[LUM#5] In search of blue gold

[LUM Magazine] - Published on April 18, 2017 in Science-Society

It provides us with pure water straight from the tap. It protects us from flooding. Welcome to the karst, this underground Swiss cheese that conceals precious groundwater reserves, especially in our region.

A real treasure lies hidden beneath our feet. Miles of tunnels and cavities carved into the limestone form a complex and little-known network concealing an invaluable resource: blue gold in abundance.

How deep do these galleries extend? How much water is stored in the cavities? These are crucial questions for hydrogeologists exploring karst. "The geometry of these structures makes it possible to estimate the volume of water available, and their morphology can provide information about the dynamics of flow within karst networks,"explains Hervé Jourde, a researcher at the Hydrosciences Montpellier laboratory. "This is valuable information: we are talking about the water resources on which local populations depend," explains the specialist.

In the depths of the Earth

In France, more than 65% of the groundwater resources used to supply drinking water to the population come from karst formations. This is the case in Montpellier, where the water that supplies the city is pumped directly from karst conduits at the source of the Lez River. "This is very high-quality water on the city's doorstep, which covers 80% of the local population's needs," explains Hervé Jourde. Will there be enough resources to supply the growing number of inhabitants each year? Can it cope with the summer peaks in demand that are typical of the region? "To find out, we need to explore this deep network in order to estimate the available volumes,"replies the researcher ( Multi-scale evaluation of the hydrodynamic properties of a karst aquifer (Lez, France), in Hydrogeological and environmental research in karst systems, 2014).

These are genuine expeditions into the bowels of the Earth, led by cave divers who immerse themselves in total darkness to map the karst."Beyond 200 meters, divers can no longer continue, so we can use robots designed for these explorations," explains the hydrogeologist, who is collaborating with researchers from the Montpellier Laboratory of Computer Science, Robotics and Microelectronics (LIRMM) and the PlongéeSout diving association through the Aleyin project.

Limiting the severity of floods

These explorations are all the more important given that karst also plays a major role in the event of flooding. "When water levels in karst are low, precipitation is first stored in fractures and conduits, which limits the risk of flooding downstream," explains Hervé Jourde, who describes karst as "a real asset in limiting hydrological risk." A better understanding of the karst network would therefore make it possible to refine flood forecasts in the event of heavy rainfall. It could even protect us from rising water levels. "We could opt for strategic positioning of pumps to regulate groundwater levels and limit the severity of flooding," explains Hervé Jourde.

Keywords: water, LUM#5, LUM Magazine, Research