Genesis and functioning of the Aix-les-Bains hydrothermal karst (Savoie, France): past research and recent advances

Abstract

Abstract Aix-les-Bains (Savoie, France) owes its name and reputation to the thermal springs that occur along the eastern shore of Lake Bourget, France largest natural lake. Although the city waters have been exploited since Antiquity, scientific investigations into the nature and characteristics of the hydrothermal karst from which they emerge did not begin until the early 19th century. The present article traces the history of these investigations and summarizes the results of more than two centuries of scientific research. Today, the only visible signs of karstification related to hydrothermal flows are to be found in the discharge zone in the Urgonian limestone anticline that rises above the city centre. These features are: – the Grotte des Serpents, which houses the Alun Spring, the system main natural discharge, – the Chevalley Aven, a blind chimney that was accidentally uncovered in 1996, – other hydrothermal springs that are too small to enter, including the Soufre Spring. Although scientific investigation of the thermal springs at Aix-les-Bains began in the early 19th century, it was not until the 1920s that scientists started examining the relationship between karstification and the state of the aquifer. E.A. Martel was the first researcher to describe the Aix-les-Bains site as an active hydrothermal karst, in a pioneering study published in 1935. Sixty years later, the discovery of the Chevalley Aven during building work on a new hydrotherapy center gave fresh impetus to research into the karstification of the Aix-les-Bains thermo-mineral aquifer. Recent studies have also investigated the deep aquifer below the karst, using data provided by boreholes. The Urgonian limestone karst at Aix-les-Bains is the site of mixing between thermal waters rising through the anticline and meteoric waters percolating from the surface. Meteoric infiltration is sufficiently high for the hydrological behavior of the thermal springs to be identical to that of exsurgences in gravity-fed, cold-water transmissive karsts. The Chevalley Aven is a shaft that descends 30 meters below the surface, thereby providing access to the ground-water at depth. Monitoring of the water quality in the aven has shown that the Legionella contamination of the springs was due to high concentrations of the bacteria in upstream passages in the karst. In 2006, dye-tracing tests confirmed the existence of a hydraulic connection between the Chevalley Aven and the Alun and Soufre Springs, the fact there is a single ascending hydrothermal conduit, which lies between the Chevalley Aven and the Alun Spring. In addition to providing a valuable source of information about the functioning of the thermo-mineral aquifer, the cavities at Aix-les-Bains are of great karstological interest, especially for the study of hypogene speleogenetic processes. The circulation of warm (40°C), sulfur-rich waters and vapors through the system has led to the development of conduits with specific morphologies and the precipitation of characteristic deposits. These features include: – “beaded” chimneys and galleries formed by the linking of spheres produced by condensation-corrosion. Diffuse karstification along bedding planes around the main conduit; – deposition of non-carbonate minerals (gypsum, native sulfur); – formation of biothems and biofilms on walls subject to condensation. The Grotte des Serpents is a horizontal cavity that formed at the upper limit of the water table. The Chevalley Aven is a hypogene chimney that was sculpted under vadose conditions by the release of sulfuric acid-rich vapors above the thermal water table. As well as a surface coating of microbial mats and the presence of bacterial flakes in the thermal water, the vadose parts of the Aix-les-Bains hydrothermal karst contain a characteristic microfauna and flora. These microorganisms are thought to play an active role in hypogene karstification processes.