Guillaume Meyzonnat

The potential of major ion chemistry to assess groundwater vulnerability of a regional aquifer in southern Quebec (Canada)

Groundwater vulnerability mapping provides useful but limited information for developing protection plans of the resource. Classical vulnerability ranking methods often do not take into account complex hydrostratigraphy and never consider groundwater flow dynamics. The objective of this work was to test the potential of major ion chemistry to assess regional-scale intrinsic groundwater vulnerability. Because it reflects water–sediment and water–rock interactions, the new vulnerability index reflects both infiltration processes and groundwater hydrodynamics. The method was applied on a regional fractured bedrock aquifer located in the Becancour region of southern Quebec (Canada). In this region, hydrogeochemistry shows that freshly recharged groundwater evolves from (Ca, Mg)–HCO3 and Ca–SO4 to Na–HCO3 type with gradually increasing confinement conditions in the fractured aquifer and tends to Na–Cl type locally by mixing with trapped marine pore-water. The new method identified recharge areas as those of the highest vulnerability and gradually decreasing vulnerability as confinement of the aquifer increased. It also highlights local discontinuities in confinement that differ from the regional pattern. Results showed a good correlation between groundwater vulnerability estimated with the new method and nitrate occurrence in groundwater. Eighty-two per cent of all samples presenting detectable nitrate concentrations were characterized by a Hydrogeochemical Vulnerability Index greater than 9 (maximum is 10). The ability of the new vulnerability method to identify areas vulnerable to detectable nitrate concentrations was much higher than that deriving from the DRASTIC method. This work confirms that major ions chemistry contains significant information about groundwater vulnerability and could be used to improve groundwater resource management.

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

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.

Examining the challenges of simulating surface water–groundwater interactions in a post-glacial environment

Although integrated models are increasingly used for water management purposes, detailed applications of these models under different conditions are necessary to guide their implementation. The objective of this study was to examine some of the challenges encountered when simulating surface water–groundwater interactions in a post-glacial geological environment. The integrated Mike SHE model was used to simulate transient-state heads and flows in the Raquette River watershed in the Vaudreuil-Soulanges region of southwestern Quebec (Canada) over a 2-year period. This application benefited from a detailed hydrogeological database recently developed for the region. Overall, flows, heads and groundwater inputs to the river were adequately simulated. A sensitivity analysis has shown that many hydrogeologic and surface flow parameters have an impact on both flow rates and heads, thus underlining the importance of using an integrated model to study watershed-scale water issues. Additional flow rate measurements to improve the quality of rating curves and continuous flow measurements in tributaries could improve model calibration. An explicit simulation of unsaturated zone infiltration processes, including soil flow, plant and evaporation processes, as well as the inclusion of the agricultural tile drainage system, could reduce simulation errors. Extending the model calibration over a longer period, including contrasting hydrological conditions, would make the model more robust in view of its use for water management under land use and climate change conditions. Nevertheless, this work demonstrated that, using data readily available for southern Québec aquifers, it is possible to build an integrated model that is representative of actual hydrological conditions. The maintenance and improvement of this model for long-term use is recommended.

High-Resolution Wellbore Temperature Logging Combined with a Borehole-Scale Heat Budget: Conceptual and Analytical Approaches to Characterize Hydraulically Active Fractures and Groundwater Origin

This work aims to provide an overview of the thermal processes that shape wellbore temperature profiles under static and dynamic conditions. Understanding of the respective influences of advection and conduction heat fluxes is improved through the use of a new heat budget at the borehole scale. Keeping in mind the thermal processes involved, a qualitative interpretation of the temperature profiles allows the occurrence, the position, and the origin of groundwater flowing into wellbores from hydraulically active fractures to be constrained. With the use of a heat budget developed at the borehole scale, temperature logging efficiency has been quantitatively enhanced and allows inflow temperatures to be calculated through the simultaneous use of a flowmeter. Under certain hydraulic or pumping conditions, both inflow intensities and associated temperatures can also be directly modelled from temperature data and the use of the heat budget. Theoretical and applied examples of the heat budget application are provided. Applied examples are shown using high-resolution temperature logging, spinner flow metering, and televiewing for three wells installed in fractured bedrock aquifers in the St-Lawrence Lowlands, Quebec, Canada. Through relatively rapid manipulations, thermal measurements in such cases can be used to detect the intervals or discrete positions of hydraulically active fractures in wellbores, as well as the existence of ambient flows with a high degree of sensitivity, even at very low flows. Heat budget calculations at the borehole scale during pumping indicate that heat advection fluxes rapidly dominate over heat conduction fluxes with the borehole wall. The full characterization of inflow intensities provides information about the distribution of hydraulic properties with depth. The full knowledge of inflow temperatures indicates horizons that are drained from within the aquifer, providing advantageous information on the depth from which groundwater originates during pumping.