Multi-technique approach for estimating groundwater transit time through the saturated zone of an unconfined granular aquifer in Quebec, Canada

Agricultural activities can generate contaminants that enter underlying granular aquifers and become transported within the groundwater to adjacent streams. This paper reports on estimation of the transit time of groundwater through a saturated granular unconfined aquifer in an agricultural region of Saguenay-Lac-Saint-Jean, Quebec (Canada). A multi-technique approach is applied, integrating analytical, hydrogeochemical, and numerical methods—to determine groundwater flow from a recharge (wetland) to discharge zone (groundwater seep). Fieldwork observations, including borehole drilling, soil/groundwater sampling, and piezometers, were combined with laboratory measurements of soil hydrogeological properties and stable (δ¹⁸O_H2O and δ²H_H2O)/radioactive (³H) isotopes in the collected groundwater. The Dupuit–Forchheimer-based analytical method used here estimated a groundwater transit time of 7.75 years, whereas the hydrogeochemical-based and three-dimensional FEFLOW numerical method produced estimates of 7.34 and 7.27 years, respectively. The similarity of the three estimates highlights the robustness of the approach, which integrates field data to produce accurate assessments of groundwater transit time. This multi-technique approach will help in the sustainable management of groundwater resources and for preparing effective environmental plans for agricultural practices in areas underlain by aquifers.