinking water and where water managers must assess its vulnerability to contamination and depletion. This work focuses on the Nelson River catchment (70 km²), located within the larger catchment of the main drinking water source in Quebec City. The objective is to quantify the links between groundwater and surface water with the 3D integrated hydrological model HydroGeoSphere and simulate coupled surface/subsurface water flow and contaminant transfer. The Nelson catchment model has been calibrated to reproduce observed surface discharges and water table level measurements. Coupled surface water and groundwater flow is then simulated over multiple years using daily meteorological data. Output variables such as distributed infiltration, preferential flow pathways, inter-seasonal changes of surface water volumes, unsaturated and saturated groundwater volumes, are analysed to assess the link between surface water and groundwater. Since this urban area is undergoing growing urbanisation, future scenarios of urban development are also simulated to evaluate the impact of soil sealing on surface/groundwater interactions. The understanding of surface/subsurface interactions in this particular context aims at assessing the vulnerability of the surface drinking water source.