Hydro-geological modeling of the Roussillon aquifer : integrating geological knowledge uncertainties and geostatistical methods in groundwater modeling
Authors:   , Dall'Alba Valentin 
Published: November 2023

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.