Search for the ‘disappeared’ river to understand the groundwater cycle
Streams are formed by the collection of rainwater, which then becomes rivers that merge to form rivers that connect seas and oceans. However, not all watercourses remain on the surface: some meet cracks in the soil and seep into it.
In limestone country, these infiltrations slowly erode the rock by chemical and mechanical action until the rivers form subterranean galleries that form a new subterranean bed: this is the “karst” of caves and cliffs discovered by speleologists.
All of these underground aquifers (aquifers and karsts) account for 99% of Earth’s liquid freshwater resources, so understanding these resources is critical to managing and protecting them.
In the context of increasing global warming and drought, these freshwater resources are of increasing interest to local communities. Pollution from surface anthropogenic activities, such as industry or agriculture, can seep into groundwater and alter the quality of drinking water.
Karst networks are carefully described by speleologists during explorations (sizes and directions of galleries, volumes of rooms, height differences, etc.), and they summarize their observations in the form of sections and plans. Fractures between defined geological features constitute strategic points of access to groundwater, which is of particular interest to hydrogeologists in community water supply issues.
It is through these void areas that surface water penetrates into the soil under the force of earth’s gravity, but also emerges again during heavy rains when the underground network of voids fills with water. Underground, some pipes are completely submerged and impassable.
Other means must be found to understand the groundwater cycle. The concept then consists of staining the water with fluorescein at the most accessible upstream point of the river and measuring the concentration of this fluorescein at various known or probable outlet points (outlet or recharge).
It was, for example, a fire at the Pernod factories in Pontarie in August 1901 that allowed an underground connection between the Dub and the Loue, releasing large amounts of absinthe into the river.
In the Meurthe-et-Moselle, the Meuse and the Vosges, the Aroffe is a river that plays hide-and-seek with the seasons and the weather between a perennial subterranean course and a temporary air current, and continues to fascinate those who travel with it. . Indeed, an underground course with a potential of at least 30 kilometers is currently being explored for just over 2.5 kilometers.
Speleologists and geologists have recently collaborated to improve the topographical and colorimetric studies already carried out on “le Fond de la Souche”, one of Aroffe’s revivals, by testing a new technique based on the measurement of electrical properties of the subsurface.
The Aroffe takes its source at Beuvezin and disappears underground through a series of fractures at Gémonville. The waters of the Aroffe reappear at the source of La Rochotte in Pierre-la-Treiche and flow into the Moselle.
During extreme rains, the thirty-kilometer underground network fills up to the visible flood on the surface with resurgences locally called “mourning” that act as a flood. The Aroffe then flows to the surface and flows into the Meuse. The purpose of the study is to try to determine the exact underground course for research purposes.
Why are we interested in reviving the Fond de la Souche?
In 1971, after work by speleologists, a 25-meter-deep fracture was opened at Fond de la Souche in Harmonville. After about 200 meters, it ends with a stream that flows into the river, which forms a large underground gallery.
Fifteen years of exploration and topographic research have led to the current layout of the Fond de la Souche network: 2,500 meters of galleries are accessible to people, but at each end they end with a siphon, a channel completely submerged in water, a difficult and tiring entrance with a significant cave. diving equipment. Although several diving campaigns have been conducted to unravel the secrets of these two flooded canals, no new unflooded gallery has been obtained.
From a hydrological point of view, various observations show that the river of the great gallery is not the Aroffe, but one of its tributaries. Does Aroffe work nearby or in a completely different area? What is its scale? Since the Fond de la Souche is only a traversable hatch, this gap remains the best place to search for the underground Aroffe.
Forty years later, these questions remain, but the techniques are evolving! Speleologists from the Union Spéléologique de l’Agglomeration Nancy and geologists from the UniLaSalle recently came together to experiment with non-destructive surface techniques to continue this exploration.
How to discover underground galleries without destroying landscapes and ecosystems?
Often, in order to directly reach a resource (water, ore, gas, etc.) deep in the basement, geologists either make holes using drills or make notches in the mountains using giant excavators.
These methods are certainly effective, but have the major disadvantages of destroying ecosystems (faunistic and floristic habitats) and landscapes, as well as chemically polluting the exploited areas. Moreover, they are quite expensive in terms of time, energy and money.
We chose to test an indirect non-destructive method to find the underground plan of the Fond de la Souche river galleries. “Electrical tomography” is a geophysical method based on the ability of material to conduct current.
Unlike air, highly conductive water allows current to flow easily. Therefore, the technique consists of measuring the electrical resistance of underground materials by injecting current along a line of electrodes planted in the ground.
The electrical resistivity of the subsurface depends on the nature of the rock, the porosity of the rock, the water content of the rock, the presence of fractures or voids. A measurement campaign was organized in the Fond de la Souche sector in 2021 to confirm or not the use of this technique for void detection.
This happened at the end of the summer season, during which the voids deprived of water are filled with air and should theoretically offer greater resistance to the current. Several electrical profiles of known galleries and their probable locations have been arranged from the projected plan of the aerial photograph.
Two of the profiles are placed perpendicular to the fracture at a depth of 25 meters, near the inlet. The first has a power penetration of 50 meters (L5), but has a lower resolution than the second, only reaching a depth of 10 meters (L6).
To the right of profile L5, at a depth of 25 meters, the characteristic orange/red spots delineate two zones of more resistive “material” consistent with the possible presence of a tension bottom cavity.
In the second electrical profile (L6), the fracture is clearly detected by electrical tomography as a brown high resistivity zone in the middle, which coincides with the topographic examination of the input shaft.
The electrical tomography technique therefore seems promising for our search for “gaps”. Although there is still a large sector to be explored to fully determine the route of the lost river, the upcoming measurement campaign will allow to determine the orientation of the galleries beyond the siphons. Research has just begun and will continue next year.
This article was produced by The Conversation and produced by 20 Minutes.