Hydrogeological modeling of the Roussillon coastal aquifer (France): stochastic inversion and analysis of future stresses

International audienceGlobal climate change-induced stresses on coastal water resources include water use restrictions, saline intrusions, and permanently modifying or damaging regional resources. Groundwater in coastal regions is often the only freshwater resource available, so an in-depth understanding of the aquifer, and the aquifer's response to climate change, is essential for decisionmakers. In this study, we focus on the coastal aquifer of Roussillon (southern France) by developing and investigating a steady-state groundwater flow model (MODFLOW 6) and calibrated with PEST++ on a Python interface (FloPy and PyEmu). Model input and boundary conditions are constrained by various scenarios of climate projections by 2080, with model results predicting the aquifer's response (and associated uncertainty) to these external forcings. Using simple assumptions of intrusion estimates, model results highlight both strong climatic and anthropogenic impacts on the water table. These include aquifer drawdowns reaching several meters locally, and the seawater interface advancing locally several hundred meters inland and rising by several meters. Intrusions of this magnitude risk endangering exploited water wells and their sustainability. Our results demonstrate the critical importance of properly characterizing the geology and its heterogeneity for understanding aquifers at risk because poor predictions may lead to inappropriate decisions, putting critical resources at risk, particularly in coastal environments. Keywords Coastal aquifer • Climate change • Seawater intrusions • MODFLOW • Inverse modeling • PEST+

Ensemble of geological models of a complex coastal aquifer for uncertainty estimation and groundwater modelling in the framework of the DEM'EAUX ROUSSILLON project

International audienceThe creation of geological and sedimentological models is a non-unique process due to the many choices that must be made during their conception. The choice of the geological and sedimentological depositional concepts, the decision regarding the modeling strategy (stochastic or deterministic), the type of algorithm used for the simulations (MPS, SGS, Kriging, TGS…) are all factors that influence the final models and the corresponding uncertainty quantification. Regarding the coastal sedimentary Roussillon aquifer (south of France), different sets of geological models have been created and are used for the hydrogeological simulations; homogeneous permeability model, depth relative permeability model, and stochastic models. The homogeneous and depth relative models are created based on the large structural envelopes delimiting the main aquifers. The stochastic models are also based on these envelopes but simulate inside complex sedimentary facies and permeability fields using the multiple-point statistics algorithm DeeSse. Different geological concepts were tested depending on the different representations and interpretations of the possible spatial distribution of the sediments. Each model has its own strengths and weaknesses. It is by combining these different approaches that we created an ensemble of geological models deemed to cover some conceptual uncertainty range. These models are then used to feed hydrogeological simulations to create an ensemble of hydrogeological models and outputs that are used to characterize the hydrogeological uncertainty and help increase the robustness of the prevision. To do so, we developed an integrated process to convert the geological model to physical model and directly used them in hydrogeological simulations. The models run on MODFLOW, using the floPy python interface which helps the automation of the process. These models can then be calibrated in a semi-automatic manner using various inversion methods such as PEST, POPEX, or using ensemble Kalman filters

Ensemble of geological models of a complex coastal aquifer for uncertainty estimation and groundwater modelling in the framework of the DEM'EAUX ROUSSILLON project

International audienceThe creation of geological and sedimentological models is a non-unique process due to the many choices that must be made during their conception. The choice of the geological and sedimentological depositional concepts, the decision regarding the modeling strategy (stochastic or deterministic), the type of algorithm used for the simulations (MPS, SGS, Kriging, TGS…) are all factors that influence the final models and the corresponding uncertainty quantification. Regarding the coastal sedimentary Roussillon aquifer (south of France), different sets of geological models have been created and are used for the hydrogeological simulations; homogeneous permeability model, depth relative permeability model, and stochastic models. The homogeneous and depth relative models are created based on the large structural envelopes delimiting the main aquifers. The stochastic models are also based on these envelopes but simulate inside complex sedimentary facies and permeability fields using the multiple-point statistics algorithm DeeSse. Different geological concepts were tested depending on the different representations and interpretations of the possible spatial distribution of the sediments. Each model has its own strengths and weaknesses. It is by combining these different approaches that we created an ensemble of geological models deemed to cover some conceptual uncertainty range. These models are then used to feed hydrogeological simulations to create an ensemble of hydrogeological models and outputs that are used to characterize the hydrogeological uncertainty and help increase the robustness of the prevision. To do so, we developed an integrated process to convert the geological model to physical model and directly used them in hydrogeological simulations. The models run on MODFLOW, using the floPy python interface which helps the automation of the process. These models can then be calibrated in a semi-automatic manner using various inversion methods such as PEST, POPEX, or using ensemble Kalman filters

Ensemble of geological models of a complex coastal aquifer for uncertainty estimation and groundwater modelling in the framework of the DEM'EAUX ROUSSILLON project

International audienceThe creation of geological and sedimentological models is a non-unique process due to the many choices that must be made during their conception. The choice of the geological and sedimentological depositional concepts, the decision regarding the modeling strategy (stochastic or deterministic), the type of algorithm used for the simulations (MPS, SGS, Kriging, TGS…) are all factors that influence the final models and the corresponding uncertainty quantification. Regarding the coastal sedimentary Roussillon aquifer (south of France), different sets of geological models have been created and are used for the hydrogeological simulations; homogeneous permeability model, depth relative permeability model, and stochastic models. The homogeneous and depth relative models are created based on the large structural envelopes delimiting the main aquifers. The stochastic models are also based on these envelopes but simulate inside complex sedimentary facies and permeability fields using the multiple-point statistics algorithm DeeSse. Different geological concepts were tested depending on the different representations and interpretations of the possible spatial distribution of the sediments. Each model has its own strengths and weaknesses. It is by combining these different approaches that we created an ensemble of geological models deemed to cover some conceptual uncertainty range. These models are then used to feed hydrogeological simulations to create an ensemble of hydrogeological models and outputs that are used to characterize the hydrogeological uncertainty and help increase the robustness of the prevision. To do so, we developed an integrated process to convert the geological model to physical model and directly used them in hydrogeological simulations. The models run on MODFLOW, using the floPy python interface which helps the automation of the process. These models can then be calibrated in a semi-automatic manner using various inversion methods such as PEST, POPEX, or using ensemble Kalman filters

Groundwater modelling of a complex multilayer coastal aquifer under climate change in the framework of the DEM'EAUX ROUSSILLON project

International audienceThe Plio-Quaternary aquifer of the Roussillon plain (multilayer aquifer of more than 350 m thick, made up of sandy layers embedded in low-permeability clay material from the Pliocene and topped by alluvial formations from the Quaternary) was extensively studied in the framework of the Dem’Eaux Roussillon project. Relying on a conceptual model of the whole groundwater system describing the main factors controlling the hydraulic behaviour of groundwater flow (recharge areas, surface-groundwater interactions, sea water intrusion risk,…) of Quaternary and Pliocene aquifers, a numerical modelling exercise of both onshore and offshore reservoirs was performed. Two parallel modelling approaches sharing a common description of the reservoir geometries, the boundary conditions and the pumping withdrawals locations were carried out, using two (not so) different modelling tools. Results comparison illustrates the uncertainty associated to numerical methods and calibration strategies applied for each modelling tool. Climate change projections were used to explore the future evolution of the groundwater resource and results of both modelling tools were compared. An analytical formulation of the increase of the hydraulic head of the sea due to swell and waves close to the coastline was provided. The analysis of its influence on the groundwater dynamics simulated by the models highlights the potential influence of storms and future sea level rise on the risk of saline intrusions for coastal sedimentary aquifers along the Mediterranean basin

Groundwater modelling of a complex multilayer coastal aquifer under climate change in the framework of the DEM'EAUX ROUSSILLON project

International audienceThe Plio-Quaternary aquifer of the Roussillon plain (multilayer aquifer of more than 350 m thick, made up of sandy layers embedded in low-permeability clay material from the Pliocene and topped by alluvial formations from the Quaternary) was extensively studied in the framework of the Dem’Eaux Roussillon project. Relying on a conceptual model of the whole groundwater system describing the main factors controlling the hydraulic behaviour of groundwater flow (recharge areas, surface-groundwater interactions, sea water intrusion risk,…) of Quaternary and Pliocene aquifers, a numerical modelling exercise of both onshore and offshore reservoirs was performed. Two parallel modelling approaches sharing a common description of the reservoir geometries, the boundary conditions and the pumping withdrawals locations were carried out, using two (not so) different modelling tools. Results comparison illustrates the uncertainty associated to numerical methods and calibration strategies applied for each modelling tool. Climate change projections were used to explore the future evolution of the groundwater resource and results of both modelling tools were compared. An analytical formulation of the increase of the hydraulic head of the sea due to swell and waves close to the coastline was provided. The analysis of its influence on the groundwater dynamics simulated by the models highlights the potential influence of storms and future sea level rise on the risk of saline intrusions for coastal sedimentary aquifers along the Mediterranean basin

Groundwater modelling of a complex multilayer coastal aquifer under climate change in the framework of the DEM'EAUX ROUSSILLON project

International audienceThe Plio-Quaternary aquifer of the Roussillon plain (multilayer aquifer of more than 350 m thick, made up of sandy layers embedded in low-permeability clay material from the Pliocene and topped by alluvial formations from the Quaternary) was extensively studied in the framework of the Dem’Eaux Roussillon project. Relying on a conceptual model of the whole groundwater system describing the main factors controlling the hydraulic behaviour of groundwater flow (recharge areas, surface-groundwater interactions, sea water intrusion risk,…) of Quaternary and Pliocene aquifers, a numerical modelling exercise of both onshore and offshore reservoirs was performed. Two parallel modelling approaches sharing a common description of the reservoir geometries, the boundary conditions and the pumping withdrawals locations were carried out, using two (not so) different modelling tools. Results comparison illustrates the uncertainty associated to numerical methods and calibration strategies applied for each modelling tool. Climate change projections were used to explore the future evolution of the groundwater resource and results of both modelling tools were compared. An analytical formulation of the increase of the hydraulic head of the sea due to swell and waves close to the coastline was provided. The analysis of its influence on the groundwater dynamics simulated by the models highlights the potential influence of storms and future sea level rise on the risk of saline intrusions for coastal sedimentary aquifers along the Mediterranean basin

Groundwater modelling of a complex multilayer coastal aquifer under climate change in the framework of the DEM'EAUX ROUSSILLON project

International audienceThe Plio-Quaternary aquifer of the Roussillon plain (multilayer aquifer of more than 350 m thick, made up of sandy layers embedded in low-permeability clay material from the Pliocene and topped by alluvial formations from the Quaternary) was extensively studied in the framework of the Dem’Eaux Roussillon project. Relying on a conceptual model of the whole groundwater system describing the main factors controlling the hydraulic behaviour of groundwater flow (recharge areas, surface-groundwater interactions, sea water intrusion risk,…) of Quaternary and Pliocene aquifers, a numerical modelling exercise of both onshore and offshore reservoirs was performed. Two parallel modelling approaches sharing a common description of the reservoir geometries, the boundary conditions and the pumping withdrawals locations were carried out, using two (not so) different modelling tools. Results comparison illustrates the uncertainty associated to numerical methods and calibration strategies applied for each modelling tool. Climate change projections were used to explore the future evolution of the groundwater resource and results of both modelling tools were compared. An analytical formulation of the increase of the hydraulic head of the sea due to swell and waves close to the coastline was provided. The analysis of its influence on the groundwater dynamics simulated by the models highlights the potential influence of storms and future sea level rise on the risk of saline intrusions for coastal sedimentary aquifers along the Mediterranean basin

Groundwater modelling of a complex multilayer coastal aquifer under climate change in the framework of the DEM'EAUX ROUSSILLON project

International audienceThe Plio-Quaternary aquifer of the Roussillon plain (multilayer aquifer of more than 350 m thick, made up of sandy layers embedded in low-permeability clay material from the Pliocene and topped by alluvial formations from the Quaternary) was extensively studied in the framework of the Dem’Eaux Roussillon project. Relying on a conceptual model of the whole groundwater system describing the main factors controlling the hydraulic behaviour of groundwater flow (recharge areas, surface-groundwater interactions, sea water intrusion risk,…) of Quaternary and Pliocene aquifers, a numerical modelling exercise of both onshore and offshore reservoirs was performed. Two parallel modelling approaches sharing a common description of the reservoir geometries, the boundary conditions and the pumping withdrawals locations were carried out, using two (not so) different modelling tools. Results comparison illustrates the uncertainty associated to numerical methods and calibration strategies applied for each modelling tool. Climate change projections were used to explore the future evolution of the groundwater resource and results of both modelling tools were compared. An analytical formulation of the increase of the hydraulic head of the sea due to swell and waves close to the coastline was provided. The analysis of its influence on the groundwater dynamics simulated by the models highlights the potential influence of storms and future sea level rise on the risk of saline intrusions for coastal sedimentary aquifers along the Mediterranean basin

Transdisciplinary characterisation of a complex coastal aquifer, for a sustainable exploitation of its groundwater resources in a Mediterranean context. The DEM'EAUX ROUSSILLON project

International audienceMore than 80 million m3 per year are pumped into the Roussillon plain coastal aquifer, covering 850 km² and located between the Pyrenean massif to the west and the Mediterranean Sea to the east, south of France. This is a multilayer aquifer of more than 350 m thick, made up of sandy layers embedded in low-permeability clayey material from the Pliocene and topped by alluvial formations from the Quaternary. Its groundwater resource is primarily used for the supply of drinking water, but also contributes to the irrigation of some 13,000 hectares. For more than 40 years, this aquifer has been undergoing a general decline in its piezometric level due to pumping and water demand is expected to increase (growing irrigation areas and climatic demand). Moreover, given its flat topography, the Roussillon plain is likely to suffer sea water intrusions and marine submersion, due to the sea level rise, which could reach 1 m by 2100. This context shaped the Dem'Eaux Roussillon project, which brought together nearly ten partners from the Occitanie region (research units, consultancies and local authorities). Its objective was to characterise the behaviour of the groundwater resource in this aquifer, in order to be able to project its future situation, in the context of climate change, rising sea levels (risk of saline intrusion) and changes in water use. A detailed characterisation of the geological reservoir highlighted the need to consider the offshore extension of this coastal aquifer. The analysis of the piezometric evolution at the scale of the Roussillon plain over the last 50 years allowed the spatialized characterization of the hydrodynamic parameters and the understanding of the vertical drainage processes that control the hydraulic equilibrium between the Quaternary and the Pliocene water tables. Two high-resolution hydro-geophysical observatories have been set up to quantify these processes and improve understanding of saline intrusions processes. Finally, a conceptual model presenting the main features of the main processes controlling the groundwater evolution and the sea water intrusion risk was obtained ready to launch a numerical modelling wor