The Upside-Down Spring: exploring the Inversac of Vise
Submarine spring, flow reversal and saltwater intrusion
An Exceptional Submarine Spring
The Vise submarine spring, located in the Thau Lagoon on the Mediterranean coast of southern France, is one of the most remarkable karst groundwater outlets in the region. Under normal conditions, groundwater flows from the karst aquifer towards the lagoon. However, under particular hydrological conditions, this natural flow can temporarily reverse.
[Picture]
Geological map of the study area. Location of the karst springs (including the Vise submarine karst spring), meteorological station and observation boreholes network.
![[Picture]](https://spaces-cdn.owlstown.com/blobs/z5imp5ob6pnojlenjykis8xwyvqq.jpeg){kind=link}
What Is an Inversac?
An inversac is a reversal of the hydraulic flow through the spring. Instead of freshwater discharging from the karst aquifer into the lagoon, lagoon water penetrates into the karst system. This spectacular reversal transforms the spring from a groundwater outlet into an entry point for surface water.
The Discovery and Observation of the Phenomenon
The investigation of the inversac phenomenon required combining hydrogeological observations, hydrodynamic measurements, water chemistry and environmental monitoring. These interdisciplinary approaches and the setup of a specific monitoring device made it possible to identify and better understand the conditions under which the normal functioning of the spring is reversed.
[Picture]
Vise submarine karst spring monitoring system. a 2D map with bathymetry of the Vise Spring, b schematic 2D vertical cross-section, c monitoring device, d in situ pictures of the monitoring device.
![[Picture]](https://spaces-cdn.owlstown.com/blobs/94xxbg5fsfnf4ss0gt6wriqd31pg.jpeg){kind=link}
[Picture]
Time-evolution of the main hydrological variables from 25/09/2019 to 31/12/2022. Flowrate, water electrical conductivity and water temperature measured at the Vise submarine spring.
![[Picture]](https://spaces-cdn.owlstown.com/blobs/vuozii67bnk20s9e4w4vcxz77svs.jpeg){kind=link}
Hydrogeological Mechanisms
- Hydraulic gradients: The reversal is controlled by changes in the hydraulic balance between the karst aquifer and the Thau Lagoon.
- Groundwater levels: Variations in groundwater pressure can reduce or reverse the hydraulic gradient normally driving freshwater towards the lagoon.
- Meteorological and hydrological forcing: Atmospheric pressure, wind conditions, lagoon water levels and recharge conditions may all contribute to triggering an inversac.
Scheme of 2D hydrogeological cross-section at the Vise submarine karst spring.
Normal flow regime: high water table with upward flow from the aquifer to the lagoon
Flow reversal regime: drought period (low water table) with high water level in the lagoon induces downward flow from the lagoon to the aquifer.
Revealing tipping points
Two tipping points are revealed by our studies:
- TP1 — inversac (flow reversal) trigger tipping point: the threshold at which the hydraulic gradient becomes insufficient to maintain freshwater flow toward the lagoon. The flow reverses, and saltwater enters the aquifer. At Issanka control point, the associated threshold is approximately 6.13 m.
- TP2 — tipping point for return to normal: the hydraulic threshold that must be reached to restore freshwater flow toward the lagoon following an flow reversal event. This threshold is higher - approximately 10.25 m at Issanka - due in particular to the column of saltwater present in the karst conduit.
The difference between the two thresholds reflects the system's hysteresis: returning to normal conditions requires more favorable hydraulic conditions than those that prevailed just before the inversac was triggered.
[Picture]
Dynamic hydrosystems states : Vise karst hydrosystem state in a diagram of flowrate at the spring (state variable) versus hydraulic gradient between aquifer at Issanka well and lagoon (control parameter).
![[Picture]](https://spaces-cdn.owlstown.com/blobs/zwe2dgxr5b497mf7n0a14pz7rm8z.jpeg){kind=link}
A Natural Laboratory for Studying Karst Systems
The Vise submarine spring provides a unique natural laboratory for studying interactions between groundwater and surface water in coastal karst environments. The inversac phenomenon demonstrates that the boundary between aquifers and surface-water bodies can be highly dynamic.
😮
The Salt Plug: Flooding Without Rain
Each inversac triggers an unexpected groundwater rise of about 2.3 m, flooding basements and underground car parks in Balaruc-les-Bains—even without rainfall.
The cause is the density contrast between lagoon water and freshwater. About 3% denser, saltwater rapidly fills the 70-m-high vertical karst conduit, increasing pressure on the confined aquifer by the equivalent of 2.3 m of water. This pressure wave propagates several kilometres within hours, raising groundwater levels and causing floods.
This pressure increase also explains why inversacs can persist for months. Saltwater acts as a “plug”, preventing freshwater from flowing out again. Even when the wind calms, the aquifer must overcome this additional 2.3 m of pressure to restore the normal flow direction.
Only heavy rainfall can break the salt plug. Recharge must raise the aquifer's water level by more than 2.3 m. This happened on March 14, 2022, when several days of rain delivered over 100 mm of precipitation, finally ending the inversac.
The Consequences of Flow Reversal
- Saltwater intrusion into the karst system
- Changes in groundwater chemistry and modification of mixing processes within the aquifer
- Flooding of basements and underground car parks
- Potential long-term impacts on groundwater quality and salinity
During an inversac event, the lagoon does not simply receive groundwater: it becomes a source of water entering the karst system. This reversal has important implications for water quality, solute transport and the functioning of the coastal aquifer.
Beyond its scientific curiosity, the inversac of the Vise spring raises fundamental questions about the vulnerability of coastal groundwater resources. As coastal environments are increasingly affected by climatic variability, sea-level change and human pressures, understanding reversible exchanges between groundwater and surface water becomes increasingly important
Related publications
Jean-Christophe Maréchal, Bernard Ladouche, Claudine Lamotte, Benoit Dewandel, Vivien Hakoun, Pierre Perrochet
Communications Earth \& Environment, vol. 6, 2025 Aug, p. 697
Bernard Ladouche, Jean-Christophe Maréchal, Claudine Lamotte, Vincent Durand, Vincent Bailly-Comte, Vivien Hakoun
Data in Brief, vol. 50, 2023 Oct