The Etang de Thau is a large salt-water lagoon that covers 70km2 on the Occitanie coast, some 30km south-west of Montpellier and close to the town of Sète. At the bottom of the lagoon, at a depth of around 30m, there is an underwater spring, called the Vise spring.
Inversac, a rare flow-reversal phenomenon that threatens the quality of the spring's freshwater resources
Under normal circumstances, this spring draws groundwater from the karst formations of the Thau aquifer and feeds freshwater into the lagoon. However, in periods of drought, if a strong wind blows across the lagoon, the water flow can be reversed and the salt water from the lagoon then filters into the spring and contaminates the karst aquifer; this phenomenon is known as inversac.
This rare hydrogeological phenomenon, which the people who live around the Thau lagoon are very familiar with, leads to the gradual salinisation of the aquifer beneath the Etang de Thau.
However, it is crucial that the Thau aquifer be protected in order to maintain the ecological balance of the area, support shellfish farming, ensure the supply of drinking water and safeguard the spa centres in Balaruc-les-Bains.
What is the reverse flow (inversac) phenomenon?
Transcription
One of the challenges for the region and the DEM'Eaux Thau project is understanding the reverse flow phenomenon.
The Vise spring emerges at the bottom of the Etang de Thau, 30m underground and 100m from the coast of the Balaruc peninsula. It's a freshwater spring. It's water from the underground aquifer, a freshwater karst aquifer, that flows completely naturally. It emerges naturally. It is vital to the water quality in the lagoon, which is why this project, DEM'Eaux Thau, is so important. The Etang de Thau is located on top of impermeable rock, like many bodies of surface water in France. But here, there are two notable features. The 1st is that under this impermeable rock, there is a karstic aquifer that is under pressure. The 2nd is a geological fault in the Vise spring that connects this pressurized aquifer with the Etang de Thau. Water is exchanged in this fault, through the Vise spring, depending on pressure differences between the pond and the water table below it. When the aquifer, which is under pressure, has high water levels, after periods of rainfall, for example, groundwater flows in the Vise fault from the bottom up. Freshwater enters the pond from the aquifer. But when there's a storm on the pond and water levels rise very quickly, the pressure and water flow through the Vise fault reverse, and salt water seeps into the aquifer. We have counted 7 of these events in the last 50 years. We were fortunate, because in the history of hydrogeology, this is the 1st time that a reverse flow was monitored with so many measuring instruments. We were able to monitor the event that started in November 2020 and lasted nearly a year and a half, ending recently, in March 2022. Brackish water intrusion damages groundwater quality. In the past, it led to the closing of a water catchment that supplied water to the town of Balaruc. It also disrupts the balance in the pond, because normally, the pond gets freshwater from the spring. With the reverse flow phenomenon, the opposite happens, so freshwater is no longer supplied to the pond. One of the objectives of the DEM'Eaux Thau project was to better understand the reverse flow phenomenon in the Vise, so it was essential to have accurate data on the spring. That was why we used innovative, custom-built instruments to monitor the underwater spring. We had to develop custom instruments to measure the water flow in the spring, water levels in the pond, conductivity, mineralization and the spring's temperature. These data are recorded in real time and sent to a web platform so all stakeholders in the region can see them in real time. All of the data that were continuously acquired during the 2020 reverse flow phenomenon were used to consider different solutions to stop the reverse flow phenomenon or prevent it. Different solutions are now being studied and discussed with groundwater resource managers.
Understanding the inversac phenomenon thanks to a unique measurement system
As part of the Dem'Eaux Thau research project, and with the help of local partners, major financial investments were made and scientific resources employed to equip the spring with a measurement system that is one of a kind in the world. This system made it possible to monitor the inversac phenomenon and its impact on the karst aquifer, leading to a better understanding of how the aquifer actually functions.
Researchers from BRGM and the University of Neuchâtel (Switzerland) recently published the results of their work in the Nature Portfolio journal Communications Earth & Environment. They demonstrated that the key parameter driving changes in the hydrosystem's regime is the pressure difference between the aquifer and the lagoon, which is regulated by changes in the density of the salt water in the lagoon. Salt water is denser than fresh water and therefore exerts greater pressure.
In the article, the authors highlighted the existence of two tipping points:
- When the pressure difference between the aquifer and the lagoon becomes very low, or even negative, the hydrosystem suddenly deteriorates and shifts from its normal regime, thus enabling salt water intrusion.
- When this occurs, a much greater pressure difference is then required to return the system to its normal state. The denser salt water that has filled the vertical karst conduit acts as a plug, which explains why the inversacs last for a long time (several months).
With climate change, the reversal of flows in underwater karst springs could occur more frequently and last for longer periods of time in the future, due to rising sea levels and less abundant aquifer recharges, thus threatening freshwater supplies.
An innovative experimental system is being installed to stop the inversac phenomenon
An international collaboration with the Laboratory of Quantitative Hydrogeology at the University of Neuchâtel has made it possible to model and reproduce the physical mechanisms leading to an inversac. As a result, an early-warning system has been set up to anticipate the risk of future inversacs and was successfully tested during the last inversac episode in October 2023.
Following on from this work, an innovative experimental system is now being installed (July 2025). The aim is to stop the inversac phenomenon. Specifically tailored to the characteristics of the site, the system is made up of several modules, installed at a depth of 27 metres, at the point where the Vise spring flows into the lagoon. This will enable tests to be carried out with a view to reducing salt-water intrusion into the aquifer while maintaining natural freshwater flows.
This scientific programme, coordinated by the Syndicat Mixte du Bassin de Thau (SMBT) and BRGM, is supported by the French government (Green Fund, DREAL), the Occitanie Region, Sète Agglopôle Méditerranée and the town of Balaruc-les-Bains.
If successful, the results of this unprecedented three-year experiment could be transferred to other sites whose water resources are threatened by salinisation.
To find out more
Read the article
Communications Earth & Environment, Nature Portfolio: Hysteresis of submarine karst springs reveals tipping points in flow reversal and saline intrusion phenomena, by Jean-Christophe Maréchal, Bernard Ladouche, Claudine Lamotte, Benoit Dewandel, Vivien Hakoun, Pierre Perrochet
