All underground aquifers (groundwater and karst) represent 99% of liquid freshwater reserves on Earth, according to our partner
The Conversation
.
In Meurthe-et-Moselle, the Aroffe is a river that plays hide and seek according to the seasons and the weather.
Its underground course with a potential of at least 30 kilometers is currently only explored over 2.5 kilometers.
This analysis was conducted by
Elise Chenot
, teacher-researcher in geology at the UniLaSalle Polytechnic Institute, and
Christophe Prévot
, professor of mathematics at the University of Lorraine.
Streams are born from the accumulation of rainwater runoff, then grow into rivers that come together to form rivers that join the seas and oceans.
But not all watercourses remain on the surface: some encounter fractures in the ground and infiltrate it.
In limestone country, these infiltrations slowly hollow out the rock by chemical and mechanical action until they form underground galleries which form the new underground bed of rivers: this is the "karst", made up of caves and chasms explored by speleologists.
All of these underground aquifers (water tables and karsts) represent 99% of liquid freshwater reserves on Earth, hence the importance of understanding these resources in order to be able to manage and protect them.
In the context of increasing global warming and drought, these freshwater reserves are attracting more and more interest from local communities.
Pollution generated by surface anthropogenic activities, for example industry or agriculture, can seep into groundwater and alter the quality of drinking water.
The karstic networks are meticulously described by the speleologists during their explorations (dimensions and orientations of the galleries, volumes of the rooms, differences in height, etc.) who summarize their observations in the form of sections and plans.
Among the geological objects identified, fractures constitute strategic points of access to groundwater which are of particular interest to hydrogeologists for questions of community water supply.
It is through these areas of voids that surface water infiltrates the soil by gravity, but also that they resurface in very rainy weather, when the network of underground cavities is filled with water.
Underground, some conduits are completely submerged and impassable.
Other means must be found to understand the circulation of groundwater.
The concept then consists of coloring the water with fluorescein at the most accessible upstream point of the river, and measuring the concentration of this fluorescein at the various known or supposed exit points (outlet or resurgence).
It was, for example, a fire at the Pernod factories in Pontarlier in August 1901 that made it possible to establish the underground connection between the Doubs and the Loue by releasing large quantities of absinthe into the river.
In Meurthe-et-Moselle, Meuse and Vosges, the Aroffe is a river that plays hide-and-seek according to the seasons and the weather between a perennial underground course and a temporary aerial course and continues to intrigue those who rub shoulders with it. .
Indeed, the underground course with a potential of at least 30 kilometers is currently only explored over 2.5 kilometers.
Speleologists and geologists have recently collaborated to refine the topographic and colorimetric studies already undertaken on one of the resurgences of the Aroffe, "le Fond de la Souche", by testing a new technique based on measuring the electrical properties of the subsoils.
The Aroffe has its source in Beuvezin and disappears underground through a succession of fractures in Gémonville.
The waters of the Aroffe reappear on the surface at the source of La Rochotte in Pierre-la-Treiche and flow into the Moselle.
In extremely rainy weather, the thirty kilometers of underground network fills up to an overflow which appears on the surface by resurgences, locally called “mournings”, which play the role of overflow.
The Aroffe then flows on the surface and flows into the Meuse.
The objective of the study is to attempt to determine the precise underground course for research purposes.
Why are we interested in the resurgence of the Fond de la Souche?
Following work carried out in 1971 by speleologists, a fracture 25 meters deep was opened at the Fond de la Souche in Harmonville.
This ends in a stream which, after about 200 meters, flows into a river forming a large underground gallery.
Fifteen years of exploration and topographical surveys have led to the current plan of the Fond de la Souche network: 2500 meters of galleries are accessible to humans but end at each end on a siphon, completely submerged conduit, difficult and tedious access with substantial cave diving equipment.
Although several diving campaigns have taken place to pierce the mysteries of these two flooded conduits, no new non-flooded gallery has been reached.
From a hydrological point of view, the various observations seem to show that the river of the great gallery is not the Aroffe but rather one of its tributaries.
Does the Aroffe run nearby or in a completely different area?
What is its magnitude?
As it is, the Fond de la Souche being the only penetrable manhole, this cavity remains the best site to search for the underground Aroffe.
Forty years later, these questions remain, but techniques are evolving!
Speleologists from the Union Spéléologique de l'Agglomeration Nancy and geologists from UniLaSalle recently met to experiment with a non-destructive surface technique to continue this exploration.
How to detect underground galleries without destroying landscapes and ecosystems?
Very often, to directly reach a resource that is found in the depths of the basements (water, ore, gas, etc.), geologists make either holes using drills, or notches in the mountains using huge diggers.
These techniques are certainly effective but have the enormous disadvantage of destroying the ecosystems (faunistic and floristic habitats) and the landscapes but also of chemically polluting the exploited sites.
Moreover, they are extremely costly in terms of time, energy and money.
To find the underground layout of the galleries of the Fond de la Souche river, we chose to test an indirect non-destructive method.
“Electrical tomography” is a geophysical method based on the ability of a material to conduct current.
Unlike air, highly conductive water allows current to flow easily.
The technique therefore consists of measuring the electrical resistivity of materials in the subsoil, by injecting current along a line of electrodes planted in the ground.
The electrical resistivity of the subsoil depends on the nature of the rock, the porosity of the rock, the water content of the rock, the fracturing or the presence of voids.
In order to validate or not the use of this technique for the detection of cavities, a measurement campaign was organized in 2021 on the Fond de la Souche sector.
This took place at the end of the summer season, during which the cavities devoid of water are filled with air and should theoretically be more resistive to the current.
From the plan projected onto an aerial photo, several electrical profiles have been arranged in relation to the already known galleries and their supposed locations.
Two of the profiles are arranged near the inlet nozzle, perpendicular to the 25 meter deep fracture.
The first has a penetration of electricity into the ground of 50 meters (L5), but a lower resolution than the second, which only reaches a depth of 10 meters (L6).
In the right part of the L5 profile, at 25 meters depth, the characteristic orange/red spots illustrate two zones of more resistive "material", compatible with the possible presence of the cavity of the Bottom of the Strain.
On the second electrical profile (L6), the fracture is clearly detected in the middle by the electrical tomography in the form of a highly resistive zone in brown, which coincides with the topographic survey of the entrance shaft.
OUR “GEOLOGY” FILE
The technique of electrical tomography therefore seems promising for our search for “voids”.
Even if there is still a vast sector to explore in order to fully identify the route of the disappeared river, a forthcoming measurement campaign will make it possible to identify the orientation of the galleries beyond the siphons.
The research has only just begun and will be continued next year.
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