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 Vise spring is regularly subject to a phenomenon called inversac, where the freshwater spring suddenly begins to absorb the 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 have been able to observe it live. This also helps explain why certain nearby towns suffer flooding during these episodes, even in the absence of rainfall.
An exceptional phenomenon, called “inversac”, threatens thegroundwater resourcesaround the Thau pond (a vast brackish water lagoon) in Occitanie. Depending on weather conditions, the underwater spring of the Vise (locally nicknamed “the Volcano” or “the Chasm”) can stop – and then momentarily ceases to supply fresh water to the lagoon.
It thus becomes a major point of infiltration of saltwater from the lagoon into the aquifer, causing chronic salinization of the precious groundwater and episodes of rainless flooding in the spa town of Balaruc.
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 will help to better understand the causes of this until now mysterious phenomenon, which can last for months: six months for the episodes that occurred in 2010 and in 2014, and even eighteen months for the one that occurred between 2020 and 2022.
In coastal areas, groundwater outlets can be underwater
Rainwater that infiltrates the surface of the earth joins the aquifers. It then travels underground to outlets on the surface: springs, wetlands, and riverbeds are all low points in the landscape toward which groundwater converges.
Xerti,CC BY-SA
In coastal areas, groundwater outlets can be submarine, located in the sea or in coastal lagoons. Playing an important role for coastal marine ecosystems, they are diffuse or point-like 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 probably the best known and is observed by thescientistsfor many years.
Further west, in the Hérault, there are several other underwater springs that emerge in the Thau lagoon. Among them, the underwater spring of the Vise, located off the coast of Balaruc, drains aaquifercoastal primordial for the inhabitants of this region with a hot and dry Mediterranean-type climate.
Provided by the author
Indeed, groundwater is extracted there for the drinking water supply of nearby villages, for irrigation as well, but above all it feeds the thermal baths of Balaruc-les-Bains, the leading spa resort in France in terms of the number of spa-goers. 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 Vise spring is located in the Thau lagoon, at a depth of 30 meters off Balaruc. This depth is exceptional for the Thau pond, whose average depth rarely exceeds 4 to 5 meters: underground water flows have dug and eroded the rock to create a large underwater cone around the emergence. This depth attracts scuba diving enthusiasts who can thus practice their sport in the heart of a remarkable site.
The spring is located at the top of a subvertical karst conduit that connects the Thau lagoon to a confined aquifer (i.e., under pressure) located deep within the Jurassic limestones. The flow rate of the spring 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 water flow from origindeepwhich also supply the spa town of Balaruc.
However, since the late 1950s, there have been temporary episodeswater flow reversalat the level of the source. The result? A sudden interruption of the arrival of freshwater from underground origin, followed by an absorption of the lagoon waters by the source. In karst hydrology, this phenomenon is known as inversac, a term that designates a karst cavity that alternately absorbs or emits water, depending on the hydrological conditions.
Most of the time, inversacs are sinkflows located along a river: depending on the water levels, the river infiltrates into the karst cavity (loss mode) or the cavity discharges water into the river (emergence mode).
In the case of theTarget, the inversac occurs in connection with the Thau lagoon. Depending on hydroclimatic conditions, the source will sometimes discharge fresh water, sometimes absorb salty water from the lagoon.
Provided by the author
A unique measurement device in the world
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 flow rates of the submarine spring. It consists of a tube about five meters long and one meter in diameter, placed on the emergence, composed of three compartments.
Provided by the author
The lower tube collects fresh water flowing from the main springs (emergence points) located at the bottom of the lagoon. Above it, an electromagnetic flowmeter measures the vertical flow within the intermediate tube. It is topped with a calming tube designed to regulate water flows and reduce turbulence to ensure a good quality flow measurement. Temperature, salinity, and pressure sensors are installed in the device to supplement the data.measures.
A landslide observed live in November 2020
Installed in June 2019 by professional divers, it allowed the live observation of a flow reversal that occurred on the morning of November 27, 2020. Indeed, at exactly 9:40 AM, the source flow abruptly reversed: from an upward flow rate of about 120 l/s, it switched to a downward flow rate of 350 l/s.
Simultaneously, the sensors revealed the presence of a completely different water: salty and cold water (like that of the lagoon) replaced, inside the measuring tube, the fresh and warm water that previously came from the aquifer. At that moment, the source began to absorb the lagoon water, which progressively invaded the aquifer.
This reversal lasted sixteen months, during which approximately 7 million cubic meters of salty water 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 reversals necessitated the closure of the Cauvy groundwater extraction, which supplied drinking water to Balaruc-les-Bains.
The successive repetition of inversacs also threatens the thermal water resource, the use of which is important for the economic activity of the sector. Several market gardeners have also stopped their activities due to the salinization of their wells, while many 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 networkwas 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 inversion began 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 happened at the very moment when, on the contrary, the groundwater level was at its lowest, due to lack of precipitation and recharge. Result: the pressure exerted by the mass of salt water in the pond became greater 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 floods without rain
One local peculiarity remained to be explained: while the inversacs occur during drought periods, when the groundwater level is at its lowest, they are systematically accompaniedof floodsin the town of Balaruc-les-Bains. This is all the more surprising since no rainfall event precedesthese floods that affect basements, cellars, and underground parking lotsof the municipality, causing extensive damage.
This hydrological curiosity is to be compared with the measurements observed in thepiezometersof the surroundings: 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 the 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 salt water over its entire height, estimated at 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 in the confined aquifer over several kilometers within a few hours, causing a rise in water levels, and thus flooding, even in the absence of rain.
This unexpected rise in water levels explains the apparent contradiction between the suddenness of the initiaton of an inversac and its very long duration. If a gust of wind on the pond causing a rise in its level of a few centimeters can trigger an inversac, 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 salt intrusion, which then acts as a kind of “plug” on the submarine spring.
As a result, only very heavy rainfall on the Aumelas plateau, in the upper Thau basin, can recharge the aquifer sufficiently to cause an increase in level exceeding 2.3 m, capable of overcoming the salt plug. This is what happened on March 14, 2022, when a multi-day rain episode, exceeding 100 millimeters, ended this inversion.
Also to read:
How climate change disrupts groundwater recharge
Since the 1950s, inversacs have been recurring and accelerating. The cause is groundwater pumping, but above all thesuccession of droughts, which cause a decrease in the water table 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 riseA ?
It is not excluded that the system could definitively switch to inversac, thus causing a complete and permanent salinization of the Jurassic aquifer. This is why aexperimental projectis underway with the Mixed Syndicate of the Thau Basin to explore possible ways to reduce the effects of an inversac and to preserve the aquifer.
This study was able to be carried out within the framework of the research projectDEM’waters Thau(2017-2022).
With an amount of 5.3 million euros, the funding of the DEM’Eaux Thau project was secured 42% by the Ministry of Higher Education and Research and the Occitanie Region (within the framework of the State-Region Plan Contract 2015-2020), 11% by the European FEDER fund, 17% by the Rhône-Mediterranean-Corsica Water Agency, 4% by Montpellier Méditerranée Metropole, 2% by Balaruc-les-Bains, and 1% by the Mixed Syndicate of the Thau Basin. The remaining 23% of the project funding was provided through the financial participation of most of the partners.
Bernard Ladouche and Claudine Lamotte do not work for, do not advise, do not hold shares in, do not receive funds from any organization that could benefit from this article, and have declared no affiliation other than their academic positions.
–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