Impact of spatial and temporal variations in groundwater on water quality at a bank filtration site

Coralie Pontoreau

The term bank filtration is used when pumping wells collect both groundwater and surface water. These systems have strong piezometric dynamics and are subject to greater seasonal chemical variations than conventional groundwater pumping but more moderate than surface water supply systems (which is their strength). Many studies have demonstrated the influence of surface water variation on the quality of pumped water and the role of dilution on contaminant attenuation. However, depending on the proportions of mixtures present in the wells, the variations observed in the groundwater can also have an impact on the quality of the pumped water. Our objective is to highlight this influence using a study site where waters (groundwater and surface water) with different physicochemical, geochemical and isotopic characteristics converge and mix at the pumping well. The study site is located next to the Lachute Fault which separates the St. Lawrence Lowlands and the Canadian Shield. Analysis of major ions in samples revealed that geology (i.e., rock type) significantly influences the geochemical facies of groundwater. First, we will investigate the presence of different chemical facies within the same lithostratigraphic unit (sandstone and dolomite interstratification of the Theresa Formation and alluvium) using physicochemical and isotopic tracers. In a second step, we will investigate the influence of pumping rates on the contribution to the mixing of these different types of water causing a temporal variation of the well chemistry.