HGS RESEARCH HIGHLIGHT – Source Water Protection in Quebec City: Using an integrated 3D hydrological model to investigate surface water-groundwater interactions
Frot, B., Gatel, L., Tremblay, Y., Delottier, H., and Therrien, R.: Source Water Protection in Quebec City: Using an integrated 3D hydrological model to investigate surface water - groundwater interactions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1312, https://doi.org/10.5194/egusphere-egu25-1312.
“The river signal was decomposed at the Chateau d’Eau water intake and Noire River hydrometric stations to quantify and evaluate the spatiotemporal variability of each component.”
Frot, B., Gatel, L., Tremblay, Y., Delottier, H., and Therrien, R.: Source Water Protection in Quebec City: Using an integrated 3D hydrological model to investigate surface water - groundwater interactions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1312, https://doi.org/10.5194/egusphere-egu25-1312.
CLICK HERE TO VIEW THE FULL POSTER.
The research, presented as a poster by Benjamin Frot at EGU 2025, explores the use of HydroGeoSphere (HGS) to investigate surface water–groundwater interactions in the Saint-Charles River watershed, which supplies drinking water to Quebec City. With a focus on source water protection, the study addresses the challenges posed by increasing urbanization, contamination from septic systems and road salts, and reduced water availability during low-flow periods. The work is part of a larger project aimed at evaluating the vulnerability of Quebec City's main surface water intake.
To accomplish this, the team developed an integrated 3D HGS model covering the 340 km² watershed, simulating coupled surface-subsurface flow and solute transport. The model integrates data on land use, geology, soil properties, and climate, and it is calibrated using stream discharge and groundwater level time series. One of the key findings is that baseflow—representing the groundwater contribution to streamflow— accounts for 74% of the annual discharge and up to 95% during winter, with considerable interannual variability. In dry years like 2021, this variability becomes even more pronounced.
The study also employs particle tracking within HGS to calculate groundwater travel times, revealing an average residence time of 17 years. To further analyze the mixing dynamics of groundwater and surface water, the researchers applied the Hydraulic Mixing-Cell (HMC) method at key monitoring points such as the Château d’Eau water intake and the Noire River hydrometric station. This decomposition of the river signal illustrated seasonal patterns in water origin and highlighted a greater contribution of deeper groundwater to certain sub-catchments, particularly during winter months.
By combining physically based modeling with novel techniques like HMC and particle tracking, the research provides a quantitative portrait of water resources across spatial and temporal scales. It also offers a decision-support tool for land-use planning and drinking water vulnerability assessment. Planned next steps include incorporating short-term hydrologic events (e.g., floods), transient climate effects based on OURANOS projections, and future urbanization scenarios from local stakeholders.
This work highlights the critical importance of considering both surface and subsurface processes in watershed management and offers a practical approach to protecting drinking water sources in a changing climate.
If you’re interested in learning more about the context of this project, read this great ArcGIS story map that the authors published here.
Abstract:
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.