Impact of temperature gradients on groundwater flow patterns at bank filtration sites in seasonal climate

Janie Masse-Dufresne

Bank filtration (BSF) is a cost-effective technology for drinking water production and has long been used in Central Europe along major rivers. Recently, this method has gained popularity worldwide and several systems are currently in operation in various climates and on sites adjacent to lakes and reservoirs. In seasonal climates, lakes typically develop thermal stratification during the summer, with temperatures >20°C at the surface and <5°C at depth, and have their water column homogenize in the fall before developing reverse thermal stratification during the winter. Although several studies have described the impact of temperature on contaminant attenuation and trace metal fate during BSF, the impact of temperature gradients on groundwater flow patterns remains poorly understood.

This study aims to quantify the spatio-temporal dynamics of water residence times in BSF. To do so, a 3D thermo-hydrogeological numerical model was developed to represent the groundwater flow patterns at a BSF site in Quebec, Canada. The model was calibrated over a one-year period. The model suggests that lake stratification controls temperature gradients in the aquifer between September and March. These gradients are quickly attenuated by the spring freshet. Because hydraulic conductivity is temperature dependent, large spatio-temporal temperature gradients lead to the development of complex flow patterns and annual changes in water residence time. In the context of climate change, it is likely that lakes in northern climates will experience prolonged thermal stratification, which will affect the operation of BSF systems.