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Renard, Philippe
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Renard, Philippe
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Directeur de Recherche
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Philippe.Renard@unine.ch
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- PublicationAccès libreHydro-geological modeling of the Roussillon aquifer : integrating geological knowledge uncertainties and geostatistical methods in groundwater modeling(2023)
; Ce travail de thèse porte sur la modélisation géologique et hydrodynamique de l’aquifère du Roussillon, en mettant l’accent sur la transition d’un modèle géologique détaillé, utilisant la méthode géostatistique de simulation multipoint (MPS), vers des modèles hydrodynamiques. La première étape de ce travail de thèse a consisté à créer les enveloppes du modèle géologique 3D du Roussillon. Les principales unités géologiques de l’aquifère du Roussillon comprennent le Pliocène marin, le Pliocène continental et le Quaternaire. La compilation d’une base de données géologiques, composés de logs géophysiques et de ligne sismiques, a permis de comprendre les structures de l’aquifère et d’interpoler les surfaces 2D qui délimitent le modèle géologique 3D. Une fois les enveloppes interpolées, la seconde étape de modélisation de ce travail s’est concentrée sur la simulation des faciès sédimentaire composant l’aquifère du Pliocène Continental. L’utilisation de l’approche de simulation multipoint (MPS) a permis de créer des modèles réalistes de faciès sédimentaire dans l’unité du Pliocène continental, en reproduisant des structures alluviales à l’échelle régionale. En complément de la simulation MPS, deux autres modèles sédimentaires ont été créés. Le premier est déterministe et se base sur l’interprétation d’essais de pompage pour caractériser les propriétés physiques du Pliocène Continental. Le second utilise une approche géostatistique appelée simulations séquentielles d’indicateurs (SIS) pour générer les propriétés hydrodynamiques de l’aquifère. Cette seconde approche géostatistique est plus couramment utilisée que le MPS et est plus simple à mettre en œuvre. La troisième étape de ce travail consiste en la définition du modèle hydrodynamique de l’aquifère du Roussillon. Le modèle hydrodynamique a été réalisé en considérant les conditions aux limites, les budgets de prélèvement asso- ciés, les observations piézométriques disponibles, et a été pré-calibrer en régime d’écoulement permanent dans une première phase de modélisation. Les modèle d’écoulement ont été réalisés avec le logiciel MODFLOW 6. La dernière étape de modélisation consiste en la création de modèle d’écoulement en régime transitoire ainsi que dans la création d’une approche de calibration des paramètres physique du modèle MPS du Pliocène Continental. Un défi important de ce travail réside dans la conciliation des modèles géologiques avec les données hydrodynamiques, ce qui nécessite une approche spécifique pour garantir de préserver les structures sédimentaires simulées, lors du processus de calibration. Il convient de noter que peu d’études existent sur la calibration des modèles MPS régionaux, et que souvent, les processus de calibration ne prennent pas en compte les éléments structuraux géologiques. La comparaison des approches de modélisation sédimentologique, effectuée en régime d’écoulement permanent et transitoire, met en avant une homogénéité des résultats entre les différentes approches. Les résultats en régime permanent sont satisfaisants pour les trois approches, mais peine à reproduire certains signaux en régime transitoire. Les problèmes des modèles en régime transitoire sont probablement dus à un problème d’initialisation du système hydrodynamique et de calibration des conditions limites. Ce travail propose donc une comparaison d’approches de modélisation sédimentologique et de leur impact sur les simulations hydrodynamiques. Il met en évidence des améliorations potentielles pour le modèle hydrogéologique de l’aquifère du Roussillon. Des données d’observation plus fiables et des informations sédimentologiques supplémentaires sont fortement recommandées, en particulier dans les zones présentant des différences significatives par rapport aux niveaux d’eau simulés, afin d’améliorer le modèle hydrogéologique. Cela permettrait de mieux comprendre le fonctionnement du système et de faciliter les ajustements locaux du modèle sédimentologique et des conditions hydrodynamiques. Malgré les difficultés rencontrées, notamment concernant la reproduction de certain signal piézométrique lors des simulations en régime transitoire, cette étude contribue à la compréhension de l’état de l’aquifère en identifiant les principales sources d’incertitude dans le modèle actuel de l’aquifère du Roussillon. ABSTRACT The presented study focuses on the geological and hydrodynamic modeling of the Roussillon aquifer. Located in southern France, near the Mediterranean Sea, the Roussillon plain covers an area of over 800 km2 and serves as the most important source of fresh water for the local community, supporting various needs such as irrigation, drinking water, and industrial usage. This aquifer is situated in one of the driest regions of France. Additionally, the aquifer experiences heavy water abstraction, mainly for drinking and agricultural purposes, leading to a steady decline in its water level over the years. The region is also affected by climatic changes, including rising sea levels and potential disruptions in precipitation patterns, which further impact the aquifer’s water availability. Balancing water management and conservation in the face of increasing population and climate change poses significant challenges for the Roussillon aquifer. The primary aim of the thesis is to enhance the geological understanding of the Roussillon aquifer and develop a hydrodynamic model to gain deeper insights into the functioning of the aquifer system. Additionally, the study aimed to create a solid foundation for investigating the potential consequences of climate change on this essential regional resource. The geological model consists of three main units, starting with the deepest unit, the Marine Pliocene unit, followed by the Continental Pliocene unit, and finally at the top the Quaternary unit. The initial phase of this work involved compiling a comprehensive geological database using onshore and offshore data sets to develop a conceptual understanding of the aquifer’s structures and to interpolate the main 2D surfaces that separate the 3D geological model. Within the Continental Pliocene layer, four subintervals were defined, and the elevation map of the three surfaces dividing these subintervals was mapped and interpolated using geophysical logs and offshore seismic data. The geological data set, although limited in resolution and coverage, served as conditioning data for the geostatistical simulation of the Continental Pliocene layer. We then used the multiplepoint simulation approach (MPS) to simulate realistic lithofacies patterns representative of the sediment spatial distribution in the Continental Pliocene layer. The 3D model of the Continental Pliocene layer was created by stacking 2D simulations controlled by vertical conditioning sampling. The results demonstrated satisfactory reproduction of sedimentary structures at the regional scale. In addition to the MPS simulation, two other approaches, a depth related approach and a Sequential Indicator Simulation (SIS) set, were used to generate hydro physical property fields for the aquifer. The depth related approach is based on the interpretation of hydraulic pumping tests, to assign hydraulic conductivity values based on the cell’s depth in the grid. The SIS employed a variogram based algorithm to simulate simple lithofacies structures (more simple compared to the MPS models). These three sets of hydraulic conductivity and specific storage values are used to feed the hydrodynamic simulations and estimate the propagated uncertainty of the sedimentological models on the hydrodynamic simulations. This work then focuses on defining the conceptual hydrodynamical model of the Roussillon aquifer. We present the main boundary conditions, their associated budgets, available piezometric observations, and the main modeling assumptions, linked to the different components of the MODFLOW 6 hydrodynamic model. In the first modeling step, a steady state calibration is performed to calibrate river parameters and mean hydraulic conductivity of simulated facies with the goal of preserving the simulated lithofacies patterns while matching the hydrodynamic observations. Once calibrated, we used these parameters for transient hydrodynamic models over a 20 years period. The three model approaches are used and compared in this study. It appears that reproducing the piezometric transient observation series presented some difficulties, with the models failing to capture the main trend of the piezometric levels on some locations. The reproduction of these piezomet ric series suffered from limited data availability, simplified river systems, and uncertainties regarding the local hydraulic conductivity and specific storage parameters. To better reproduce the piezometric series, this work ends with a short study on the use of the ES-MDA approach to attempt local corrections of the hydraulic conductivity and specific storage parameters. These initial tests faced limitations, as many forward models failed to converge during the process, limiting the applicability of the calibration process. Overall, this work proposes a unique regional comparison of sedimentological modeling approaches and their influence on hydrodynamic simulations. It also identifies directions for improving the aquifer model’s performance. Obtaining more reliable observation data series, as well as more onshore sedimentological information, especially in areas with significant deviations from simulated water levels, is highly recommended for improving the Roussillon hydrogeological model. This would aid in better understanding the system’s behavior and facilitate localized modifications of the sedimentological model and the hydrodynamic conditions. Despite the challenges faced, the study contributes to understanding the aquifer’s transient state, emphasizing the importance of sedimentological models in hydrodynamic studies, and identifying major sources of uncertainty in the current model of the Roussillon aquifer. - PublicationAccès libreDeterministic and probabilistic numerical modelling towards sustainable groundwater management: application to seawater intrusion in the Korba aquifer (Tunisia)(2008)
; This PhD endeavours numerical groundwater modelling considering heterogeneous and uncertain hydraulic parameters. It is made of three parts. First, we investigated the effects of dimensionality and heterogeneity of the hydraulic conductivity on dispersive seawater intrusion (SWI) processes. Multiple 2D and 3D unconditional simulations of hydraulic conductivity fields sharing the same statistics were generated then used to solve density-dependent flow and solute transport equations with a finite element code. Monte Carlo simulations were analysed in terms of dimensionless criteria including the penetration length and width of the saltwater wedge. Results showed that the 2D heterogeneity is affecting more strongly the SWI processes than the 3D heterogeneity. The saltwater wedge length in the 2D models is smaller than in the 3D ones while there is more mixing in 2D models. Most important, results showed that there is a critical ratio between advection and dispersion processes which is controlling the behaviour of SWI in heterogeneous porous medium. The second part of the thesis dealt with deterministic and probabilistic modelling and long term forecasts of SWI in the Korba aquifer (Tunisia). The study started by the development of a 3D density-dependent flow and solute transport model of the regional Korba aquifer. Then, two geostatistical models of the exploitation rates and of the hydraulic conductivities within the aquifer were built by combining incomplete direct data and secondary information including aquifer physical parameters. The effects of the uncertainty on the spatial distribution of the pumping rates and the uncertainty on the hydraulic conductivity field on the 3D density-dependent model were analysed separately and then jointly. To circumvent the large computing time required to run hundreds of 44-years transient models, the simulations were made in a parallel fashion on the EGEE Grid infrastructure as well as on a local Linux cluster. The deterministic numerical model allowed to estimate the current over-exploitation of the Korba aquifer to 135%. It also allowed to estimate the time lapse needed to turn back the initial head and slat distributions (before exploitation start) to about 150 years. The results of the stochastic simulations showed that both uncertainties led to a zone representing 12% of the aquifer area, where the groundwater heads and salt concentrations are not known with accuracy. Most important, results showed that reducing the pumping rates progressively by 50% until 2048 will not result in a recession of the saltwater wedge ; instead an additional 9.5% of the surface of the aquifer will be contaminated in 2048. In the third part of the thesis, the performances of kriging, stochastic simulations and sequential self-calibration inversion are assessed when characterizing a non-multi-Gaussian synthetic 2D braided channel aquifer. In a first step, the performance of the three methods was compared in terms of reproducing the original reference transmissivity or head fields. In a second step, the methods were compared in terms of accuracy of flow and transport (capture zone) forecasts. Results showed that the errors remain large even for a dense data network. In addition, some unexpected behaviours are observed when large transmissivity datasets are used. We also observed an increase of the bias with the number of transmissivity data and an increasing uncertainty with the number of head data. This was interpreted as a consequence of the use of an inadequate multi-Gaussian stochastic model.