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

The Dem'Eaux Thau project studies the hydrosystem in the Etang de Thau region. To that end, we need to know the surroundings, or the geological container, of these water flows and mixtures. The region is characterized by its geological evolution over a very long time, tens of millions of years, or even up to 100 million years. The other main feature of the region is that it is made up mainly of limestone that can be dissolved by meteoric water, which creates porosity and ducts through which water will flow. The region is covered in large limestone outcrops which serve as refilling zones. Water seeps in to a great depth, passing beneath areas covered with much more recent sediment, even the Etang de Thau itself, and the water flows deep down along dozens of kilometers before re-emerging from certain known springs, such as those at Vise and Issanka. Our study shows that this flow is on karst built up throughout geological history over tens of millions of years. From there, we need to understand how the flow is organized and how water masses mix within the system. The Thau hydrosystem is particularly complex. First, there is cold water at around 15°C, with low mineral content, from karstic massifs in the north of the hydrosystem. Then there is water that is highly mineralized, which comes from the sea and the Thau lagoon. Finally, there is thermal water, or warm water, at around 50°C, with high mineral content, although less than seawater, which rises up to the Balaruc-les-Bains peninsula. As part of the Dem'Eaux Thau project, a geological model was produced whose structure was transferred to a hydrogeological model simulating flows throughout the relevant area. This model identifies the routes taken by water in the aquifer, which is important to determine where water comes from in various locations. The model simulates how the aquifer fills up in high and low periods. Then, over a period of time, we can also use the model to run simulations. Here we see comparisons between simulated values in red and observed values in green. We see that the dynamic of aquifer filling over time corresponds pretty well, which gives us confidence in the model's calibration. The tool provides us with a better understanding of water flow in the aquifer, enabling a better assessment of the aquifer's water resources. After making field observations and setting up a model, what comes next? As part of the Dem'Eaux Thau project, with help from Synapse Informatique and the support of BRGM and SMBT, we developed a platform that collects data in real time, which is useful for managing the western aquifer. It contains all the data from wells, water levels, river levels and water flow in springs. It allows us to share useful data for the region. By way of example, for the Vise spring, in Balaruc, I can access the entire flow data history, with comparisons, histories and data from as recently as today. Having real-time data to share is a new thing. Thanks to what we've learned from this project about how the reverse flow phenomenon is triggered, our teams developed a forecasting tool to try and build scenarios to assess exactly when one could be triggered. On this platform, managers will find several parameters: groundwater and lagoon levels, temperature, salinity and various scenarios. Collecting prospective data on climate and environmental changes will enable them to assess when the reverse flow will be triggered. Being able to make those forecasts is a new thing. Dem'Eaux Thau is interesting because groundwater levels in the west can be analyzed in real time, which allows us first to anticipate the reverse flow phenomenon we've seen recently and predict and anticipate resource sharing between different uses. For the Local Water Commission, this in-depth knowledge is very important. Thanks to this detailed knowledge, we are now ready to take action, anticipate and plan for the future.