Source: French to English Tester Published on: 2026-05-07
Source: The Conversation – France in French (2)– By Jean-Christophe Maréchal, Research Director – Hydrogeologist, BRGM
At the bottom of the Thau lagoon, in Occitanie, the source of the Vise is regularly subject to a phenomenon called “inversac,” where the freshwater spring suddenly starts to absorb brackish water. This mechanism, until recently little known, exposes the freshwater resources of the coasts to a risk of salinization. Thanks to a unique experimental setup, French researchers were able to observe it live. Which also helps explain why certain nearby towns suffer from flooding during these episodes, even in the absence of rain.
An exceptional phenomenon, called “inversac”, threatens thegroundwater resourcesaround the Thau pond (a vast brackish water lagoon) in Occitanie. Depending on the weather conditions, the underwater spring of the Vise (locally nicknamed “the Volcano” or “the Chasm”) can stop – and then momentarily stops bringing fresh water into the lagoon.
It thus becomes a major infiltration point of saltwater from the lagoon into the aquifer, causing chronic salinization of the precious groundwater and episodes of flooding without rain in the Balaruc thermal spa.
With colleagues, we were able to follow the phenomenon live thanks to specific instruments, a world first, which was notably the subject of ascientific publicationin 2025. This allows for a better understanding of the causes of this hitherto mysterious phenomenon, which can last for months: six months for the episodes that occurred in 2010 and 2014, and even eighteen months for the one that occurred between 2020 and 2022.
In coastal areas, the outlets of groundwater can be underwater
Rainwater that infiltrates the earth’s surface joins the aquifers. It then circulates underground to surface outlets: springs, wetlands, and riverbeds are all low points in the landscape towards which groundwater converges.
Xerti,CC BY-SA
In the coastal zone, the outlets for groundwater can be submarine, located in the sea or in coastal lagoons. Playing an important role for coastal marine ecosystems, they are diffuse or point-source depending on the type of aquifer. In the latter case, most often in contextkarstic, these are underwater sources. They are particularly numerous in the Mediterranean thanks to the strong presence of limestone rocks conducive to karst phenomena.
In France, theunderwater source of Port-Miou, offshore from the cove, is undoubtedly the best known and is observed by thescientistsfor many years.
Further west, in Hérault, there are several other underwater springs that emerge in the Thau lagoon. Among them, the underwater spring of the Vise, located off Balaruc, drains aaquiferprime coastal area for the inhabitants of this region with a hot and dry Mediterranean-type climate.
Provided by the author
Indeed, the groundwater there is extracted for the drinking water supply of neighboring villages, for irrigation as well, but above all it feeds the thermal baths of Balaruc-les-Bains, the leading thermal spa in France by number of visitors. Unfortunately, this precious freshwater resource is threatened by an exceptional phenomenon: the “inversac.”
The inversac, or when water flows reverse and a spring absorbs brackish water
Well known to fishermen and oyster farmers, the source of the Vise is located in the Thau lagoon, at a depth of 30 meters off the coast of Balaruc. This depth is exceptional for the Thau pond, whose average depth rarely exceeds 4 to 5 meters: groundwater flows have dug and eroded the rock to create a large underwater cone located around the emergence. This depth attracts scuba diving enthusiasts who can thus practice their sport in the heart of a remarkable site.
The source is located at the top of a subvertical karst conduit that connects the Thau lagoon to a confined aquifer (that is, under pressure) located deep within the Jurassic limestones. The flow rate of the source is generally around 100-150 liters per second (l/s), and increases with the water level in the aquifer after heavy rains. It is fresh and warm water (20 °C), as it is heated by the flow of water from origin.deepwhich also supply the Balaruc thermal spa.
However, since the late 1950s, temporary episodes have been observedreversal of water flowsAt the source level. The result? A sudden interruption of the supply of freshwater of underground origin, followed by an absorption of the lagoon waters by the source. In karst hydrology, this phenomenon is known as “inversac,” a term designating a karst cavity that is alternatively absorbent or emissive, depending on hydrological conditions.
Most of the time, inversacs are losing streams located along a river: depending on the water levels, the river infiltrates into the karst cavity (losing condition) or the latter discharges water into the river (gaining condition).
In the case of theAim, the inversac occurs in connection with the Thau lagoon. Depending on hydroclimatic conditions, the source will sometimes discharge fresh water, sometimes absorb the salty water from the lagoon.
Provided by the author
A world-unique measurement device
To better understand this phenomenon, the hydrogeologists of BRGM, in collaboration with the company ANTEA, have designed an unprecedented device specifically dedicated to measuring the flows of the submarine spring. It is a tube approximately five meters long and one meter in diameter, placed on the emergence, composed of three compartments.
Provided by the author
The lower pipe collects fresh water exiting from the main springs (emergence points) located at the bottom of the lagoon. Above, an electromagnetic flowmeter measures the vertical flow within the intermediate pipe. It is topped by a calming tube designed to regulate water flows and reduce turbulence to ensure good flow measurement quality. Temperature, salinity, and pressure sensors are installed in the device to complete themeasures.
An inversac observed live in November 2020
Installed in June 2019 by professional divers, it allowed the live observation of a reversal that occurred on the morning of November 27, 2020. Indeed, at exactly 9:40 a.m., the source’s flow abruptly reversed: from an upward flow of about 120 l/s, it switched to a downward flow of 350 l/s.
Simultaneously, the sensors revealed the presence of a completely different water: a salty and cold water (like the lagoon) has replaced, inside the measurement tube, the fresh and warm water that previously came from the aquifer. At that moment, the source began to absorb water from the lagoon, which gradually invaded the aquifer.
This backflow lasted sixteen months, during which approximately 7 million cubic meters of saltwater infiltrated, causing the intrusion of 200,000 tons of salt into the aquifer. In 2014, the progressive salinization of the water table caused by the succession of backflows necessitated the closure of the Cauvy groundwater intake, which supplied drinking water to Balaruc-les-Bains.
The successive repetition of inversacs also threatens the thermal water resource, whose use is important for the economic activity of the sector. Several market gardeners have also stopped their activity due to the salinization of their wells, while numerous domestic boreholes have been affected. Salt has also risen into the soils, threatening green spaces by weakening a large number of trees in the municipality.
In addition to the measures at the source of the Vise, aobservation networkhas been installed to monitor the water levels of the lagoon and the aquifer.
The detailed analysis of all these data made it possible to show that the reversal occurred at a very particular moment, when a marine storm accompanied by a marine gust caused a rise in the level of the pond. And this, at the very moment when, on the contrary, the level of the aquifer was at its lowest, due to lack of precipitation and recharge. Result: the pressure exerted by the salty water mass of the pond became higher than the pressure of the aquifer: the water flows then suddenly reversed. This is the mechanism that we described in our article published in the journalNature Communications Earth and Environment.
“Salt plug” and flooding without rain
There remained to explain a local peculiarity: while the inversacs occur during dry periods, when the water table level is at its lowest, they are systematically accompaniedof floodingin the town of Balaruc-les-Bains. This is all the more surprising given that no rain episode precedesthese floods that affect basements, cellars, and underground parking lotsof the municipality, causing extensive damage.
This hydrological curiosity should be compared with the measurements observed in thepiezometersabout: each inversac is followed by a rapid increase in groundwater levels of about 2.3 m. This rise is explained by the density contrast between waters: the lagoon’s saltwater is about 3% heavier than the freshwater.
Thus, at the moment of the inversac, the vertical karst conduit fills in a few minutes with saltwater over its entire estimated height of about 70 m. This results in a sudden increase of 2.3 m in the pressure exerted by the lagoon on the aquifer. This pressure wave then rapidly propagates through the confined aquifer over several kilometers within a few hours, causing a rise in water levels, and therefore floods, even in the absence of rain.
This unexpected rise in water levels explains the apparent contradiction between the sudden onset of a reversal and its very long duration. If a gust of wind over the pond causing its level to rise by a few centimeters can trigger a reversal, a meteorological lull is not enough for the system to return to its initial normal state. To restore upward flows in the karst conduit, the pressure of the aquifer must overcome this overpressure of 2.3 m caused by the intrusion of salt, which then acts like a kind of “plug” on the submarine spring.
As a result, only a very heavy rain on the Aumelas plateau, on the heights of the Thau basin, can sufficiently recharge the aquifer to cause a level increase greater than 2.3 m, capable of overcoming the salt plug. This is what happened on March 14, 2022, when a rainy episode lasting several days, exceeding 100 millimeters, ended this inversac.
Also to read:
How climate change disrupts groundwater recharge
Since the 1950s, inversacs have been repeating and accelerating. The cause is groundwater pumping, but above all thesuccession of droughts, which cause a drop in the groundwater level. Until now, the system always returns to its normal state after a few months, but what will happen in the future when natural recharge declines andthat the sea level will rise ?
It is not excluded that the system definitively shifts to inversac, thus causing a complete and permanent salinization of the Jurassic aquifer. This is the reason why aexperimental projectis underway with the Mixed Syndicate of the Thau basin to explore possible ways to reduce the effects of a reversac and to preserve the aquifer.
This study was able to be carried out as part of the research projectDEM’eau Thau(2017-2022).
With an amount of 5.3 million euros, the funding of the DEM’Eaux Thau project was provided 42% by the Ministry of Higher Education and Research and the Occitanie Region (within the framework of the 2015-2020 State-Region Planning Contract), 11% by the European FEDER fund, 17% by the Rhône-Méditerranée-Corse Water Agency, 4% by Montpellier Méditerranée Metropolis, 2% by Balaruc-les-Bains, and 1% by the Mixed Syndicate of the Thau Basin. The remainder of the project funding (23%) was contributed thanks to the financial participation of most partners
Bernard Ladouche and Claudine Lamotte do not work for, advise, own shares in, or receive funds from any organization that could benefit from this article, and have declared no affiliation other than their academic position.
–ref. The Thau inversion, or when an underwater spring absorbs salty water and threatens groundwater –https://theconversation.com/linversac-de-thau-or-when-an-underwater-spring-absorbs-salty-water-and-threatens-groundwater-281672