Assessing drilling location in karstic coastal aquifer through approach inspired from common risk segment (CRS) mapping

Coastal karstic aquifers represent major challenges for the groundwater resources. With the growing interest in alternative energy, they represent also valuable assets for geothermal exploitation or carbon storage. The characterization of such aquifers is difficult due to their intrinsic complexity (karstification and structuration) and natural or induced saline intrusion processes. This study presents a workflow inspired by the CRS mapping techniques used in oil and as exploration to compute risk map for the presence of salt and freshwater aquifer. To do so, it is proposed to decompose the hydrogeological system into four main elements: (1) equivalent permeability at the geological unit scale, (2) geological unit thickness, directly linked to the storage capacity, (3) groundwater level that enables the definition of unsaturated zone, and (4) saline intrusion. The equivalent permeability has been assessed through the determination of karst conduit functionality, fracture intensity and effective porosity. The geological unit structure has been reproduced from structural cross-section in a 3D geological numerical model. Groundwater level and saline intrusion have been determined from field campaign and well monitoring. Each element has been summarized as a CRS map and then combined as a CCRS to define the best location for a future well drilling.