De la pluie à l'eau souterraine. Apport du traçage naturel (ions majeurs, isotopes) à l'étude du fonctionnement des aquifères volcaniques. (Bassin d'Argnat, Auvergne, France)

摘要

Hydrogeological characteristics of the Chaîne des Puys are linked with its climatic, geological, morphological and environmental characteristics. These specifities constrain origin, quantity quality and flow of water in volcanic aquifers. As the chemical and isotopic signatures of water reflect processes occurring during the water cycle, their variations into and between the various compartments specifies the role of these factors. The subject of this thesis deals with the chemical (major ions) and isotopic (δ18O, δ2H, δ13C) characterization of different stages of the water cycle to define the operation of sub-volcanic aquifers of the Chaîne des Puys. In this perspective, this work focused on two themes: 1) the definition of the input signal of regional aquifers, a prerequisite to any hydrogeological study, and 2) studying the functioning of the Argnat catchment, located in the North of the Chaine des Puys. The study of the mechanisms of acquisition of the rain chemistry (Na+, K+, Mg2+, NH4+, Ca2+, Cl-, NO3-, SO42- , PO43-, HCO3-) based on its long-distance travel (rainout) and the influence of meteorological parameters on the local chemical input during rain (washout), shows that the majority of rain comes from the Atlantic and the Mediterranean Basin and is predominately influenced by terrigenous (Ca2+, HCO3-) and anthropogenic (NO3-,NH4+,SO42-) elements, often associated (neutralization process). The local influence on the chemistry is largely due to the terrigenous sediments from the basin of the Limagne, but soil moisture, wind direction and speed can promote recharge of cloud by anthropogenic aerosols from Clermont-Ferrand. The evolution of chemical (major ions) and isotopic (δ18O, δ2H, δ13C) parameters between the different compartments of the Argnat basin emphasizes the role of the unsaturated zone in the supply of the saturated zone in volcanic aquifers. Thus it appears that the scheme of a vertical circulation is not simple and that the behavior of the unsaturated zone is close to that of a perched aquifer. In this case, the existence of a perched aquifer can only be explained by superimposed circulations within the basalt flow which is heterogeneous in terms of physical characteristics and therefore the hydrodynamic. From 10 sampling points, the study of the transfer into the saturated zone from upstream to downstream, given the geological context and topography, allows estimate the relative contribution of supply from high and low altitudes. Those from low altitude significantly modify the water chemistry, particularly through chlorides and nitrates contributions. The interpretation of δ13C measurements associated with calculations of pCO2 confirms the presence of deep origin CO2 and precises its geographic distribution within the basin Argnat.