Coastal aquifers characterization along a land sea continuum – AQUIMER

The recent identification of large quantities of fresh to low saline waters in coastal submarine sedimentary prisms extending under the sea (Offshore Freshened Groundwaters, OFG) requires to revisit our understanding of the dynamics of coastal aquifers, and to consider how these volumes of freshwater can be integrated into aquifer management. Knowledge of the submarine part of these systems is made difficult by their position at sea, which limits possible acquisitions. It is therefore necessary to set up 1) tools adapted to marine conditions to characterize the distribution of groundwater salinity in the offshore domain, and 2) an approach for integrating acquisitions and modeling to better observe and understand these distributions and their evolution. The AQUIMER project will study a potential large submarine aquifer, the Roussillon plio-quaternary aquifer (southern France), which is relatively well known onshore and extends several tens of kilometers offshore. AQUIMER will implement tools and methods to characterize this complex aquifer in terms of groundwater volumes, salinity distribution, circulation dynamics and mixing with seawater. To achieve these objectives, geophysical acquisition devices, on land and at sea, at different scales and resolutions, will be used to image variations in the electrical resistivity of the aquifer complex up to 15 km from the coast. In addition, AQUIMER will establish a new workflow to make the best use of resistivity as a proxy for the distribution of groundwater masses, and identify the position of its interface with seawater in its submarine part. A predictive geological model will integrate all available geological data to define the distribution of reservoir and lithological properties. The calibration of electrical resistivity signals from different facies and geological levels by onshore acquisitions on existing borehole observatories where geological and hydro-geophysical data are well constrained will enable a synthetic 3D resistivity model to be established from the geological model. This work will enable us to constrain sediment-related variations in the resistivity signal, so as to better individualize the signal related to pore water salinity. Finally, a hydrogeological model based on the geological model and paleoclimatic data will simulate the evolution of the aquifer over a long time span (at least since the last glacial maximum), including the emplacement of groundwater in the aquifer and the evolution of the mixing interface with seawater (including the trapping of offshore freshwater). This model will be used to build an electrical resistivity model (fluid and matrix) on which geophysical data acquisition will be simulated. The inversion of these data will be compared with measured resistivities to test the hypothesis of off-shore freshwater trapping and thus better constrain the dynamics of freshwater-saltwater flow and mixing in the system as a whole. This new workflow has the potential to be applied to similar shallow coastal aquifers around the Mediterranean and over many margins in the world.