Far and deep into the brackish groundwater of coastal karst: crossing CTD time series monitoring in submarine spring and during cave diving exploration (Port-Miou and Bestouan-SE France)

International audience ; Mediterranean coastal karsts are known to exist below the current sea level. They were initiated or enlarged when the sea level was lower, mainly during the quaternary, pliocene or messinian periods. Indeed, the sea acts as a boundary condition in the aquifer, imposing a base level that forces the groundwater to: 1) flow deeper during recession times, or 2) find a new outlet at a higher elevation during transgression when the sea level rises. The karst network is now submerged, and the Mediterranean coastal karst aquifers discharge usually in point source in springs close to the sea level. Moreover, coastal karstic aquifers are encroached by saline intrusion. Saline intrusion has the shape of a saltwater wedge, locally modified by preferential flow in karst conduits. SGD to the sea at springs is usually brackish, with a varying salinity correlated to the groundwater flow rate. This work aims to go insight into the saline intrusion observation in karst aquifer. We recorded salinity, temperature and depth (CTD) time series at the Port-Miou spring (France), and during cave diving exploration in large, long and deep karst conduit. Indeed, when karst conduits connected to a submarine spring are large enough to be explored by cave divers, the in-situ CTD time series recorded at the spring over time can be compared to the spatial variation of saline intrusion within the conduit. Mixing zones of seawater intruded and groundwater in the karst network of conduits are shown to be very deep, more than a depth of 233 meters below the sea level and more than 2km upstream the seashore. The case study proposed is located in south-east of France, near Marseille, and is one of the main submarine spring of Europe discharging to the Mediterranean Sea.