Underwater spring in the Thau Lagoon (source of the Vise) (Hérault).
© BRGM
Inversac, or when water flow reverses and a spring absorbs brackish water
Rainwater infiltrates the earth's surface and flows into aquifers. It then flows underground to surface outlets: springs, wetlands and riverbeds are all low points in the landscape towards which groundwater converges.
In coastal areas, groundwater outlets may be underwater, located in the sea or in coastal lagoons. As they play an important role in coastal marine ecosystems, they are either diffuse or discrete depending on the type of aquifer. In the latter case, most often in a karstic context, these are underwater springs. They are particularly numerous in the Mediterranean thanks to the presence of limestone rocks that are ideal for karstic phenomena.
In the Hérault department, several underwater springs emerge in the Thau lagoon. These include the underwater spring of La Vise, located off the coast of Balaruc, which drains a coastal aquifer of vital importance to the inhabitants of this region with its hot, dry Mediterranean climate. Groundwater is drawn from here to supply drinking water to neighbouring villages and for irrigation, but above all for the thermal spa at Balaruc-les-Bains, France's leading establishment in terms of the number of people taking the waters. Unfortunately, this precious freshwater resource is threatened by an exceptional phenomenon called the inversac.
The Vise spring is located at the top of a subvertical karstic conduit that connects the Thau lagoon to a captive aquifer, that is, under pressure, deep in the Jurassic limestone. Under normal circumstances, this spring drains groundwater from the Thau karstic aquifer and supplies freshwater to the lagoon.
However, since the end of the 1950s, there have been temporary episodes of reversed water flows. In periods of drought, after a strong wind blows over the lagoon, the water flows can reverse, and the salt water from the lagoon seeps through the spring to contaminate the karstic aquifer. This is the inversac phenomenon.
In the case of the Vise, the inversac is linked to the Etang de Thau. Depending on the hydroclimatic conditions, the spring will either discharge fresh water into or absorb salt water from the lagoon.
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.
Measuring device installed at the source of the Vise. (a) Position of the device on the griffin at the underwater source of the Vise, (b) detailed view of the device.
© BRGM - Jean-Christophe Maréchal
A unique measurement system
To gain a better understanding of this phenomenon, BRGM hydrogeologists, in collaboration with ANTEA, have designed a unique device for measuring the flow of the underwater spring. This is a tube placed on the emergence, made up of three compartments.
The lower tube collects the fresh water issuing from the main risings (emergence points) on the floor of the lagoon. Above, an electromagnetic meter measures the vertical flow within the intermediate tube. It is surmounted by a stilling tube designed to regulate the water flow and reduce turbulence in order to ensure good quality discharge measurement. Temperature, salinity and pressure sensors are installed in the device to complete the measurements.
“Salt plugs” and floods without rain
What remained to be explained was a local quirk: while the inversacs take place during periods of drought, when the water table is at its lowest, they are systematically accompanied by flooding in the town of Balaruc-les-Bains. This is all the more astonishing given that no rainfall episode precedes these floods, which hit the town's basements, cellars and underground car parks, causing extensive damage.
Each inversac is followed by a rapid rise in groundwater levels of around 2.3 m. This rise is explained by the difference in density between the waters: the salt water in the lagoon is around 3% heavier than the fresh water. Consequently, at the moment of inversion, the vertical karstic conduit fills with salt water over its entire height, estimated at around 70 m, in just a few minutes. The result is a sudden 2.3 m increase in the pressure exerted by the lagoon on the aquifer. This pressure wave then spreads rapidly through the confined aquifer, covering up to several kilometres in a matter of hours, causing water levels to rise, leading to flooding, even when no rain has fallen.
To restore upward flow in the karstic conduit, the pressure of the aquifer must overcome the 2.3 m overpressure caused by the salt intrusion, which acts as a sort of “plug” on the underwater spring.
Since the 1950s, the inversacs have recurred more frequently. This is due to the pumping of groundwater, but above all to the succession of droughts, which cause the water table to drop. So far, the system always returns to normal after a few months, but what will happen in the future when natural recharge is likely to decline and sea levels rise?
It is possible that the system might then switch definitively to inversac, causing complete and definitive salinisation of the Jurassic aquifer. To forestall this, an experimental project is under way with the Syndicat Mixte du Bassin de Thau to explore possible ways of reducing the effects of an inversion and preserving the aquifer.
