Contribution of chemical and isotopic contents to the characterization of groundwater circulation in a thermo-mineral system. A case study in Auvergne, Massif Central (France): A case study in Auvergne, Massif Central (France)

International audienceThe numerous thermo-mineral springs of Auvergne originate from the regional extensive dynamics, connected to the formation of the Alps, and which gave rise, in Auvergne, to a succession of horsts and grabens. These thermo-mineral springs are known and used since the Antiquity, as shown by numerous Gallo-Roman sites, and still have an economic role, regionally demonstrated by the presence of 10 thermal spas in Auvergne. The general circulation of these thermo-mineral waters corresponds to an infiltration of meteoric waters reaching deep zones of the crystalline basement via a dense network of faults. At this depth, waters acquire a high temperature and high content of gaseous CO2 which confer them a high power of dissolution. Thermo-mineral waters emerge from crystalline, sedimentary or volcanic rocks and present HCO3/Na or (Cl-HCO3)/Na types in connection with the regional geology. The present study concerns 16 springs located in Auvergne which were sampled and analyzed for physico-chemical parameters (T°C, EC, pH, CO2), major ions (Na, Ca, Mg, K, HCO3, Cl, SO4, NO3) and carbon-13. The study allows classifying springs according to their geologic context of circulation and emergence. Thermo-mineral springs appearing in crystalline basement are generally less mineralized than those emerging from sedimentary layers, mainly marly-calcareous. Isotopic determinations of carbon-13 highlight the endogenous origin of CO2 resulting from a mantelic degassing. The use of geothermometers allows calculating deep temperatures of groundwaters’ circulation. They vary between 134°C and 278°C that lead to the deduction of a depth of groundwaters’ circulation of 3 to 8 km, according to the local geothermic gradient between 3.3°c and 4°C/100m (Genter et al., 2003). An inventory on the chemistry of regional thermomineral springs have previously been realized (Négrel, 2004; Serra et al., 2003; Pauwels et al., 1997) and allows to define the origin of waters and to integrate the data of this study into a regional context. Besides, a time-following of the physico-chemical, chemical (major ions and traces) and isotopic parameters (oxygen-18, deuterium and carbon 13) was set up on the Ours’ thermomineral spring since January 2013 in order to verify the supposed stability of the chemical composition of this specific type of groundwater