A closer look over the Thau hydrosystem, a new monitoring network to characterize and prevent seawater intrusion in the submarine Vise spring (Balaruc-les-Bains, France)

The Thau hydrosystem lies at the interface between terrestrial and marine (lagoon) environments along the Mediterranean coast. Together with this set up, the karstic nature of this hydrosystem makes it subject to localized seawater intrusion phenomena in the Balaruc-les-Bains peninsula. This phenomenon is linked to a sudden flow reversal at the submarine Vise spring, during which saltwater from the Thau lagoon invades the karst aquifer in its discharge area. This process and the nature of the aquifer have strong impacts on the local field of hydraulic heads (sudden rise up to 2m high) as well as on groundwater quality (rise of salt concentrations close to seawater content). The inversion phenomena has already been detected 7 times over the last 50 years. Yet, our current understanding of the triggering mechanisms remains limited, notably because of a lack of precise monitoring of groundwater fluxes at the lagoon – aquifer interface. To address this issue a new monitoring network (observatory) was installed at the submarine Vise spring and across the hydrosystem. Here, we present the observatory and we show unique first results from the detailed monitoring of a flow reversal phenomenon, which started in November 2020. Submarine spring flow, electrical conductivity and groundwater head time-series gathered both offshore and on land at different depth allowed following: the sudden rise of the water level in the wells and changes of temperature and salinity associated with the transport of salt water in the karstic aquifer. Overall, this observatory provides valuable data needed to characterize and understand the processes triggering the flow reversal phenomenon in a hydrosystem at the terrestrial – marine interface.