Présentations Colloques

Coastal aquifers constitute vulnerable water resources threatened by the increasing concentration of population density on the coast worldwide and thus the related water demand. Freshwater stored in coastal aquifers is particularly susceptible to degradation because of seawater proximity. Seawater intrusion, e.g. saline water intrusion into freshwater aquifers, especially threats groundwater resources because of enhanced pumping for water supply, change of land-use, irrigation and industrial activities, in addition to climate variations or sea-level fluctuations. **Here we present a multi-isotope approach (stable isotopes of the water molecule, strontium isotopes, boron isotopes and sulfur and oxygen of sulfates) to constrain salinization sources and processes in two coastal multilayer aquifers. The first one is located in Brazil, in the city of Recife where groundwater is overexploited in some areas as complementary resource for water supply. Results highlight that the saline component in deep strategic aquifers is partly inherited from the Pleistocene marine transgression with very low groundwater renewal. Present day marine intrusion only occurs in very limited areas on the seashore were overpumping leads to reverse the natural groundwater flux, inducing seepage from saline groundwater from the surficial aquifer. The second one is located in South of France in the coastal part of the Roussillon sedimentary basin seasonally facing increase of water abstraction for tourism. Salinization sources and processes were investigated in detail along a 120 m vertical profile using a Westbay monitoring system located a few hundred meters from the coast (sea and lagoon). Groundwater sampling in both high and low productive layers allows pointing out the high diversity of geochemical processes involved+ there is no evidence of present day seawater intrusion neither of present day (>1950) recharge within the system containing paleo-groundwater with a remaining paleo-seawater.**