The assessment of groundwater quality in shallow aquifers is of high societal relevance given that large populations depend directly on these water resources. The purpose of this study was to establish links between groundwater quality, groundwater residence times, and regional geology in the St. Lawrence Lowlands fractured bedrock aquifer. The study focuses on a 4500 km 2 watershed located in the St. Lawrence Lowlands of the province of Quebec in eastern Canada. A total of 150 wells were sampled for major, minor, and trace ions. Tritium (3 H) and its daughter element, 3 He, as well as radiocarbon activity (A 14 C) were measured in a subset of wells to estimate groundwater residence times. Results show that groundwater evolves from a Ca-HCO 3 water type in recharge zones (i.e., the Appalachian piedmont) to a Na-HCO 3 water type downgradient, toward the St. Lawrence River. Locally, barium (Ba), fluoride (F), iron (Fe), and manganese (Mn) concentrations reach 90, 2, 18, and 5.9 mg/L respectively, all exceeding their respective Canadian drinking water limits of 1, 1.5, 0.3, and 0.05 mg/L. Release of these elements into groundwater is mainly controlled by the groundwater redox state and pH conditions, as well as by the geology and the duration of rock-water interactions. This evolution is accompanied by increasing 3 H/ 3 He ages, from 4.78±0.44 years upgradient to more than 60 years downgradient. Discrepancies between calculated 3 H/ 3 He and 14 C water ages (the latter ranging from 280 ± 56 to 17,050 ± 3410 years) suggest mixing between modern water and paleogroundwater infiltrated through subglacial recharge when the Laurentide Ice Sheet covered the study area, and during the following deglaciation period. A linear relationship between 3 H activity and corrected 14 C versus Mg/Ca and Ba support a direct link between water residence time and the chemical evolution of these waters. The Ba, F, Fe, and Mn concentrations in
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 3D 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 km 2 , 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 3 H/ 3 He residence time (3 to 60 years), 17 14 C 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 14 C 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. Résumé La compréhension de la dynamique régionale (> 100 km) de l'eau souterraine est essentielle au développement d'une règlementation orientée vers le développement durable de cette ressource. Les modèles d'écoulement de l'eau souterraine sont de plus en plus utilisés pour supporter ces stratégies. Par contre, pour être utilisés à des fins de règlementation, ces modèles doivent être calés et validés. L'objectif de ce travail est d'évaluer les avantages et les limites de l'utilisation de l'âge isotopique de l'eau souterraine et de la géochimie des ions majeurs pour valider un modèle régional de l'écoulement de l'eau souterraine dans le climat continental humide du sud du Québec (Canada). En utilisant les données acquises dans le cadre de projets de caractérisation hydrogéologiques récents, un modèle 3D régional en régime permanent a été construit avec MODLOW afin de représenter l'aquifère fracturé de la région du Centre-du-Québec (Québec, Canada). Le modèle couvre une superficie de 7...
There is growing concern worldwide about the exposure of groundwater resources to pharmaceutically active compounds (PhACs) and agricultural contaminants, such as pesticides, nitrate, and Escherichia coli. For regions with a low population density and an abundance of water, regional contamination assessments are not carried out systematically due to the typically low concentrations and high costs of analyses. The objectives of this study were to evaluate regional-scale contaminant distributions in untreated groundwater in a rural region of Quebec (Canada). The geological and hydrogeological settings of this region are typical of post-glacial regions around the world, where groundwater flow can be complex due to heterogeneous geological conditions. A new spatially distributed Anthropogenic Footprint Index (AFI), based on land use data, was developed to assess surface pollution risks. The Hydrogeochemical Vulnerability Index (HVI) was computed to estimate aquifer vulnerability. Nine wells had detectable concentrations of one to four of the 13 tested PhACs, with a maximum concentration of 116ng·L for benzafibrate. A total of 34 of the 47 tested pesticides were detected in concentrations equal to or greater than the detection limit, with a maximum total pesticide concentration of 692ng·L. Nitrate concentrations exceeded 1mg·L N-NO in 15.3% of the wells, and the Canadian drinking water standard was exceeded in one well. Overall, 13.5% of the samples had detectable E. coli. Including regional-scale sources of pollutants to the assessment of aquifer vulnerability with the AFI did not lead to the identification of contaminated wells, due to the short groundwater flow paths between recharge and the sampled wells. Given the occurrence of contaminants, the public health concerns stemming from these new data on regional-scale PhAC and pesticide concentrations, and the local flow conditions observed in post-glacial terrains, there is a clear need to investigate the sources and behaviours of local-scale pollutants.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.