Dating tools applied to mineralized Eocene groundwater, for a sustainable management of the Entre-deux-Mers drinking water, Aquitaine Basin, South western France.
Résumé
In the south-west of France, the Eocene aquifer is one of the main resources for irrigation, thermo-mineral water, and mainly for drinking water in the Bordeaux region. This aquifer is characterized by the presence of a large mineralized area, centered on the Entre-deux-Mers region, between the Garonne and the Dordogne rivers, where the groundwaters show strong mineralization and anomalous levels of critical elements, such as sulphates and fluoride, leading to difficulties of resource exploitation for drinking water supply. Combined geochemical analysis (major and trace elements) and classical isotopic methods using δ18OH20 and δ2HH20, δ34SSO4 and δ18OSO4 is carried out. In addition, an innovative isotopic method using strontium isotopes (87Sr/86Sr) and more exploratory isotopic methods using boron (δ11B), lithium (δ7Li), uranium (234U/238U), thorium (230Th/234Th) and radium (228Ra/226Ra) isotopes are applied on the mineralized area. U/Th results are compared to former 13C /14C results. Moreover several 3H analyses have been performed on 11 samples, including one spring which age and original aquifer were unknown. These dating tools may help to better understand the complex circulations in the Eocene aquifer, of primary importance for drinking water, and its connections to the others aquifer layers. The results of this dating approach coupled with hydrodynamics models, hydrogeological knowledge and a better understanding of the deposit conditions and mineralization may help to better manage this strategic resource of drinking water.
Domaines
Sciences de l'environnement
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