The influence of water/rock − water/clay interactions and mixing in the salinization processes of groundwater

Study region: Groundwater from the Precambrian Shield rock and Pleistocene deposit aquifers in Saguenay-Lac-Saint-Jean region ( >13 000 km2) in the province of Quebec, Canada. Study focus: Interpretations are based on the combination of hierarchical cluster analysis (HCA) results, principal component analysis (PCA), binary plots investigations ([Na+, Ca2+, Br−] vs. Cl−; Ca2+ vs. HCO3−; Ca2+ vs. Na+) and Piper diagram investigations. The HCA and PCA was applied on 321 samples to specifically enable the identification of two very distinct salinization paths that produce the brackish groundwater in the study area. New hydrological insights for the region: The results show that each of the two salinization paths exerts a major and different influence on the chemical signature of groundwater. Groundwater present in the crystalline bedrock naturally evolve from a recharge-type groundwater (Ca-HCO3-dominant) to a type of brackish groundwater (Ca-(Na)-Cl-dominant) due to water/rock interactions (plagioclase weathering and mixing with deep basement fluids). Groundwater evolution in confined aquifers is dominated by water/clay interactions. The term water/clay interactions was introduced in this paper to account for a combination of processes: ion exchange and/or leaching of salt water trapped in the regional aquitard. Mixing with fossil seawater might also increase the groundwater salinity. PCA revealed that Ca2+, Sr2+, Ba2+ are highly correlated with groundwater from bedrock aquifers, while Mg2+, SiO2, K+, SO42− and HCO3− are more representative of the regional confining conditions

Insights on pumping well interpretation from flow dimension analysis : the learnings of a multi-context field database

The flow dimension parameter n, derived from the Generalized Radial Flow model, is a valuable tool to investigate the actual flow regimes that really occur during a pumping test rather than suppose them to be radial, as postulated by the Theis-derived models. A numerical approach has shown that, when the flow dimension is not radial, using the derivative analysis rather than the conventional Theis and Cooper-Jacob methods helps to estimate much more accurately the hydraulic conductivity of the aquifer. Although n has been analysed in numerous studies including field-based studies, there is a striking lack of knowledge about its occurrence in nature and how it may be related to the hydrogeological setting. This study provides an overview of the occurrence of n in natural aquifers located in various geological contexts including crystalline rock, carbonate rock and granular aquifers. A comprehensive database is compiled from governmental and industrial sources, based on 69 constant-rate pumping tests. By means of a sequential analysis approach, we systematically performed a flow dimension analysis in which straight segments on drawdown-log derivative time series are interpreted as successive, specific and independent flow regimes. To reduce the uncertainties inherent in the identification of n sequences, we used the proprietary SIREN code to execute a dual simultaneous fit on both the drawdown and the drawdown-log derivative signals. Using the stated database, we investigate the frequency with which the radial and non-radial flow regimes occur in fractured rock and granular aquifers, and also provide outcomes that indicate the lack of applicability of Theis-derived models in representing nature. The results also emphasize the complexity of hydraulic signatures observed in nature by pointing out n sequential signals and non-integer n values that are frequently observed in the database

Numerical modeling for determining the local vertical hydraulic gradient at the wall of a tunnel

The determination of the flow rate in the underground excavation is a very important parameter in the design of the structures. Among all parameters that have an impact on the inflow rate to the tunnel, the hydraulic gradient is one of the most effective one that, according to Darcy’s law, controls the tunnel inflow rate. An empirical-numerical equation is proposed for the determination of the vertical hydraulic gradient in the wall of a tunnel excavated below a water table. The horizontal hydraulic gradient is not supposed to have a significant impact on the tunnel inflow rate as its value is very low. By contrast, the existing vertical hydraulic gradient is among the most effective parameters. On the other hand, in the case of an underground excavation, no equation exists for the determination of the hydraulic gradient that considers more than one parameter, i.e., the depth. Using the results of the numerical simulations, it was deduced that the depth of the tunnel, the ratio between principal hydraulic conductivities, and their relevant directions are the most effective parameters that have a significant influence on the hydraulic gradient and inflow rate to the underground tunnels. The resultant hydraulic gradients in the vicinity of the wall of the tunnel were obtained using the RS2 Rocscience software. The mathematical relationship between the input data, i.e., the depth (z), ratio between hydraulic gradients (a) and their relevant directions (α), and the result of the simulation, i.e., the hydraulic gradient (iz), have been derived by curve fitting. Finally, for each orientation of the principal hydraulic conductivity, an equation is proposed for the calculation of the hydraulic gradient. La détermination du débit dans une excavation souterraine est un paramètre très important dans la conception des structures. Parmi tous les paramètres qui ont un impact sur le débit d'entrée dans le tunnel, le gradient hydraulique est l'un des plus influents qui, selon la loi de Darcy, contrôle le débit d'entrée du tunnel. Une équation empirico-numérique est proposée pour la détermination du gradient hydraulique vertical dans la paroi d'un tunnel creusé sous une nappe phréatique. Le gradient hydraulique horizontal n'est pas censé avoir un impact significatif sur le débit d'entrée du tunnel, car sa valeur est très faible. En revanche, le gradient hydraulique vertical existant a un impact significatif. En outre, dans le cas d'une excavation souterraine, aucune équation n'existe pour la détermination du gradient hydraulique qui prend en compte plus d'un paramètre, c'est-à-dire la profondeur. En utilisant les résultats des simulations numériques, il a été déduit que la profondeur du tunnel, le rapport entre les principales conductivités hydrauliques et leurs directions pertinentes sont les paramètres les plus efficaces qui ont une influence significative sur le gradient hydraulique et le débit d'entrée dans les tunnels souterrains. Les gradients hydrauliques résultants au voisinage de la paroi du tunnel ont été obtenus à l'aide du logiciel RS2 Rocscience. La relation mathématique entre les données d'entrée, c'est-à-dire la profondeur (z), le rapport entre les gradients hydrauliques (a) et leurs directions pertinentes (α), et le résultat de la simulation, c'est-à-dire le gradient hydraulique (iz), ont été dérivés par ajustement de courbe. Enfin, pour chaque orientation de la conductivité hydraulique principale, une équation est proposée pour le calcul du gradient hydraulique

Transient Electromagnetic (TEM) Surveys as a First Approach for Characterizing a Regional Aquifer: The Case of the Saint-Narcisse Moraine, Quebec, Canada

Geological contexts that lack minimal stratigraphic and piezometric information can be challenging to produce an initial hydrogeological map in remote territories. This study proposes an approach to characterize a regional aquifer using transient electromagnetic (TEM) surveys. Given the presence of randomly dispersed boreholes, the Saint-Narcisse moraine in the Mauricie region of Quebec (Canada) is an appropriate site for collecting the required geophysical data, correlating the stratigraphic and piezometric information, and characterizing regional granular aquifers in terms of stratigraphy, geometry, thickness, and extent. In order to use all TEM results (i.e., 47 stations) acquired in the moraine area, we also correlated 13 TEM stations, 7 boreholes, and 6 stratigraphic cross-sections to derive an empirical and local petrophysical relationship and to establish a calibration chart of the sediments. Our TEM data, combined with piezometric mapping and the sedimentary records from boreholes and stratigraphic cross-sections, revealed the compartmentalization of a multi-kilometer morainic system and indicated the presence of two large unconfined granular aquifers overlying the bedrock. These aquifers extend more than 12 km east to west across the study area and are between 25 and >94 m thick. The TEM method provides critical information on groundwater at a regional scale by acquiring information from multiple stations within a short time span to a degree not possible with other existing methodologies

The Specific Length of an Underground Tunnel and the Effects of Rock Block Characteristics on the Inflow Rate

The specific length of a tunnel (STL) and a new analytical model for calculating the block surface area of the rock mass are introduced. First, a method for determining the appropriate length of a tunnel for a numerical simulation is described. The length is then used to examine the correlation between the inflow rate to the tunnel and the block volume, the block surface area, and the fracture intensity (P32) through analytical and numerical modeling. The results indicate that the length of the tunnel should at least be equal to the least common multiple (LCM) of the apparent spacings of the joint sets at the wall of the tunnel to obtain the more reliable and immediate results for the inflow rate to a tunnel that is excavated in a fractured rock mass. A new analytical model was developed to calculate the block surface area and determine the essential joint set parameters, which include the dip, dip direction, and spacing. The determination of the rock block characteristics through numerical modeling requires considering the intact block for calculations. The results indicated that the inflow rate to the tunnel increased with an increase in fracture intensity and a decrease in block volume and surface area. The STL and the analytical model used for calculating the block surface area are validated through numerical simulations with 3DEC software version 7.0

Assessing groundwater recharge and transpiration in a humid northern region dominated by snowmelt using vadose-zone depth profiles

Profiles of the stable isotope ratios of pore water within the vadose zone provide fingerprints of the history of water percolation into a soil. These profiles, combined with profiles of the volumetric water content, can determine the timing and amount of water that has percolated during specific periods. This study aims to: (1) understand water percolation at two sites in Quebec (Canada) that experience thick snow coverage during the winter season; (2) calculate groundwater recharge rates using the peak-shift method; and (3) estimate the transpiration rate based on the water balance budget. A 7-m-deep borehole was drilled at two sites: one site is sparsely covered by vegetation (S1), while the second underlies a pine forest (S2). For all subsamples, δ18O and δ2H from the soil pore water were analyzed, volumetric water content of the cores was measured, and grain-size analyses to estimate the hydraulic properties were performed. For both boreholes, the winter–spring and summer–autumn periods were determined. Given the limited evapotranspiration occurring during the winter–spring period, recharge rates were high at both sites (71 and 75%), while the summer–autumn period had lower recharge rates of 63% (S1) and 41% (S2). A transpiration rate of 0.7 mm/day was estimated for the pine trees covering site S2. This study provides new field observations for estimating recharge based on water stable isotope profiles in a humid northern region dominated by snowmelt. Moreover, it confirms the accuracy of the peak-shift method for assessing groundwater recharge and estimating transpiration. Les profils du taux d’isotopes stables de l’eau des pores de la zone vadose fournissent des empreintes de l’histoire de la percolation de l’eau dans le sol. Ces profils, combinés à des profils de teneurs volumétriques en eau, peuvent déterminer le moment et la quantité d’eau qui a percolé pendant des périodes particulières. Cette étude a pour objectifs: (1) de comprendre la percolation de l’eau sur deux sites du Québec (Canada) qui connaissent un épais manteau neigeux pendant l’hiver; (2) de calculer les taux de recharge en appliquant la méthode du « décalage du pic »; (3) d’estimer le taux de transpiration sur la base du bilan hydrologique. Un forage de 7 m de profondeur a été réalisé sur les deux sites: le premier site est couvert d’une végétation clairsemée (S1), tandis que le deuxième se situe sous une forêt de pins (S2). Pour tous les échantillons, le δ18O et le δ2H de l’eau des pores du sol ont été analysés, la teneur volumétrique en eau des carottes mesurée et des analyses de la granulométrie conduites afin d’estimer les propriétés hydrauliques. Pour les deux forages, les périodes hiver–printemps et été–automne ont été définies. Etant donnée la faible évapotranspiration durant la période hiver–printemps, les taux de recharge sont élevés sur les deux sites (71 et 75%), tandis que la période été–automne montre des taux de recharge plus faibles, de 63% (S1) et 41% (S2). Un taux de transpiration de 0,7 mm/jour a été estimé pour la forêt de pins couvrant le site S2. Cette étude fournit des observations de terrain inédites pour estimer, sur la base de profils d’isotopes stables de l’eau, la recharge d’une région nordique humide dominée par la fonte des neiges. De plus, elle confirme la précision de la méthode du « décalage du pic » pour évaluer la recharge et estimer la transpiration

Constraining a flow model with field measurements to assess water transit time through a vadose zone

The modeling of thick vadose zones is particularly challenging because of difficulties in collecting a variety of measured sediment properties, which are required for parameterizing the model. Some models rely on synthetic data, whereas others are simplified by running as homogeneous sediment domains and relying on a single set of sediment properties. Few studies have simulated flow processes through a thick vadose zone using real and comprehensive data sets comprising multiple measurements. Here, we develop a flow model for a 7‐m‐thick vadose zone. This model, combining the numerical codes CTRAN/W with SEEP/W, includes the measured sediment hydraulic properties of the investigated vadose zone and incorporates the actual climate and subsurface conditions of the study site (precipitations, water‐table elevations, and stable isotope data). The model is calibrated by fitting the simulated and measured vertical profiles of water content. Our flow model calculates a transit time of 1 year for the travel of water through the 7‐m vadose zone; this estimate matches stable isotope‐based results obtained previously for this site. A homogeneous sediment domain flow model, which considers only a single set of sediment properties, produces a transit time that is approximately half the duration of that of the heterogeneous flow model. This difference highlights the importance of assuming heterogeneous material within models of thick vadose zones and testifies to the advantage gained when using real sediment hydraulic properties to parametrize a flow model

Résultats du projet d’acquisition de connaissances sur les eaux souterraines du territoire municipalisé de Lanaudière, de l’est de la Mauricie et de la Moyenne-Côte-Nord, PACES-LAMEMCN – section Mauricie-Est

Le Centre d’études sur les ressources minérales (CERM) de l’Université du Québec à Chicoutimi (UQAC) a réalisé la première caractérisation régionale des aquifères et des eaux souterraines du territoire municipalisé de la région de l’est de la Mauricie. Cette étude a été effectuée dans le cadre du projet d’acquisition de connaissances sur les eaux souterraines des territoires municipalisés de Lanaudière, de l’est de la Mauricie et de la MoyenneCôte-Nord (PACES-LAMEMCN), géré par le ministère provincial de l’Environnement et de la Lutte contre les changements climatiques (MELCC). Ce rapport présente les résultats des trois phases du PACES-LAMEMCN section Mauricie-Est échelonnées sur quatre années de travail (2018 – 2022)

Résultats du projet d’acquisition de connaissances sur les eaux souterraines du territoire municipalisé de Lanaudière, de l’est de la Mauricie et de la Moyenne-Côte-Nord, PACES-LAMEMCN – section Lanaudière

Le Centre d’études sur les ressources minérales (CERM) de l’Université du Québec à Chicoutimi (UQAC) a réalisé la première caractérisation régionale des aquifères et des eaux souterraines du territoire municipalisé de la région de Lanaudière. Cette étude a été effectuée dans le cadre du projet d’acquisition de connaissances sur les eaux souterraines des territoires municipalisés de Lanaudière, de l’est de la Mauricie et de la Moyenne-Côte-Nord (PACES-LAMEMCN), géré par le ministère provincial de l’Environnement et de la Lutte contre les changements climatiques (MELCC). Ce rapport présente les résultats des trois phases du PACES-LAMEMCN section Lanaudière échelonnées sur quatre années de travail (2018 – 2022)

Characterization of general and singular features of major aquifer systems in the Saguenay-Lac-Saint-Jean region

The hydrogeology of the municipalized territory (13,210 km2) of the Saguenay-Lac-Saint-Jean (SLSJ) region has been studied as part of the groundwater knowledge acquisition program entitled Programme d’acquisition de connaissances sur les eaux souterraines (PACES) launched in 2008 by the Government of Quebec, Canada. This study involved the collaboration of numerous contributors (municipal policymakers, government agencies, watershed organizations and universities) to meet the program’s multiple and wide-ranging requirements. The key deliverables included a numerical geodatabase, 38 regional-scale maps and a scientific report, all elaborated after 4 years of data gathering and compilation, fieldwork and information synthesis. In addition, numerous collateral research projects were undertaken by undergraduate and graduate students. The results of the SLSJ-PACES project provided new insights into regional groundwater resources and led to a generalized conceptual model of regional hydrostratigraphic features and groundwater quality. This paper summarizes the particular aspects of the major aquifers in the SLSJ region as uncovered by PACES, and presents the emerging challenges for updating and improving the region’s hydrogeological knowledge and ensuring the sustainable management of regional groundwater resources. Le gouvernement du Québec, au Canada, a lancé un programme scientifique d'acquisition de connaissances sur les eaux souterraines intitulé le «Programme d'acquisition des connaissances sur les eaux souterraines» (PACES) visant à améliorer les connaissances sur les ressources en eaux souterraines. La région du Saguenay-Lac-Saint-Jean (SLSJ, Québec, Canada, 13 210 km2) est parmi les premières régions ayant bénéficié de ce programme. Pour réaliser le projet, plusieurs partenaires (décideurs municipaux, organismes gouvernementaux, organisations de bassins versants et universités) ont collaboré afin de répondre aux exigences multiples et variées du projet PACES. Le livrable principal est constitué d’une géodatabase numérique, à laquelle s’ajoute 38 cartes d'échelle régionale, et un rapport scientifique. Le tout a été livré après 4 ans de travail incluant de la compilation, des levés sur le terrain et la synthèse des résultats. En outre, de nombreux projets de recherche parallèles ont été menés par des étudiants de premier cycle et des cycles supérieurs. Les résultats du projet PACES au SLSJ apportent de nouvelles perspectives sur les ressources en eaux souterraines à l'échelle régionale et soutiennent un modèle conceptuel généralisé des caractéristiques hydrostratigraphiques, de même que de la qualité des eaux souterraines pour la région. Cet article résume les aspects singuliers des principaux aquifères de la région SLSJ, et permet de discuter des défis qui émergent du PACES-SLSJ pour maintenir les connaissances hydrogéologiques à jour et pour les rendre disponibles dans une optique de gestion durable des ressources en eaux souterraines