Benefits and limitations of using isotope-derived groundwater travel times and major ion chemistry to validate a regional groundwater flow model: example from the Centre-du-Québec region, Canada

Abstract

Understanding groundwater dynamics at the regional scale (> 100 km) is essential to the development of sustainable water management regulations. Groundwater flow models are increasingly used to support these strategies. However, in order to be reliable, these models need to be calibrated and validated. The objective of this work is to evaluate the benefits and the limitations of using isotope-derived groundwater travel times and major ion chemistry to validate a regional-scale groundwater flow model in the humid continental climate of southern Québec (Canada). A three-dimensional regional-scale steady-state groundwater model was created using MODFLOW for the fractured bedrock aquifer of the Centre-du-Québec region (Québec, Canada), using data acquired during recent aquifer characterization projects. The model covers an area of 7452 km2, from the unconfined Appalachian Mountains to the confined St. Lawrence Platform. Groundwater travel times were simulated for 211 wells using particle tracking. The groundwater flow model was calibrated using 11,775 regionally distributed heads and 15 baseflow values. The model was validated using 23 3H/3He residence time (3 to 60 years), 17 14C residence time (226 to 22,600 years), and the major ion compositions from 211 wells. Results indicate that the model is able to satisfactorily simulate ³H/³He isotopic residence time, while 14C isotopic residence times are generally underestimated. These results suggest substantial mixing between groundwater recharged during the last deglaciation and recently recharged water. Regional groundwater flow is limited or absent, and most of the recharge discharges to the river network as baseflow. The analysis of travel times indicates a statistically distinct mean travel time for the different groundwater types. Median travel time is 68 years for recently recharged groundwater (Ca-HCO3), 274 years for semi-confined groundwater (Na-HCO3), and 738 years for confined groundwater (Na-Cl). This confirms that groundwater chemistry is a broad indicator of groundwater travel time.