Bull. Soc. géol. Fr., 2010, t. 181, n o 4, pp. 315-326 HOBLEA GE NE SIS AND F. etFUNC al. TIO NING OF THE AIX-LES-BAINS HY DROTHERMAL KARST (SAVOIE, FRANCE) Genesis and functioning of the Aix-les-Bains hydrothermal karst (Savoie, France): past research and recent advances FABIEN HOBLEA1, STÉPHANIE GALLINO-JOSNIN1 and PHILIPPE AUDRA2 Key-words. – Thermal-sulfidic karst, Dye-tracing, Hypogenic caves, Condensation-corrosion, Aix-les-Bains, France. Abstract. – Aix-les-Bains (Savoie, France) owes its name and reputation to the thermal springs that oc cur along the eastern shore of Lake Bourget, France largest natural lake. Although the city waters have been exploited since Antiquity, scientific investigations into the na ture 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 chim ney 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 aqui fer. 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 groundwater at depth. Monitoring of the water quality in the aven has shown that the Legionella contami nation of the springs was due to high concentrations of the bac teria 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 be tween the Chevalley Aven and the Alun Spring. In addition to providing a valuable source of information about the functioning of the thermo-mineral aqui fer, the cavities at Aix-les-Bains are of great karstological interest, especially for the study of hypogene speleogenetic processes. The circulation of warm (40oC), 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 chim ney 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 ac tive role in hypogene karstification processes. Genèse et fonctionnement du karst hydrothermal d’Aix-les-Bains (Savoie, France) : historique des recherches et avancées récentes Mots-clés. – Karst thermal, Traçage, Cavités hypogènes, Condensation-corrosion, Aix-les-Bains, France Résumé. – Cet article dresse un historique et une synthèse des connaissances sur le karst hydrothermal d’Aix-les-Bains (Savoie, France). 1. Ecole Na tio nale d’Ingénieurs de Sfax, Dé parte ment de Génie géologique, BP W, 3038 Sfax, Tunisie, e-mail: Farhat.Rekhiss@enis.rnu.tn 2. La boratoire Eau, Energie et Environnement, ENIS, BP W, 3038 Sfax, Tunisie, e-mail: smguidiche@ya hoo.ca 3. Faculté des Sciences de Tunis, Département de Géologie, Campus Universi taire, 1060 Tunis, Tunisie Manuscrit déposé le 17 fé vrier 2008; accepté après ré vision le 27 avril 2009. Bull. Soc. géol. Fr., 2010, no 4 316 HOBLEA F. et al. Les manifestations visibles de la karstification liée aux circulations hydrothermales se limitent actuellement aux alentours de la zone d’émergence, sur le flanc du dôme anticlinal de calcaires urgoniens qui domine le centre-ville. Ces phénomènes sont: – la grotte des Serpents, qui contient la source d’Alun, principale émergence na turelle du système, – le gouffre Chevalley, cheminée aveugle tré panée fortuitement en 1996, – d’autres sources et griffons hydrothermaux impénétrables, principalement la source de Soufre. L’approche historique montre que les premières approches scientifiques des sources thermales d’Aix remontent au début du XIXe siècle. Ce n’est qu’à partir des années 1920 que les scientifiques s’intéressent aux interactions entre la karstification et l’état de l’aquifère. En 1935, E.A. Martel est le premier karstologue à s’intéresser au site d’Aix comme exemple de karst hydrothermal ac tif. Soixante ans plus tard, la découverte du gouffre Chevalley sur le site des nouveaux thermes, relance les études sur la karstification de l’aquifère thermo-minéral d’Aix, en complément des investigations de l’aquifère profond menées grâce aux forages d’exploitation. L’Urgonien karstifié de l’anticlinal d’Aix-les-Bains constitue une zone de mé lange des re montées d’eaux thermales et des infiltrations de surface. Du fait de ces infiltrations dans les cal caires, les sources thermales ont un comportement hydrologique identique aux émergences des karsts transmissifs à eaux gravitaires froides. Le gouffre Chevalley offre à 30 m de profondeur un regard sur la nappe thermale. Le suivi de la qua lité bac tériologique de l’eau dans le gouffre a révélé l’état avancé de co lonisation des conduits karstiques par les lé gionnelles, en amont des sources chroniquement contami nées. La liaison hydraulique entre le gouffre Chevalley et les sources d’Alun et de Soufre a été dé montrée par traçage en 2006. Celui-ci a également montré l’unicité du tronc hydrothermal ascendant, situé entre le gouffre Chevalley et la source d’Alun. Outre leur con tribution à la compréhension du fonctionnement de la partie supérieure de l’aquifère ther mo-minéral, les cavités aixoises offrent également un grand intérêt karstologique, notamment pour l’étude des processus de spéléogenèse hypogène. Celle-ci, du fait de la température (autour de 40 oC) et du chimisme des eaux sulfureuses et des vapeurs générées, façonne des conduits à la morphologie et aux dépôts caractéristiques: – cheminées et galeries ganglionnaires formées d’une succession de coupoles liées principalement à une spéléogenèse par condensation-corrosion. Karstification diffuse sur les plans de stratification autour du drain principal ; – formation et dé pôts de minéraux non carbonatés (gypse, soufre natif) ; – formation de biothèmes et biofilms pariétaux dans les par ties exondées soumises à la conden sation. La grotte des Serpents est une cavité horizontale formée en limite supérieure de la surface piézométrique. Le gouffre Chevalley serait une cheminée hypogène modelée en régime vadose sous l’effet des dégazages chargés d’acide sulfurique au-dessus de la nappe thermale. Outre la ba régine formant un voile et des flocons bactériens dans l’eau thermale, les cavités du karst hydrothermal d’Aix-les-Bains contiennent dans leur partie exondée une microfaune et des micro-organismes particuliers. Ces derniers sont soupçonnés d’être actifs dans les processus de karstification hypogène. INTRODUCTION Aix-les-Bains (Savoie, France) lies on the eastern shore of Lake Bourget, the largest natu ral lake in France. The city owes its name and its rep u tation to its ther mal springs, which have been exploited since Antiquity by successive gen erations of spa baths. Ther mal waters rise to the sur face, either natu rally or through boreholes, in three main areas. The most important area is the city center, which includes the Soufre and Alun hyperthermal springs. The other two areas are lo cated either side of the city cen ter, with the Raphy-St Simon hypothermal spring to the north and the Domaine de Marlioz spring to the south (fig. 1). The origin and properties of these thermo-min eral waters have long at tracted the cu rios ity of sci en tists, who quickly noted that the springs in the city center were related to the Grotte des Serpents (also known as the Grotte de St Paul), a relatively vo luminous cavern within the Urgonian limestone. For many years, this cavern, which contains the main feed spring for the spa, was the only known manifestation of the karstification of the upper part of the Aix-les-Bains thermo-mineral aquifer. During the 19th century it was both the focal point for studies of the aquifer and an attraction for tour ists, who came to mar vel at the fantastical features of the cave. Un til the end of the 20 th century, stud ies of the re la tion ship be tween the un derground wa ters and the karst were primarily undertaken with the aim of Bull. Soc. géol. Fr., 2010, no 4 pro tecting the thermal waters as a re source. Con sequently, research ers fo cused on as pects such as the in flu ence of meteoric inputs on the quantity and quality of the thermal waters. Karst studies at Aix-les-Bains were given new im pe tus and a new direction in 1996 with the uncovering of a previously unknown cavity during work on the foundations for a new hydrotherapy center being built at Chevalley. This discovery raised a number of questions about the underground system below Aix-les-Bains and opened the way for new research into the formation and functioning of this unusual karst : – what is the relationship between the vertical Chevalley Aven and the sub-horizontal exsurgence in the Grotte des Serpents? – are these iso lated features or is the Aix-les-Bains anticline rid dled with other cavities sim ilar to the Chevalley Aven? – is this an example of a clas sic, an cient, in her ited karst that is being reused by hot waters circulating from depth, or was it formed uniquely by the hydrothermal activity, and thus completely hypogenic? If the latter case is true, it is one of only a very small number of such karsts in France. These questions, which find elements of answer in this paper have implications for both karst research and the management of the thermal resource and related facilities. GENESIS AND FUNC TIO NING OF THE AIX-LES-BAINS HY DROTHER MAL KARST (SAVOIE, FRANCE) 317 FIG. 1. – Location of Aix-les-Bains and the 3 thermomineral spots. FIG. 1. – Localisation d’Aix-les-Bains et des 3 zones d’émergences thermominérales. MORPHOLOGICAL AND STRUCTURAL SETTING (fig. 2) Aix-les Bains and Lake Bourget (alt. 233 m) are lo cated to the west of the sub-Alpine front, here represented by the Bauges Mountains, whose western ramparts rise above the city to an altitude of 1550 m. The city and the lake lie in the molasse basin that formed on the western edge of the sub-Alpine front, in a synclinal depression in the folded sediments of the southern Jura. To the west, the basin is bounded by the anticlines of the Montagne de l’Épine, Dent du Chat, and Montagne de Charvaz; to the east, it is bounded by the lower hills of La Chambotte (850 m) and the Aix-les-Bains anticline. As it only rises to an al titude of 350 m, the Aix-les-Bains anticline is mostly concealed under molasse sediments and by urban development. The Aix-les-Bains anticline has been thrust over the Le Bourget syncline, along major faults injected with gypsum, to form a double-kink folded structure [Gallino, 2007] (fig. 3). It con tains a con tin u ous se ries of sed imen tary rocks from the Trias to the Mid dle Cretaceous, in which Triassic evaporites are overlain by an alternating se quence of marls and lime stones. The uppermost part of the series, a 100 to 200 m thick se quence of mas sive Barremian-Aptian lime stones of Urgonian facies, contains the karst described in the present article. PROVENANCE AND PHYSICAL-CHEMICAL CHARACTERISTICS OF THE THERMAL WATERS Aix-les-Bains ther mal waters have intrigued geologists and hydrogeologists since the beginning of the 19th century, and nu mer ous hy poth e ses have been ad vanced to ac count for their origin. Recent reviews of this research show the wide diversity of provenances that have been suggested [Iundt et al., 1987; Nicoud, 1994; Hobléa, 1999; Gallino, 2007]. Today, a general consensus appears to have been reached, accepting the Montagne de la Charvaz, on the west bank of Lake Bourget, as the source of the thermal waters. First proposed by Nicoud in [1980], and refined by Carfantan [1992] (fig. 4), this hypothesis has been con firmed by sub se quent re search, most no ta bly iso tope stud ies [Iundt et al., 1987; Gallino, 2007]. Meteoric waters falling on the Montagne de la Charvaz, (700 to 1150 m asl) infiltrate the Jurassic (Kimmeridgian) limestones of the Le Bourget syncline, following the dip of the strata to a depth of over 2000 meters and heating up to a temper a ture of around 70 oC. The thermosiphon effect then causes the waters to rise through the opposite side of the syncline, where they encounter the thrust faults of the Aix-les-Bains anticline. At this point, the waters are brought into contact with sulfate evaporites that are reduced to sulfides by bacteria [Gallino, 2007]. The waters continue to rise through the Jurassic and Cretaceous rocks to the surface, where they percolate into the Urgonian limestones, which form the ‘outlet aquifer’. The waters that emerge from the Urgonian aquifer have a sodium sulfide facies (5 mg/l of H2S in Alun water) and a temperature of either 33 to 46oC, for the springs in the city center (treatment of rheumatism and gout), or of around 11oC, for the waters at the Domaine de Marlioz (treatment of re spira tory prob lems). Bottled min eral wa ter is pro duced to the north of Aix-les-Bains, but this water has an en tirely differ ent prove nance – the Ju rassic lime stones of the Chambotte anticline. The aquifer from which these hypo thermal waters emerge is sep arated from the Urgonian aqui fer by a major regional fault hidden under the Quaternary for ma tions. EARLY IDEAS ABOUT THE KARST PHENOMENA AT AIX-LES-BAINS: THE TIME OF MYSTERIES (16 TH-19 TH CENTURIES) There is very little surface evidence of the karst un derlying Aix-les-Bains, as it is covered by a layer of till deposited during the last glacial period and by the urbanization of the area. Aix-les-Bains has been re nowned for its thermal waters since Gallo-Ro man times; however, the lack of an obvious natural en trance meant that the Grotte des Ser pents (or Grotte de St Paul) remained inaccessible and mysterious for centuries. Cabias provides the ear liest record of a karst Bull. Soc. géol. Fr., 2010, no 4 318 HOBLEA F. et al. FIG. 2. – Geological setting of Aix-les-Bains at the contact of southern Jura (at the west and northwest) and sub-alpine massifs (Bauges at the east). FIG. 2. – Le contexte géologique d’Aix-les-Bains au contact du Jura méridional (ouest et nord-ouest) et des massifs subalpins (Bauges à l’est). opening when in 1623, he described the hot vapors that could be seen emerg ing from “Les Puits d’Enfer” (the Pits of Hell) [Cabias, 1623]. Numerous 19th century medical docu ments and tourist guides describ ing the city waters men tion “Les Puits d’Enfer”, and many attempts were made by doctors, chemists and the merely curious to unveil the mystery of these steamy and malodorous shafts. The earliest account of such an exploratory incursion was written by Dr Dacquin in 1773, who reported that: “hav ing en tered via an opening at ground level, I heard a noise similar to that produced by rushing water (…). I was prevented from going very far by the small size of the passage and, most importantly, by the high temperature of the vapors, which were suffocating me and which forced me to precipitously back my way out.” [Dacquin, 1808]. A report by the doctor and Bull. Soc. géol. Fr., 2010, no 4 chemist J.-M. Socquet, recently uncovered by Gauchon [2009], provides the first known scientific descrip tion of Aix-les-Bains underground cavities [Socquet, 1803]. In addition to their anecdotal interest, the reports of the early ex plor ers are valu able for re con struct ing the ge om etry of the Alun Spring [Gauchon, 2009] prior to the building of an adit (between 1853 and 1855) designed to improve extraction of the spring water. According to Despine (the younger) [1837], the cave was accessible via two natural and almost vertical passages that opened onto the Chemin de St Paul (today known as the Rue du Puits d’Enfer), approximately 100 meters from the natu ral exsurgence, the Alun Spring. The entrance to the first cavity, named the “Grotte des Ser pents” because of the large number of snakes that had made it their home, was GENESIS AND FUNC TIO NING OF THE AIX-LES-BAINS HY DROTHER MAL KARST (SAVOIE, FRANCE) 319 FIG. 3. – Geological cross-section through the Bourget synclinorium showing the double-kink structure of the buried anticline of Aix-les-Bains [Gallino, 2007]. FIG. 3. – Coupe géologique à travers le synclinorium du Bourget montrant la structure en double kink de l’anticlinal enfoui d’Aix-les-Bains [Gallino, 2007]. covered by a grid built into the supporting wall of the Chevalley building (then owned by the Mollard family). The second passage, the Puits d’Enfer, emerged in the mid dle of the Chemin de St Paul, where its en trance was covered by a slab of rock. These openings gave access to the ends of a low chamber that slopes towards a section that is submerged. Reports from Soquet’s successors, who ex plored the passages at periods of low water during the 19th cen tury, suggest that the water level in this room fluc tuated [Ordinaire, 1875]. To all contemporary observers, a direct link between the water in the cave and the Alun Spring ap peared self-evident. As a re sult, an en gineer named François decided to extract the water directly from the cave and thereby optimize access to the resource. His solution, an 83 meters tunnel into the Grotte des Serpents, lowered the water level in the cave by 7.8 me ters, and caused the natu ral spring to dry up [Martel, 1935]. The new water catchment point, which replaced the Alun Spring, was named the Alun Pool. The ar tificial tunnel gave easy ac cess to the now dry cave, allowing it to be opened to tourists. 20TH CENTURY RESEARCH: THE INFLUENCE OF KARSTIFICATION ON THE QUALITY OF THE WATERS During the 1920s, the Institut d’Hydrologie et de Climatologie car ried out de tailed stud ies (with monitor ing of the exsurgences) of the relationship between meteoric infiltration and the con tamination of the ther mal waters. These stud ies in dicated that the karstification of the Urgonian limestone was one factor in the vulnerability of the waters to con tamination. FIG. 4. – Repre sentation by Carfantan [1992] of the actual accepted hypothesis for the thermal water circulation proposed by Nicoud [1980]. FIG. 4. – Repré sen tation par Car fantan [1992] du schéma de circulation des eaux des ther mes d’Aix selon Ni coud [1980]. Bull. Soc. géol. Fr., 2010, no 4 320 HOBLEA F. et al. E. A. Martel, the founder of scientific speleology in France, visited the cave at the beginning of the 1930s. The expertise he had gained from studies of thermal karsts all across Eu rope allowed him to rec og nize the cave as a karst phenomenon. He made special note of the “roof and wall features formed by cor ro sion, ero sion, hy dro dy namic pressure. These features resemble those formed by the action of wa ter in nat u ral lime stone cav erns and un derground rivers. It goes without saying that (chemical) corrosion has been the dominant process due to the presence of hydrogen sulfide.” [Martel, 1935, p. 795]. Also at this time, an engineer called Schneider, with as sistance from a ge ologist called Moret, who was France leading specialist in thermo-min eral aqui fers, car ried out a study of the springs in preparation for the build ing of the new “Thermes Nationaux” [Moret, 1946]. Schneider’s conclusions, based on meticu lous monitoring of the flow rate of the springs (construction of overflow weirs) and analy ses of the hydrological functioning of the system and the morphology of the Grotte des Serpents, show that he, too, recognized the system to be a karst aquifer: “The path followed by the waters has been enlarged by dissolution and friction over the centuries. (…). Hence, the Grotte des Ser pents, where the waters of the Alun Spring used to flow, and the cave that used to contain the Soufre exsurgence (…) con tinue to a certain depth. The thermal springs, at least in the neighborhood of the surface, must be considered to resemble a string of beads, consisting of irregular cavities linked by narrower sections.” [Schneider, 1935] (fig. 5). This in sightful hypothesis was confirmed 60 years later following the discovery of the Chevalley Aven. Hydrogeological studies into the quality (reports of the SOGREAH in the 1960s) and origin of the thermal waters [Lepiller, 1975; Nicoud, 1980] continued after the Second World War. The results showed the karstification of the Urgonian limestone to be an insurmountable problem in terms of the contamination of the thermal waters, and the springs were grad ually abandoned in favor of deep boreholes. Subsequent research also concentrated on the deep waters [Iundt et al., 1986, 1987; Muralt, 2003], leading to the drilling of four boreholes for the Thermes Nationaux between 1989 and 2003. All four of these boreholes focus on the main thermal springs around the city center. With a guaranteed supply of high-quality water, it became possible to envisage replacing the aging “Thermes Nationaux”. As a result, in the middle of the 1990s a project was launched to build a new hydrotherapy center at Villa FIG. 5. – Re presen ta tion by the engineer Schnei der [1935] of the kars ti fica tion of the upper part of the thermal aqui fer of Aix-les-Bains. FIG. 5. – Repré sentation par l’ingé nieur Schneider [1935] de la karsti fica tion de la partie su périeure de l’a quifère hydrother mal d’Aix-les-Bains. Bull. Soc. géol. Fr., 2010, no 4 Chevalley, near the top of the Aix-les-Bains anticline. Nicoud’s pre pa ratory geo log ical sur vey high lighted the risk that ex cavations for the spa foundations would encounter hidden karst cavities and this is exactly what happened. In June 1996 a mechanical shovel uncovered a karst shaft while remov ing blast rubble produced during work to level the new building foundations. Work was immediately stopped, the site was cordoned off and the “Départemental” Caving Committee was asked to map the cavity as the first step in producing a geotechnical survey [Hobléa, 1996]. Ex ploration of the shaft, which had been named the Chevalley Aven, was quickly impeded by the narrowness of the passages and the poor air quality. The hot, noxious vapors in the cave became increasingly suffocating with depth, and the cavers suffered similar symptoms of nausea and dizziness to those reported by the early explorers of the Grotte des Serpents when it contained water. Several 2-hour ex ploratory sessions, using Dräger counters to monitor H2S and CO2 concentrations, were needed to progress a few tens of me ters. However, the small size of the pas sages, the vertical nature of the system and the solid ity of the rock indicated that the cav ity pre sented very lit tle risk to the sta bility of the build ing. Nevertheless, the questions of whether or not there were other sim ilar cav ities in the area, and of how to stabilize the shaft that had been un covered remained to be an swered. The first question was quickly resolved by a geo-radar sur vey of the en tire site, which revealed the presence of about a dozen anomalies, and the drilling of reconnaissance boreholes, which showed that none of these anomalies corresponded to karst cavities of any significant size. The second question proved more problematical, as it had been intended to sink a foundation pile for the new spa building at the ex act position of the Chevalley Aven. The classic solution to such a problem is to fill the cavity with concrete but this was not possible here, as exploration of the cavity showed that it ended at a depth of about 30 meters in a sump containing water at 42oC. As a re sult, the developer had to find an architectural solution to the foundation problem, and the cavity was spared. The entrance to the cavity and thermal aquifer is now enclosed in the pipe room in the basement of the build ing. Re search begun dur ing the initial ex plo ration of the cavity [Hobléa, 1997] had to be interrupted until building work had been completed. At this time, the Chevalley Aven was regarded as a second window onto the Aix-les-Bains ther mal aqui fer and it was interpreted as a water-table bal ance chim ney for the ther mal drain that takes the waters to the Alun Spring [Hobléa, 1999]. However, this hypothesis remained to be proven and it was necessary to model the water circulation between the various access points and exsurgences. It was also necessary to determine the point of upwelling of the thermal waters, which then diffuse through the karstified Urgonian limestone. Direct observations by a cave diver did not give the hoped for results. Whether the diver entered the system from the Chevalley Aven or from the Alun pool, his progress was soon blocked by constrictions in the passage that could not be overcome in the restricted time (15 min utes) available to him because of the high water temperature. He nevertheless reported that the beaded morphology contin ued for at least ten meters, which was as far as the poor visibility caused by the bacterial flocs and the sloughing of the altered walls allowed him to see. GENESIS AND FUNC TIO NING OF THE AIX-LES-BAINS HY DROTHER MAL KARST (SAVOIE, FRANCE) The pressure relation between the Alun Spring and the Chevalley sump was proved in December 1996, when water was pumped into the pool to clean the catchment. Scientists had to wait another ten years, until the waters from the Alun Spring were no longer used for spa treatments, before they could carry out dye-tracing tests. Despite the layer of till overlying the Urgonian karst at Aix-les-Bains, the sealing of the topsoil by urbanization and the draining and pro tection works carried out since the 19th century, perco lating surface waters still cause serious problems for the exploitation of the spa waters. Decreases in the temperature of the waters and, most importantly, recur rent pol lu tion and con tam i nation prob lems, led to the springs being abandoned at the beginning of the 2000s in favor of deep boreholes. Waters from these boreholes are mixed to reconstitute the mineral composition of the spring water, which is the only composition approved for medical use. FIG. 6. – Map of the dye-tracing of 2006. FIG. 6. – Carte du traçage de 2006. 321 LATEST ADVANCES: A REFERENCE LOCALITY FOR THE STUDY OF THERMO-KARSTIC CIRCULATION AND HYPOGENE KARSTOGENESIS PROCESSES Physi cal proof of a hy drau lic con nection be tween the Chevalley Aven and the Alun and Soufre Springs (fig. 6) was finally obtained in April 2006 by a dye-tracing test [Gallino, 2006]. The dye-tracing data indicated that the water in the Chevalley sump rapidly divides to feed the two springs independently, via two conduits with different hy drau lic gra dients (fig. 7). This suggests that there is a complex relationship between the karst drain age sys tem and the deeper ther mal drains on which it is su per im posed. Si mul ta neous com par ative analyses of the spring temperatures [Galino, 2006] have shown that the thermal up-flow tube is located between the Alun Spring and the Chevalley Aven, and not upstream or between the Alun and Soufre Springs (fig. 8). When the new hydrotherapy center was finished, scientists were once again given access to the cavities, allowing them to study the processes of hypogene speleogenesis in the cavity, as well as the cir cu lation of the waters. Contrary to Martel’s observation (see above), the forms and formations that can be seen in the Grotte des Serpents and the Chevalley Aven are spectacularly differ ent from those found in “clas sic” cavities. It is true that most can be en coun tered in divid u ally in any endokarst. How ever, their density, arrangement, and spatial distribution create a very char ac ter is tic un derground land scape. Common features of the two cav ities include the abun dance and density of the roof spheres (almost 1000 spheres in 170 m2 in the Grotte des Serpents), and the “string of beads” form of some of the conduits, produced by a succession of meter-scale spher ical cavities separated by decimeter-scale necks [Hobléa, 1999] (fig. 9 b). In both cavities, gypsum formations and wall biofilms are associated with the abundant so-called baregine (micro bial mats and flakes) present in the thermal waters of the Alun pool and the Chevalley sump (microbial mats of the sulfur bacteria Beggiattoa [Maillard and Shep herd, 1977]). The wall biofilms developed under the water table can produce soft FIG. 7. – Tracer plume of the dye-tra cing of 2006. FIG. 7. – Courbe de res ti tution du traçage de 2006. Bull. Soc. géol. Fr., 2010, no 4 322 HOBLEA F. et al. FIG. 8. – Inter preta tion of the com plex geometry of the kars tic channels under Aix-les-Bains thermal center [Gal li no, 2006]. FIG. 8. – Inter pré tation de la géométrie complexe du drainage kars tique sous le quar tier des ther mes d’Aix-les-Bains [Gallino, 2006]. and slimy stalactites (biothems), sam ples of which were an alyzed at the University of Babes-Bolyai in Romania and found to be rich in actinomycetes, a group of filamentous bacteria [Manolache, 2001]. DNA anal yses revealed the pres ence of four differ ent spe cies, in clud ing Micro monospora chalcea (77.5% of the strains) and Micromo nospora coerulea (17.5%). These species can grow in aer o bic envi ron ments and be long to the fam ily of mi cro organ isms ca pable of par tic ipat ing in the dis so lu tion/pre cip itation cycle of calcium car bon ate. Actinobacteria play an im por tant role in the for ma tion and com po sition (hydromagnesite) of moonmilk [Cañaveras et al., 1999; 2006]. The Chevalley biofilms and biothems, combined with the presence of magnesian calcite and sulfur on the walls of the cavity surrounding the Alun pool, strongly suggest that micro bial ac tivity plays a sig nificant role in hypogene speleogenesis [Audra and Hobléa, 2007]. It is interesting to note that Socquet recorded the presence of moonmilk in 1803 in his descrip tion of the Grotte des Ser pents: “the rock that forms the vault is covered by a layer of muddy paste, one finger thick, which is produced by the successive softening of the surface of the rock. This rock is constantly in contact with, and penetrated by a constant stream of very hot vapors, which condense on the rock in the same way as vapor condenses on the walls of a still. When tested with a ther mometer, the rock walls exhibit a tem pera ture of 21 degrees.” [Socquet, 1803, p. 43]. Epistemologically, Socquet’s work was ahead of its time in terms of hypogene speleogenesis, and the chemist seems to have un der stood the prin ci ples of al ter ation by con den sation-cor ro sion in sul fur-rich thermal envi ron ments as they are seen to day (below). The Grotte des Serpents and the Chevalley Aven show very different modes of spatial development (fig. 9). The Grotte des Serpents (fig. 10) is a mostly horizontal passage from which emerge a number of vertical conduits, some of which are cut by the topographic surface (e.g., Puits d’Enfer). Conversely, the Chevalley Aven (fig. 11) is a vertical shaft with short horizontal or oblique branches. Cold water from the sur face in filtrates both the Grotte des Ser pents and the Chevalley Aven. These meteoric waters flow through conduits that have a beaded profile close to the Aven and the Grotte but a more classic profile, produced by the flow of water under gravity, near the surface. On the basis of these observations, the features of the Aix-les-Bains karst were ini tially in ter preted as “transition thermal karst” phenomena [Dublyansky, 1997]. Such phenomena have been postulated to occur when karstification pro cesses close to the regional base level are mag nified by the mixing of rising thermal waters and percolating meteoric waters. The omnipres ence of spher ical forms was thought to be due to the convectional movement of the water. Thus, the Chevalley Aven was interpreted as a water-table balance chimney that absorbs the fluctuations in load due to variations in meteoric inputs [Hobléa, 1999]. This interpretation has been revised in the light of data from re cent top o graphic sur veys and mor pho log ical stud ies, and most of the speleogenesis is now attributed to condensa tion-cor ro sion pro cesses [Dreybrodt et al., 2005; Audra et al., 2007; Klimchouk, 2009]. The cavities at Aix-les-Bains have become the French reference locality for the study of active hypogene speleogenesis processes [Audra et al., 2010]. Theses cavities are subject to convection currents, driven by the high ther mal gra dient be tween the ther mal waters and the rock walls, which are cooled by their proximity to the surface. When the warm air co mes into contact with the cool rock, condensation produces large quantities of a corrosive, sulfuric acid-rich fluid (fig. 12 a). This condensation is concentrated in the spheres, causing the formation of a thin layer of altered rock and biofilms (fig. 12 b). Conversely, evapo ration is the predominant process at the base of the spheres, especially at the necks between two adjoin ing spheres (fig. 12 c). These necks are characterized by the for mation of “popcorn” concretions of calcite and by the substitution FIG. 9. – Cross section showing the location and relationship between the Chevalley and the Serpents caves (+/– 0 = local topometric reference for each cave). FIG. 9. – Coupe indiquant la dis posi tion relative du gouffre Cheval ley et de la grotte des Ser pents (+ /– 0 = cote de ré fé rence pour les dénivelés de chacune des cavités). Bull. Soc. géol. Fr., 2010, no 4 GENESIS AND FUNC TIO NING OF THE AIX-LES-BAINS HY DROTHER MAL KARST (SAVOIE, FRANCE) 323 FIG. 10. – Map and pictures of the Serpents caves showing representative hypogenic morphologies developed on a subhorizontal bedding-plane. FIG. 10. – Plan et images de la grotte des Serpents montrant des morphologies hypogènes caractéristi ques développées sur un joint de stratification subhorizontal. of calcite by gypsum. Gypsum also forms crusts on the walls above the pool of the Alun Spring, where there is substan tial emis sion of H2S gas. The spa tial dis tribution of these processes is determined by the initial pattern of thermal conductivity within the surrounding rock. The thermal conductivity of the rock governs where sphere development initiates, and the spheres then develop along the axes of max imum thermal gra dient, thereby pro duc ing the char acteristic beaded shapes first described in the Hungarian hy drothermal karst [Szunyogh, 1982, 1990]. A number of issues relating to the Aix-les-Bains karst are still to be resolved. For example, how much of the hypogene speleogenesis can be attributed to condensation-corrosion processes in the vadose zone of the karst and how much is due to phreatic processes? Condensation-corrosion processes are currently considered to be much more effective than phreatic processes in terms of producing the forms and features of the Aix-les-Bains karst cavities. Two centuries af ter Dr. Socquet took his first measure ments, a new monitoring program has been set up to answer this ques tion. EFFECTS OF THE KARSTIFICATION ON THE THERMAL WATERS Although hypogene speleogenesis processes operate more quickly than classic karst processes, cavern formation does not proceed quickly enough for the known cav ities at Aix-les-Bains to pres ent an immediate danger to the sta bil ity of the ground. However, these processes do affect the thermal waters, so a detailed understanding of how they op erate and evolve is essen tial for the man agement of this re source. The development of the karst has had two major effects on the thermal waters used by the hydrotherapy center. First, karstification of the upper part of the Urgonian limestone allows Bull. Soc. géol. Fr., 2010, no 4 324 HOBLEA F. et al. FIG. 11. – Cross-section and pictures of the verti cal-developped Cheval ley Cave, from the entrance (top) to the Ther minator sump (down), showing spheres with biofilms. FIG. 11. – Topographie en coupe et images du gouffre Cheval ley à développement ver ti cal, de puis le puits d’entrée (en haut) jus qu’au si phon Ther mi nator (en bas), en pas sant par les clo ches de cor rosion à biofilms. meteoric infiltration waters to mix with the thermal waters, thereby altering the temperature and mineral composition of the thermal resource. Second, during the 1990s, water from the Alun Spring was frequently found to be con tam i nated by Legionnella pneumophila. At tempts to solve this prob lem by emptying and disinfecting the Alun pool every December proved futile, but it was not until the discovery of the Chevalley Aven, which is upstream from the Alun Spring, that the reasons for this came to light. Samples taken from the sump at the bottom of the Chevalley Aven were infested with Legionnella, which proliferates in this type of environment. As a result, Legionnella transported from upstream infected the Alun pool as soon as it was refilled. This finding was the coup de grace for the use of the springs. Guided visits to the Grotte des Serpents were also stopped when analyses of the atmosphere in the cave Bull. Soc. géol. Fr., 2010, no 4 showed that the guides were being exposed to radon levels above the max imum lim its set by France employment laws. CONCLUSION: A RESOURCE WITH GREAT HERITAGE AND SCIENTIFIC VALUE The karst phenomena identified to date only concern the upper part of the thermal aquifer, close to the springs in the city center. Although some geological reports vaguely suggest that all the lime stone strata of the ther mal aquifer are karstified [Iundt et al., 1986], no trace of deep karstification has yet been reported from the boreholes that have been sunk to ab stract the ther mal waters (Reine Hortense bore hole – 1104 m in 1990, and Chevalley borehole – 2200 m in 1992). However, all the elements we collected sug gest that GENESIS AND FUNC TIO NING OF THE AIX-LES-BAINS HY DROTHER MAL KARST (SAVOIE, FRANCE) 325 FIG. 12. – Sy nopsis of the condensa tion-corrosion pro cess after Szunyogh’s theoreti cal mo del ([1990], fig. 12 a) and the experimental mo del of the Cheval ley Aven (fig. 12 b & c). FIG. 12. – Synopsis du proces sus de condensation-corrosion d’a près le mo dèle théorique de Szu nyogh ([1990], fig. 12a) et le mo dèle expéri men tal du gouffre Cheval ley (fig. 12 b & c). these caves are only of hypogenic origin and not old caves filled and re shaped by ther mal water and gaz. Not all the scien tific rich ness of the Aix-les-Bains hy drothermal karst has yet been revealed, as is shown by the recent discovery that aluminum sulfates are forming on the aluminum frame of the win dow enclosing the Alun pool in the Grotte des Ser pents. These sul fates include the very rare jurbanite, which has been found for the first time in a karst cave [Audra and Hobléa, 2007]. Although the waters and underground landscapes of the Aix-les-Bains hydrothermal karst no longer form a directly exploitable resource, they have played an essential role in the area develop ment. As a result, the city must be en cour aged to recognize the rarity and exemplarity of the karst ensemble, which has been proven by over two centuries of detailed and dedicated scientific investigations, and manage it as an integral part of Aix-les-Bains her itage. It is there fore vital that access to the cav ities be pre served, whatever the future holds for the hydrotherapy center that con trols them. As well as allowing tech nical in spec tions (quan ti ta tive and qual ita tive mon i tor ing of the ther mal and in filtra tion waters, de tection of leaks in the cir culation system of the Chevalley hydrotherapy center), access to the cavities is es sen tial if scien tific re search is to continue. This research currently includes the fields of geology, hydrogeology, karstology, biospeleology (discovery of Coecobrya caeca in the sed iments of the Chevalley Aven. 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