Bank filtration: a Quebec-wide overview using isotopic and geochemical methods

Laurence Labelle

Bank filtration (BF), the placement of a groundwater well near surface water such as a lake or river, reduces the risk of overexploitation of groundwater and acts as a natural and inexpensive pretreatment of surface water. Because the ability of BF sites to mitigate contaminants is sensitive to changes (in surface water quality, redox conditions, and hydraulic conditions), it is critical to identify wells in BF situations and map their vulnerability to these changes. Since November 2019, as part of this project funded by the Ministère de l’Environnement et de la Lutte contre les changements climatiques (MELCC), water samples from nearly 50 municipal wells in the Laurentian and Nicolet regions are sampled monthly to weekly at raw water. Stable isotopes of the water molecule (δ¹⁸O-H₂O and δ²H-H₂O) and electrical conductivity (EC) were chosen as tracers to quantify surface water-groundwater interactions. A simple analytical model will be developed for each site that will quantify (i) surface to groundwater mixing ratios at the pumping well and (ii) surface water residence time in the banks on an annual basis. By combining these results with the hydro-geo-morphological context of each site, it will be possible to identify i) the intensity and variability of surface/groundwater connection types and ii) the influence of pumping sequences and meteorological effects. This project will propose a typology of these sites with respect to their sensitivity to anthropogenic and climatic changes, as well as present the advantages of geochemical and isotopic tools as methods of monitoring raw water quality for BF sites in Quebec.