On Wednesday and Thursday, the study of the Public Territorial Basin Establishment (EPTB) of Vidourle entered the field phase, with the injection of dyes at three strategic points, in tributaries of the Vidourle, and the installation tracking devices. Chance of the calendar, the cave divers had chosen the same afternoon to go up from the resurgence of the Vidourle to the Grand aven de Sauve by the karst with a Navscoot, an instrument full of technology which allows the mapping of the conduits.
The EPTB has launched the markets, the service providers have been chosen, the speleologists have started to share their data… The field work can begin. As part of the study conducted by the manager of the Vidourle basin (reread here), two days of implementation of monitoring and injection of tracings took place, Wednesday and Thursday, between Sauve and Pompignan. “We inject a little coloring to fully understand the impact on the drinking water supply withdrawalsexplains Marie Savéan, in charge of water resources at the EPTB Vidourle. We want to see how water flows from three different places.”
For the moment, therefore, it is only a question of “micro-tracings, to set larger tracings to understand the delimitation between the Vidourle and Lez basins”, explains Guilhem Mestre, in charge of logistics at the Cenote consulting firm, which installed the instruments for monitoring these tracings. Three sites were chosen for the injection, “the Bégué-Ponchon sinkhole, in the town of Pompignan, identifies Marie Savéan, as well as the upstream loss and the downstream loss of the Artigues stream”. Either the entry into the Rieumassel, a direct tributary of the Vidourle, for this last tracing. “We want to see how it circulates from these three different places by analyzing for a month whether the tracking devices detect the coloring”continues Marie Savéan.
Devices that Cenote was responsible for installing before the injection of coloring products at the Lacan catchment, which supplies Pompignan with drinking water, in the Grand aven de Sauve and at the drinking water catchment of Sauve, a source taken before it throws into the Vidourle, downstream of its resurgence. “We inject three products”, explains Guilhem Mestre. The best known being fluorescein, green, but also another red dye and another “detectable but not visible. Before, we put activated carbon in the developers. Today, the instruments measure the concentration of dyes at the level of the milligram per liter. They detect fluorescence on a certain wavelength.”
“In the Great Avencontinues Guilhem Mestre, we installed a probe, connected to a box, which records the data.” Under the metal slab of the Sauve drinking water supply, it was a question of seeing, once the installation was done, if the box managed to communicate with the outside via its small antenna. “We have a light that notices fluorescence, that takes readings every fifteen minutes, plus a mechanism that puts water in a bottle every 24 hours.” If the teams were afraid of running out of water at one of the losses to start the tracing correctly, carrying out the tracing during the low water period is no coincidence. “In autumn, argues Marie Savéan, we have too high water and the tracings are lost.” “Here, with the flow rate and the concentration, we will know if we have recovered all the dye”pushes Guilhem Mestre.
“Almost two kilometers to the Grand Aven”
Frank Vasseur, speleologist and diver
Coincidence of calendars and possible appointments, another team was busy, Wednesday afternoon, next to this small world centered on the capture point. The speleologists Frank Vasseur and Damien Vignoles had planned an ascent from the resurgence of the Vidourle to the Grand Aven by underground route, using the Navscoot (see box). “There are almost two kilometers to the Grand Aven”explains Frank Vasseur. “It takes about three hours to go there and back, anticipates Damien Vignoles, customary for the journey. We are going to rescan the walls, from here to the Grand Aven, to map it.” “And so that hydro-geologists can also use the datacontinues Doriane Morata, also a speleologist but remained on the bank this time. The preliminary work was to remove the previous passage wires before the passage of the Navscoot.”
The Navscoot, to map the karstic bottoms
There are only two in the world, including one developed in Montpellier. “The research was funded by the Region, within the LIRMM (computer science, robotics and microelectronics laboratory of Montpellier), with the companies Syera and Reeds, explains Benoît Ropars, specialist in microelectronic systems at Reeds. The objective is to do 3D mapping and reconstruction.” If the Region has shown interest in financing the project, this is due “to the interest of development and the problems of water in the region”. And to the karstic terrain, very widespread in Occitania, especially in its former departments of Languedoc-Roussillon. The machine is made up of “a whole series of sensors, continues Benoît Ropars. We have a vertical sonar and another horizontal, a system to measure the speed, another which makes it possible to store the parameters and the measurements of the sonar, and a central inertial to measure angles of inclination.” While the Navscoot has undergone many improvements since it was first launched more than five years ago, a new robot version “is being developed in Montpellier, in order to embed all the system sensors on an autonomous robot “. And who could therefore carry out explorations without human presence. Even if we should always be able to find a volunteer speleologist to accompany him…
Always voluntary, this additional crossing will deliver other information to the EPTB Vidourle on the underground dynamics of the river, even if the public body was not the sponsor of the day. With a downside, quickly recalled by Damien Vignoles, “the part of the karst, penetrable by man, remains negligible in karstic environments”. Micro-fissures will therefore continue to keep the secrets they hide from view. Exploration and coloring will therefore not prevent a part of mystery from remaining.