Factors affecting river turbidity in a degrading permafrost environment : the Tasiapik River, Umiujaq (Nunavik)

This study focuses on spatiotemporal changes in water turbidity in relation to permafrost to document the impact of meteorological conditions and water flow on hydro-sedimentary processes in northern regions. Starting in June of 2019, water turbidity data were collected at six sites along the Tasiapik River (Nunavik). A statistical analysis was completed based on records of water turbidity, precipitation, water flow, and air temperature. Our results show a significant correlation between air temperatures and turbidity, with a correlation of up to r = 0.59. These correlations depend on the location of the site along the river and the time of the study period (June–October 2019). The flow rate was the primary factor that caused variations in the turbidity of the Tasiapik River. Our results showed that following an increase in flow rate, there was an almost simultaneous increase in turbidity due to erosion of the banks. The duration and intensity of precipitation events are also important factors affecting the process of sediment transport. Even though meteorological conditions play an important role in turbidity variation, other characteristics of the site such as the topography and the existence of thermokarst lakes are additional factors that influence the dynamics of sediment transport in the Tasiapik River.Les travaux menés en Arctique et Subarctique démontrent une accélération de la dégradation du pergélisol durant les dernières décennies, provoquant des tassements importants du sol et par le fait même, un accroissement du fluage d’eau chargée de sédiments vers les lacs et les rivières. Cette étude vise à mieux comprendre la variation spatio-temporelle de la turbidité fluviale en contexte périglaciaire dans le but de faire avancer les connaissances sur les impacts des conditions météorologiques et du débit sur les processus hydrosédimentaires des régions nordiques. Des données de turbidité de l’eau de la rivière Tasiapik, située à 5 km à l’est du village d’Umiujaq (Nunavik), ont été enregistrées de juin à octobre 2019 dans six sites distincts. Des analyses statistiques réalisées sur ces enregistrements indiquent qu’il existe une corrélation significative (r = 0,59) entre les températures de l’air et la turbidité de la rivière. Ces relations sont plus ou moins importantes selon l’emplacement du site le long de la rivière et selon le moment de la période d’étude. Le débit était le principal facteur à l’origine des variations de la turbidité de la rivière Tasiapik. Nos résultats ont montré qu’à la suite d’une augmentation du débit, il y a eu une augmentation presque simultanée de la turbidité due à l’érosion des berges et de la quantité des sédiments en suspension. La durée et l’intensité des précipitations sont également des facteurs importants ayant fait varier la turbidité de la rivière. Bien que les conditions météorologiques jouent un rôle important dans la variation de la turbidité, il s’est avéré que les caractéristiques du site telles que la topographie et la présence de lacs thermokarstiques sont des facteurs importants dans la dynamique du transport sédimentaire de la rivière Tasiapik

Numerical evaluation of grouting scenarios for reducing water inflows in underground excavations – Goldcorp’s Éléonore mine study case

Water inflows through fracture networks are one of the many challenges that the Éléonore mine has to face. Although pregrouting of pilot holes during mine development has been proven to efficiently reduce water inflows into mine excavations, the actual design methods are empirical and can be optimized to increase grouting efficiency and decrease the associated costs. Optimization of the amount of cement needed for pre-grouting is achieved by designing the grouting approach based on the location of major faults around the excavations. Here, a base case finite-element numerical model and associated sensitivity analyses are used to simulate groundwater inflows into a stope, based on the Éléonore mining site characteristics. Simulations are conducted for testing various grout injection scenarios for various major fault locations around the stope. Sensitivity analyses have shown that for a fault located above the stope, the inflow reduction is greater when the zone between the fault and the stope is grouted instead of directly grouting the fault itself. Also, in the case of a fault intersecting a stope, the results have demonstrated that the fault itself should be grouted as widely as possible, instead of sealing only the immediate surroundings of the stope

Numerical simulation of hydrocarbon fuel dissolution and biodegradation in groundwater

The behaviour of hydrocarbon fuels in contaminated groundwater systems is studied using a multicomponent reactive transport model. The simulated processes include residual NAPL dissolution, aerobic and anaerobic biodegradation with daughter-product transport, and transport of a reactive carrier with mixed equilibrium/kinetic sorption. The solution algorithm is based on a three-dimensional Galerkin finite element scheme with deformable brick elements and capacity for a free watertable search. Nonlinearities are handled through Picard iteration. Convergence is rapid for most applications and mass balance errors for all phases are minimal. The model is first applied to simulate a pilot scale diesel fuel dissolution experiment in which humic acid is used as a natural organic carrier to enhance dissolution and to promote biodegradation of the aqueous components. The pilot scale experiment is described by Lesage et al. (1995) and Van Stempvoort et al. (2000). The conceptual model includes 8 unique components dissolving from 500 mL of residual diesel fuel within a 3D saturated domain. Oxygen-limiting competitive aerobic biodegradation with a dynamic microbial population is also included. A mixed 2-site equilibrium/kinetic model for describing sorption of the carrier to the aquifer solids was adopted to reproduce the observed breakthrough of the humic acid and organic components. Most model parameters were obtained independently with minimal calibration. Batch sorption data were found to fit well at the pilot scale, however biodegradation and dissolution rates were not well known and had to be fitted. Simulations confirmed the observed 10-fold increase in effective solubility of trimethylnaphthalene, and increases on the order of 2-5 for methyl- and dimethylnaphthalene. The simulated plumes showed almost complete attenuation after 5 years, in excellent agreement with observed data. A sensitivity analysis showed the importance of carrier concentrations, binding coefficients, dissolution and biodegradation rates. Compared to a dissolution scenario assuming no carrier, the humic acid-enhanced dissolution case decreased the remediation time by a factor of about 5. The second application of the model involves simulating the effect of ethanol on the persistence of benzene in gasoline-impacted groundwater systems. The conceptual model includes a 4-component residual gasoline source which is dissolving at the watertable into a 3D aquifer. Comparisons are made between dissolved plumes from a gasoline spill and those from an otherwise equivalent gasohol spill. Simulations have shown that under some conditions, a 10% ethanol component in gasoline can extend the travel distance of a benzene plume by at least 150% relative to that from an equivalent ethanol-free gasoline spill. The increase is due to preferred consumption of oxygen by ethanol and a corresponding reduction in the biodegradation rate of benzene while the two plumes overlap. Because of differences in retardation however, the ethanol and benzene plumes gradually separate. The impact therefore becomes limited because oxygen rapidly disperses behind the ethanol plume and benzene degradation eventually resumes. A sensitivity analysis for two common spill scenarios showed that background oxygen concentrations, and benzene retardation had the most significant influence on benzene persistence. A continuous gasohol spill over 10 years was found to increase the benzene travel distance by over 120% and a pure ethanol spill into an existing gasoline plume increased benzene travel distance by 150% after 40 years. The results are highly relevant in light of the forthcoming ban of MTBE in California and its likely replacement by ethanol by the end of 2002

Three-dimensional numerical modeling of cryo-hydrogeological processes in a river-talik System in a continuous permafrost environment

In continuous permafrost environments, understanding complex river-talik system dynamics is fundamental for the sustainable use of talik aquifers as a source of drinking water in remote communities. A conceptual model of a river-talik system was previously developed based on field investigations in the floodplain of the Kuuguluk River at Salluit, Nunavik (Quebec), Canada, including geophysical surveys and monitoring of hydraulic heads and riverbed temperatures. This conceptual model is here used to develop a 3D numerical model for simulating the governing cryo-hydrogeological processes and dynamic system behavior. The numerical simulations, supported by the field data, show that the width and thickness of the river talik is highly correlated to the width of the overlying riverbed. In summer, the river talik is hydraulically connected to the riverbed, and groundwater from the talik aquifer is contributing to river baseflow. In winter, when the river and riverbed freeze, the river talik becomes hydraulically isolated from the riverbed. Under such conditions, the river talik acts as a tube-like conduit system which focusses groundwater flow. Increasing hydraulic heads at constrictions in the talik can be sufficient to fracture the frozen riverbed and ice cover, leading to groundwater overflows and icing formation. This study presents an integrated field and modeling approach for assessing the potential of talik aquifers as reliable sources of drinking water in northern communities

A conceptual model for talik dynamics and icing formation in a river floodplain in the continuous permafrost zone at Salluit, Nunavik (Quebec), Canada

Icing occurs each winter along the floodplain of the Kuuguluk River in the continuous permafrost zone at Salluit in Nunavik (Quebec), Canada. The source of successive water overflows which thicken and enlarge this ice cover over time is suprapermafrost groundwater discharging from a talik below the riverbed. Electrical resistivity tomography was used to delineate the talik, while water level and temperature dataloggers were used to assess the thermo-hydraulic conditions of the riverbed. The mean annual riverbed temperature was 1.8°C in 2016 while the mean annual air temperature was −6.0°C. Hydraulic heads below the ice cover as high as 2.8 m and events of abrupt decreases in hydraulic head due to suprapermafrost groundwater overflow through cracks in the ice cover were monitored. An analytical solution based on beam mechanics theory was used to assess the water pressure-induced stresses which are sufficient to fracture the ice cover. A detailed conceptual model of the talik and icing dynamics is proposed to explain the cryo-hydrogeological processes taking place in this complex groundwater–river system. The groundwater pressure buildup in the talik during the winter is due to constricted flow of suprapermafrost groundwater in the talik. These results have implications for understanding the dynamics of river taliks and their use as potential water supplies in northern communities

Field evidence of CO2 sequestration by mineral carbonation in ultramafic milling wastes, Thetford Mines, Canada

Two experimental small-scale cells have been constructed in the field to better understand passive mineral carbonation under natural atmospheric conditions in the ultramafic milling wastes of Thetford Mines (Québec, Canada). The magnesium-rich milling wastes mainly consist of poorly sorted grains and fibers of lizardite and chrysotile, with smaller amounts of antigorite, brucite and magnetite. These wastes could serve as significant and long-term CO2 sinks but the mechanisms and rates of mineral carbonation are not well understood. In this paper, the design of the experimental cells along with observations on gas composition, gas pressure, soil temperature, volumetric water content and mineral composition are presented with the objective to better understand the mineral carbonation processes under natural conditions and to propose a conceptual model for mineral carbonation at the field cell scale. Low CO2 concentrations (5–50 ppm) measured in the experimental cells and the presence of hydromagnesite suggest that natural and passive mineral carbonation is a significant process occurring in the magnesium-rich wastes (4 kg/m3/year of sequestrated CO2). In the proposed conceptual model, atmospheric CO2 (∼400 ppm) dissolves in the hygroscopic water of the piles, where weathering of magnesium silicates forms magnesium carbonates. Water saturation in the cells is relatively stable over time and varies between 0.4 and 0.65, which is higher than optimal saturation values proposed in the literature, reducing CO2 transport in the unsaturated zone. Gas-phase CO2 concentrations along with gas flow rate measurements in the cells suggest that the reaction is most active close to the surface and that diffusion of CO2 is the dominant transport mechanism in the wastes. Although the carbonation reaction is exothermic, no evidence of thermal convection has been observed in the experimental cells. This conceptual model will serve to support a reactive transport model in order to quantify and optimize the amount of CO2 that can be captured in chrysotile milling waste piles

Coupled cryo-hydrogeological modelling of permafrost dynamics near Umiujaq (Nunavik, Canada)

A two-dimensional (2D) cryo-hydrogeological numerical model of groundwater flow, coupled with advective-conductive heat transport with phase change, has been developed to study permafrost dynamics around an ice-rich permafrost mound in the Tasiapik Valley near Umiujaq, Nunavik (Québec), Canada. Permafrost is degrading in this valley due to climate warming observed in Nunavik over the last two decades. Ground temperatures measured along thermistor cables in the permafrost mound show that permafrost thaw is occurring both at the permafrost table and base, and that heat fluxes at the permafrost base are up to ten times higher than the expected geothermal heat flux. Based on a vertical cross-section extracted from a 3D geological model of the valley, the numerical model was first calibrated using observed temperatures and heat fluxes. Comparing simulations with and without groundwater flow, advective heat transport due to groundwater flow in the subpermafrost aquifer is shown to play a critical role in permafrost dynamics and can explain the high apparent heat flux at the permafrost base. Advective heat transport leads to warmer subsurface temperatures in the recharge area, while the cooled groundwater arriving in the downgradient discharge zone maintains cooler temperatures than those resulting from thermal conduction alone. Predictive simulations incorporating a regional climate-change scenario suggest the active layer thickness will increase over the coming decades by about 12 cm/year, while the depth to the permafrost base will decrease by about 80 cm/year. Permafrost within the valley is predicted to completely thaw by around 2040.Un modèle numérique cryo-hydrogéologique bidimensionnel (2D) de l’écoulement de l’eau souterraine, couplé avec un transfert de chaleur advectif-convectif à changement de phase, a été développé pour étudier la dynamique du pergélisol autour d’un monticule fortement gelé dans la Vallée de Tasiapik près de Umiujaq, Nunavik, Québec (Canada). Le pergélisol se dégrade dans cette vallée du fait du réchauffement climatique observé à Nunavik au cours des deux dernières décennies. Les températures du sol mesurées sur des câbles de thermistance au sein du monticule de pergélisol montrent que la fonte de ce dernier se produit à la fois à sa surface et à sa base et que les flux de chaleur de la base sont jusqu’à 10 fois supérieurs au flux de chaleur géothermique attendu. Sur la base d’une coupe verticale extraite d’un modèle géologique 3D de la vallée, le modèle numérique a d’abord été calé sur les températures et les flux de chaleur observés. En comparant les simulations avec et sans écoulement d’eau souterraine, on montre que le transfert de chaleur advectif lié à l’écoulement de l’eau souterraine dans l’aquifère sous le pergélisol joue un rôle crucial dans la dynamique du pergélisol et peut expliquer le flux de chaleur élevé constaté à sa base. Le transfert de chaleur advectif conduit à des températures de subsurface plus chaudes dans la zone de recharge tandis que l’arrivée d’une eau souterraine froide dans la zone de décharge aval assure le maintien des températures plus froides que celles résultant de la conduction thermique seule. Des simulations prédictives intégrant un scénario de changement climatique régional suggèrent que l’épaisseur de la couche active s’accroîtra durant les décennies à venir d’environ 12 cm/an, tandis que la profondeur de la base du pergélisol diminuera.Se ha desarrollado un modelo numérico bidimensional (2D) criohidrogeológico del flujo de agua subterránea, junto con el transporte de calor advectivo-conductivo con cambio de fase, para estudiar la dinámica del permafrost alrededor de un montículo de permafrost rico en hielo en el valle de Tasiapik, cerca de Umiujaq, Nunavik (Québec), Canadá. El permafrost se está degradando en este valle debido al calentamiento climático observado en Nunavik en las últimas dos décadas. Las temperaturas del suelo medidas a lo largo de los cables del termistor en el montículo del permafrost muestran que el deshielo del permafrost está ocurriendo tanto en la capa como en la base del permafrost, y que los flujos de calor en la base del permafrost son hasta diez veces más altos que el flujo de calor geotérmico esperado. Basado en una sección transversal vertical extraída de un modelo geológico 3D del valle, el modelo numérico fue calibrado primero utilizando temperaturas observadas y flujos de calor. Comparando simulaciones con y sin flujo de agua subterránea, se ha demostrado que el transporte de calor advectivo debido al flujo de agua subterránea en el acuífero del subpermafrost juega un papel crítico en la dinámica del permafrost y puede explicar el alto flujo de calor aparente en su base. El transporte conveniente de calor conduce a temperaturas subterráneas más cálidas en el área de recarga, mientras que el agua subterránea más fría que llega a la zona de descarga con pendiente descendente mantiene temperaturas más frías que las que resultan de la conducción térmica sola. Las simulaciones predictivas que incorporan un escenario regional de cambio climático sugieren que el espesor de la capa activa aumentará en las próximas décadas en unos 12 cm/año, mientras que la profundidad hasta la base del permafrost disminuirá en unos 80 cm/año. Se predice que el permafrost dentro del valle se descongelará completamente hacia 2040.Um modelo numérico crio-hidrogeológico bidimensional (2D) crio-hidrogeológico, juntamente com transporte de calor condutor-advetivo com mudança de fase, foi desenvolvido para estudar a dinâmica do pergelissolo em torno de um monte de pergelissolo rico em gelo no vale Tasiapik perto de Umiujaq, Nunavik (Quebec), Canadá. O pergelissolo está se degradando neste vale devido ao aquecimento climático observado em Nunavik nas últimas duas décadas. As temperaturas do solo medidas ao longo dos cabos do termistor no monte de pergelissolo mostram que o degelo do pergelissolo está ocorrendo no topo e na base de pergelissolo e que os fluxos de calor na base do pergelissolo são até dez vezes mais altos que o fluxo de calor geotérmico esperado. Com base em uma seção transversal vertical extraída de um modelo geológico 3D do vale, o modelo numérico foi primeiro calibrado usando temperaturas e fluxos de calor observados. Comparando simulações com e sem fluxo de águas subterrâneas, é mostrado que o transporte de calor devido ao fluxo de água subterrânea no aquífero subpergelissolo desempenha um papel crítico na dinâmica do pergelissolo e pode explicar o alto fluxo de calor aparente na base do pergelissolo. O transporte de calor advectivo leva à temperaturas mais baixas do subsolo na área de recarga, enquanto as águas subterrâneas resfriadas que chegam à zona de descarga de nível inferior mantêm temperaturas mais baixas do que aquelas resultantes apenas da condução térmica. Simulações preditivas incorporando um cenário regional de mudança climática sugerem que a espessura da camada ativa aumentará nas próximas décadas em cerca de 12 cm/ano, enquanto a profundidade da base do pergelissolo diminuirá em cerca de 80 cm/ano. Prevê-se que o pergelissolo no vale descongele completamente por volta de 2040

Pumping optimization under uncertainty in an island freshwater lens using a sharp-interface seawater intrusion model

Pumping optimization under uncertainty is a powerful approach for the management of groundwater resources, and its implementation would be valuable in island aquifers where freshwater lenses are affected by seawater intrusion. Sharp-interface numerical models are especially well suited for the task as they offer fast simulation times, but to date they have not been used because of a lack of guidelines and due to the specific challenges associated with this approach. This study presents a methodology for pumping optimization under uncertainty for island freshwater lenses using a sharp-interface model (MODFLOW-SWI2) and demonstrates it for a real case (Magdalen Islands, Quebec, Canada). The total pumping in a well field was maximized while avoiding well salinization due to upconing. To do so, the sharp interface simulated below the pumping wells was corrected successively for cell-to-well upconing and for dispersion. Pumping optimization under uncertainty was then conducted using PESTPP-OPT, considering parameter and observation uncertainty, and was repeated for 23 reliability levels to illustrate a large range of risk-averse, tolerant and neutral stances. The maximum pumping was obtained as a function of risk of well salinization. This approach enabled quantification of the tradeoffs between pumping and risk, allocation of pumping amongst wells, and an examination of the ability of the well field to meet the water demand while maintaining an acceptable level of risk. Ultimately, this framework allows groundwater managers to select the final pumping scenario themselves, depending on their attitude toward risk

Semi-automated filtering of data outliers to improve spatial analysis of piezometric data

The identification and removal of data outliers remains a major challenge for spatial analysis of piezometric data. In this context, a simple semi-automated procedure for filtering outliers of depth to static water level was developed and used as a part of a regional groundwater-mapping project in the Québec Metropolitan Community, Québec, Canada. Following a few basic steps of data control, potential outliers were detected using two simple automated steps: (1) identifying water levels that are deeper than the 99th percentile of a high-reliability dataset compiled by groundwater professionals and assumed to adequately represent depths to static water level, and (2) using moving averages within a search radius of 250 m calculated around each well. All detected potential outliers were visually examined in a geographic information system and compared to neighbouring data before being kept or discarded. To evaluate the efficiency of the procedure, exploratory statistics, histograms and semi-variograms of the initial, intermediate and filtered datasets were compared to the high-reliability dataset. Objective interpolation was then performed using ordinary kriging. A cross-validation analysis showed a less biased and more accurate interpolation after applying the proposed outlier filtering procedure. Qualitative knowledge of the hydrogeological settings is an important component of this procedure which combines advantages of both manual and automated processing, making the procedure adaptive and easy to use. The final outcome of the proposed procedure is an improved interpolation map of depth to static water level along with minimised and low squared estimation errors.L’identification et la suppression de données aberrantes restent un enjeu majeur pour l’analyse spatiale de données piézométriques. Dans ce contexte, une simple procédure semi-automatique pour filtrer des données aberrantes de profondeur du niveau d’eau statique a été développée et utilisée dans le cadre d’un projet de cartographie hydrogéologique régionale, de la communauté métropolitaine de Québec (Canada). En suivant quelques étapes basiques de contrôle de données, les données potentiellement aberrantes ont été détectées en utilisant deux étapes automatisées simples : (1) en identifiant les niveaux d’eau dont la profondeur est supérieure au 99ème percentile d’un jeu de données fiables, compilées par des hydrogéologues et considérées valides pour représenter la profondeur du niveau d’eau statique, et (2) en utilisant les moyennes glissantes dans un périmètre de 250 m autour de chaque puits. Toutes les données potentiellement aberrantes détectées ont été examinées dans un système d’information géographique et comparées aux données voisines avant d’être conservées ou écartées. Afin d’évaluer l’efficacité de la procédure, des statistiques exploratoires, des histogrammes et des semi-variogrammes des jeux de données initiaux, intermédiaires et filtrés ont été comparés au jeu de données fiables. Une interpolation objective a ainsi été réalisée en utilisant un krigeage ordinaire. Une analyse discriminante a montré une interpolation moins biaisée et plus précise, après avoir appliqué la procédure proposée de filtrage de données aberrantes. La connaissance qualitative des conditions hydrogéologiques est une composante importante de cette procédure qui associe les avantages de traitements à la fois manuels et automatisés, ce qui rend cette procédure adaptable et facile à utiliser. Le résultat final de la procédure proposée est une carte d’interpolation améliorée de la profondeur du niveau statique de l’eau, ainsi que des erreurs quadratiques minimisées et faibles