Typologie des aquifères évaporitiques du trias dans le bassin lémanique du Rhône (Alpes occidentales )

The typology of evaporitic aquifers is part of the AQUITYP research project, whose principal aim is the hydrogeological characterization of different aquifer types between the Jura and the Swiss Alps. This study has been directed by the Geology laboratory of the Swiss Federal Institute of Technology in Lausanne (GEOLEP) since 1981. More than one hundred sources were studied. These sources are part of the vast observation network laid out in the Rhone Basin, above Lake of Geneva, and are linked to the evaporitic aquifers of Trias, which developed at the base of the important tectonic unit of the Western Swiss Alps: Median Prealps, Helvetic, Ultrahelvetic and Penninic. These tectonic considerations permitted the division of the aquifers of Rhone Trias into four secondary evaporitic aquifers. This division seemed necessary in order to arrive at a typological synthesis of the hydrogeological behaviour and the geochemical characteristics of each one of these types. No general hydrogeological study of the cold waters of Triassic evaporites of this regions had ever previously been carried out. The emphasis in this study was placed on the selenitic sources whose annual average temperature is less than 12°C. The behaviour of the physical-chemical parameters of these waters were studied as a function of time and space at the scale of one tectonic unit as well as the four together. In addition to the major and minor components, a large number of trace elements in the groundwaters were detected for the first time. According to degree of karstification, the gypseous rocks constitute the principal aquifers of Trias. These were analyzed very intensively, using several methods to determine the components and the chemical elements of these rocks, according to whether they were soluble in water, weak or strong acid or after alkaline fusion. The gypsum leaves a very strong chemical imprint on the waters. This is characterized by a total average mineralization of 1.8 g/l with 50 to 90% SO4 (average level = 1041 mg/l) and Ca (average level = 419 mg/l). Other karstic rocks mineralize more or less strongly in these waters; they are calcareous dolomitic rocks of Trias (dolomite, dolomitic limestone and cellular dolomite). Levels of HCO3 (average level = 251 mg/l) and Mg (average level = 66.6 mg/l) were observed, each which may represent 10 to 20% of the total chemical composition. The carbonated rocks make up the secondary aquifers associated with the gypseous karst. Approximately 80% of the sources showed similar chemical compositions, where Ca, Mg, SO4 and HCO3 formed the major ions. In order to better characterize the chemical composition of these waters, theoretical hydrochemical facies based on the minor and trace elements detected in the waters were created. The ion strontium found in the water showed itself to be a marker of the gypsum bearing the same name as did the sulfate and calcium ions. It is linked to the presence of authigenic crystals of celestine, char