Located 30 kilometers southwest of Montpellier, near the town of Sète (Hérault), Etang de Thau is a brackish water lagoon of about 70 square kilometers connected to the sea. An underwater spring, called “la Vise”, whose outlet is 30 meters deep, normally supplies fresh water to this vast lagoon, thereby contributing to the health of the ecosystem and the quality of shellfish production. But along comes a gust of wind after a period of drought and the process is reversed. Salt water from the pond seeps through spring channels and pollutes the karst aquifer which is exploited by several surrounding areas.
This reversal phenomenon has been repeated eight times since the 1960s, and each time over a longer period of time. The year 2020-2022 lasts almost sixteen months. A team from the Geological and Mining Research Agency (BRGM) and the University of Neuchâtel (Switzerland) has documented this mechanism.
As part of the Dem’Eaux Thau project, coordinated by the Thau Basin Mixed Syndicate and BRGM, hydrogeologists first stepped up observations in the field, from 2017 to 2022. Drilling, coring, geophysical campaigns, geochemical analysis and deployment of experimental devices at the Vise outlet by divers made it possible to identify meteorological, climatic and hydrological conditions favorable for the hazard. And to offer managers a tool to assess the risks that trigger inversac. A possible link to climate change is one hypothesis.
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