Characterization of the hydrogeological boundary separating two aquifers: : a multi-disciplinary approach combining geological, geochemical and hydrodynamic data (Aix-les-Bains, France)

• Autores: Stéphanie Gallino, Marc Dzikowski, Jean-Yves Josnin, D. Gasquet
• Localización: Bulletin de la Société Géologique de France, ISSN 0037-9409, Vol. 181, Nº. 4, 2010 (Ejemplar dedicado a: Hydrothermalisme en domaine continental / coord. por D. Gasquet), págs. 305-313
• Idioma: inglés
• Títulos paralelos:
  • Caractérisation d'une limite hydrogéologique entre deux aquifères contigus au moyen de données géologiques, géochimiques et hydrodynamiques (Aix-les-Bains, France)
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• Resumen
  • français

    Cette étude permet au moyen de divers outils (géologie, géochimie, hydrodynamisme) de délimiter deux aquifères contigus: celui des eaux minérales de Raphy Saint Simon (RSS) et celui des eaux thermales des Thermes nationaux d'Aix-les-Bains (TN) et de discuter les relations entre ces deux aquifères.

    La réinterprétation des profils de sismique réflexion et la cartographie géologique ont permis de différencier deux grands types de structure anticlinales: i/au nord, 1'anticlinal posséde une structure d'amplitude plus faible et un flanc oriental recoupé par deux failles de rétrochevauchement; ii/au sud, 1'anticlinal est plus large et une écaille tectonique est coincée dans la rampe de chevauchement. La continuité des structures étant impossible entre ces deux compartiments sur une aussi courte distance conduit à positionner un accident d'orientation N065°E, la faille de Raphy Saint Simon, entre la terminaison périclinale de la Chambotte et la colline de Tresserve.

    La signature géochimique (ions majeurs et isotopes) des forages profonds est différente bien qu'ils captent leur eau dans un même horizon géologique carbonaté. En effet. le forage RS4 présente des concentrations en sulfates supérieures à celles du forage RS5 (100 mg.L-1 contre 20 mg.L-1). La signature isotopique en 34S des eaux du forage RS4 (+18,9{per thousand} vs CDT) est proche de celles des eaux thermales (+31,5{per thousand} vs CDT) tandis que celle du forage RS5 (+8,9{per thousand} vs CDT) est plus proche de celles des pyrites (-0,8{per thousand} vs CDT). Cette différence de chimisme impose done que ces deux forages se situent dans deux compartiments géologiques différents séparés par la faille de Raphy Saint Simon.

    D¿un point de vue hydrodynamique, la présence de cette faille permet de maintenir une différence de niveaux plus basse de 30 mètres dans le compartiment nord que dans le compartiment sud. Néanmoins, cette faille permet les transfers de pression puisque les variations de débit d¿exploitation sont observables d¿un compartiment à l¿autre. Enfin, des phénoménes propres aux émergences minérales (RSS) permettent de confirmer la présence de deux aquifères à fonctionnement distinct: en profondeur (500 m) le forage RS5 (bloc nord) enregistre des chutes de niveaux lors d¿événements pluvieux tandis que RS4 (bloc sud) n¿en présente pas. En outre, le puits de Raphy Saint Simon (RSSW) (bloc nord) possède une cyclicité en phase avec la pression atmosphèrique alors que les émergences thermales des TN (Bloc sud) n¿en preséntent pas.

  • English

    Aix-les-Bains has three installations that use deep water: the Marlioz and Thermes Nationaux spas, to the south, and the Raphy Saint Simon (RSS) mineral water plant, to the north. The spas draw their water exclusively from deep boreholes, whereas the RSS bottling plant has a natural spring (RSS well) and two boreholes (RS4 and RS5), both of which are more than 500 m deep. Although the spas and mineral water plant are only a few kilometers apart and they catch their waters in the same calcareous layers, their waters have distinct physico-chemical characteristics, suggesting that they are derived from two different but adjoining aquifers. The present study used geological, geochemical and hydrodynamic data in order to determine the boundary between the thermal water and mineral water aquifers, and to investigate the relationship between them.

    Geological mapping and a reinterpretation of seismic profiles produced in the 1970s for oil exploration were used to investigate the structure of the area between Aix-les-Bains and La Chambotte (8 km north of Aix). This relatively small area was found to contain two very different types of anticlinal structure: a fault-bounded anticline to the north and a box fold to the south. Dip measurements for both anticlines revealed sub-vertical western flanks, less steeply dipping eastern flanks and sub-horizontal central sections. The central section of the northern anticline is much narrower than the central section of the southern anticline. Both anticlines have been thrust over their adjoining synclines; however, the thrust plane is much steeper in the north than it is in the south. As a result, equivalent strata are at a higher altitude to the north of Aix-les-Bains than they are to the south of the city. In addition, the eastern flank of the northern anticline is intersected by two backthrust faults and the southern anticline is affected by a peel thrust. Given the extremely rapid transition from one anticlinal form to another, these two structures cannot be contiguous; however, their juxtaposition can be explained by the presence of a fault oriented N065°E. This fault is not visible at outcrop but it must lie between the end of the La Chambotte anticline and the hill at Tresserve. It has been named the Raphy Saint Simon Fault (RSSF).

    Water samples from wells and springs within a 300 m radius of the RSS well field had identical or similar major ion concentrations to the mineral waters; nevertheless, two distinct chemical facies were recognized on the basis of differences in magnesium and sulfate concentrations. For example, the sulfate concentrations of water samples from RS5 and the RSS well were five times lower than those of RS4 (20 mg.L-1 vs. 100 mg.L-1). Differences were also found in the sulfur isotope signatures of samples from the two boreholes, even though both boreholes abstract their waters in the Upper Kimmeridgian limestone. The sulfur isotope signature of RS5 (8.9{per thousand} vs. CDT) is closer to that of pyrite (-0.8{per thousand} vs. CDT), whereas the signature of RS4 (18.9{per thousand} vs. CDT) is closer to that of the thermal water (31.5{per thousand} vs. CDT). These differences in sulfur isotope and major ion concentrations show that the recharge waters for the two boreholes have different sources and that the boreholes must therefore be in different geological blocks separated by the RSSF.

    Monitoring of the dynamic water levels in the boreholes and the flow rate of the RSS well allowed us to determine the hydrogeological and geological limits. Variations in the water yields of the boreholes in one block were observable in the adjoining block. Although the fault allows the transfer of pressure between the two blocks, it also leads to the water level in the northern block being 30 meters lower than the water level in the southern block.

    Some phenomena were only observed in the northern, mineral-water block. For example, during rainfall events water levels were seen to drop suddenly in RS5 but not in RS4. In addition, we observed cyclical variations in the flow rate of the RSS well, as well as variations related to atmospheric pressure. These phenomena were not observed at the natural thermal water outlets in the southern block.

    Geological, geochemical and hydrodynamic data all indicate the existence of a N065°E-trending fault near the RSS well field. This fault marks the boundary between the mineral water aquifer to the north and the thermal water aquifer to the south. It maintains a difference in the water levels of the two aquifers, but allows the transfer of pressure between them