Reference, completed project

Dem'Eaux Thau: management of groundwater resources in a coastal karst aquifer

Where do the thermal waters of Balaruc-les-Bains come from? What is the source of the drinking water that supplies the city of Sète? Why are there changes in the flow of the submarine Vise spring? The goal of the Dem'Eaux Thau research project (2017-2022) was to improve our understanding of the structure and functioning of a complex karst hydrosystem in the Thau Basin area, west of Montpellier. It has enabled the development of groundwater management tools at different spatial and temporal scales.

24 May 2023

DEM'Eaux Thau: a tool for managing the region's groundwater

Where do the thermal waters of Balaruc-les-Bains come from? What is the source of the drinking water that supplies the city of Sète? The goal of the DEM'Eaux Thau research project was to improve our understanding of the structure and functioning of a complex karst hydrosystem in the Thau Basin area. Find out more about the general context of the study carried out between 2017 and 2022 to the west of Montpellier.

Objectives and needs

The underground water resources of the karst limestone formations of the Montpellier western fold are of major interest for the region, for the supply of drinking water (town of Sète, Syndicat Intercommunal d'Aduction en Eau Potable Balaruc-Frontignan, Syndicat Intercommunal d'Adduction d'Eau des communes du Bas Languedoc), but also for economic development, in particular shellfish farming/fishing in the lagoon, as well as for the spa on the Balaruc-les-Bains peninsula (the leading spa in France with more than 50,000 guests coming to "take the waters" in 2019), and irrigation. The site is located at the convergence of underground waters coming from various surface and deep reservoirs: cold karst waters of the Causses d'Aumelas and the Gardiole, waters of marine origin (lagoon and sea) and hot and mineralized thermal waters. These different reservoirs interact with each other through complex processes whose determinants are not all understood.

View (from left to right) of the Balaruc peninsula, the Etang de Thau, Sète and the Mediterranean (Mont Saint Clair, Sète, 2020).

This karst aquifer is subject to temporary brackish water intrusion phenomena (called “inversac”) via the submarine Vise spring in the Thau Lagoon, less than 200 m west of the Balaruc-les-Bains peninsula. Since the end of the 1960s, seven inversac phenomena have been detected. During this phenomenon, which can last several months (about 6 months in 2010 and 2014, and nearly 18 months in 2020-2022), the Vise spring, instead of supplying fresh water for the aquatic life of the Thau lagoon, absorbs brackish water from it, leading to progressive and recurrent salinisation of the aquifer. This phenomenon led, among other consequences, to the abandoning (in 2014) of the Cauvy spring in Balaruc-les-Bains as a supply of drinking water.

Thus, given the different issues at stake in this coastal area, the different stakeholders need a groundwater management tool.

What is the reverse flow (inversac) phenomenon?

At the bottom of the Thau Lagoon, there is a submarine spring, called La Vise. This spring usually supplies fresh water to the pond, which is extremely beneficial for aquatic wildlife. However, in the late 1960s, the spring began absorbing brackish water from the lagoon, and the phenomenon has occurred several times since then. This has led to the gradual salinisation of the aquifer. Our experts explain this phenomenon, known as inversac in French, which was studied as part of the DEM'Eaux Thau research project.

Work programme

This scientific study was conducted by a multidisciplinary team in order to characterise the geology, geomorphology, hydrodynamics, hydrogeology and geochemistry of the Thau hydrosystem. The researchers then pooled and discussed their different observations to build a comprehensive conceptual model of the functioning and dynamics of the aquifer. This was achieved in various ways by means of geophysical imagery, drilling a borehole in Sète and constructing a multi-borehole platform in Balaruc-les-Bains, including one shaft nearly 780 m deep, designing and installing custom-made equipment for the Vise spring, and unprecedented sampling campaigns (180 samples, more than 3,000 analyses).

Project outputs

The results of the various tasks carried out by the project partners are summarised in the form of reports or publications.

Focus on some of the results

The integration of all the historical geological data with the new data acquired by the project (stratigraphic, lithological and structural) and geophysical data (gravimetry, seismic reflection, passive seismic observations) made it possible to build a 3D geological model of the area covered by the Dem'Eaux Thau project, using the Geomodeller tool.

The study of the hydrogeological functioning of the Thau hydrosystem using the global modelling approach (with Gardenia software), on the one hand qualified the hydrogeological functioning of the karst system which developed in the Jurassic formation, and on the other hand, provided an assessment of the groundwater situation within the identified compartments of the hydrogeological system.

The geochemical analyses provided new insights into water circulation and mixing within Jurassic reservoirs and have led to a better understanding of water-rock interactions. The chemical, isotopic and dating information acquired during the project on the sampled sources and boreholes contributed to the revision of the conceptual scheme of the way the thermal reservoir functions.

A diagram illustrating the water supply to boreholes F8 and Sète/Stade Michel. The thermal waters (30% sea water, 70% karst water) rise from the thermal reservoir through the Thau fault. An additional contribution of salt water (+16% for F8 and +18% for Sète/Stade Michel), represented by the green arrows, comes from the Mediterranean and/or the leading edge of the Jurassic formation.

The understanding and modelling of the inversac phenomenon has enabled the development of a prototype tool to assess the risk of this phenomenon being triggered, based on various variables recorded on the Balaruc-les-Bains peninsula.

The list below will be updated with links to the documents as they are produced.

All the results of the DEM'Eaux Thau project

The goal of the DEM'Eaux Thau research project was to improve our understanding of the structure and functioning of the complex karst hydrosystem in the Thau Basin area. A particular focus was La Vise, the lagoon's underwater spring, which has been affected by the inversac phenomenon. The project involved creating a web platform to observe the aquifer and spring, a 3D hydrogeological model and an aquifer management tool. All the project’s results are now available for consultation.

Financing

The project has a budget of 5.3 million euros, 42% of which is financed by the Ministry of Higher Education and Research and the Occitania Region (as part of the 2015-2020 State-Region Plan Contract), 11% by the European ERDF fund, 17% by the Rhône-Méditerranée-Corse Water Agency, 4% by Montpellier Méditerranée Métropole, 2% by Balaruc-les-Bains and 1% by the Syndicat Mixte du Bassin de Thau. The remaining 23% of the project's funding comes from contributions made by most of the partners.