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Over time, coastal aquifers, which constitute a great part of available freshwater resources from sedimentary basins in France, have been subjected to changes in hydraulic gradients and hydrodynamic properties, mainly due to discharge–recharge phases in response to sea-level variations and/or human influences. This work aims to understand the salinization process originating from the recharge–discharge conditions as recognized at three sites: the calcareous Dogger aquifers along the English Channel (Caen area); along the Atlantic coast (Marais Poitevin), and the Astian sandy aquifer (Cap d’Agde). In addition to conventional hydrogeological and hydrochemical techniques, the main tools used for investigation are those of isotope geochemistry.

For the three study sites, the evolution of isotopic signatures along a flow path, depending on the mineralogy of the aquifer matrix, is linked to water–rock interactions such as cation exchange and equilibrium with aluminosilicates. Residence times of these fresh groundwaters are from Modern (Atlantic site) up to the 14C detection limit (English Channel site). Groundwater of the Astian aquifer (Mediterranean) belongs to the Holocene, as determined by 14C analyses.

The saline waters identified in the three study sites have a marine origin and were modified either by interaction with organic-rich layers, by cation exchange, or by deep carbon input. The salinization process has been associated with marine overflow onto a plain and to an upward leakage of water rich in CO2. The marine intrusion registered in the English Channel and Atlantic aquifers is associated with the Flandrian transgression; for the Astian aquifer, the salinization is related to mixing with older water.

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