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, characteristic of gypseous conduits. The average concentration of Sr in the waters is 7.9 mg/l. In 70% of the cases, it was higher than that of K or Na. The cold selenitic waters are slightly mineralized with K and Na, as well as with Ca. Their average levels were less than 7 mg/l. The significant marking of the gypsum on the water has also been confirmed by the discovery of trace elements which are characteristic of this rock: these are Rb, Ni, Cu, Sc and Y. Other elements, such as Al and Li, seem to come from the carbonated karst or from the clays which are often present as filling material in the microdiscontinuities of the gypsum. The selenitic waters may also flow through fractured quartzite. This was observed in the Penninic. The approach based on hydrological flow patterns and time variations in the physical-chemical parameters of the water showed that, in spite of variable flows linked to snow thaw, the temperature and the mineralization remained stable. This signals that the water remained a sufficient amount of time in the aquifer for thermal and geochemical equilibrium. The hydrogeological behaviour of the aquifers depends on the model of the hydrogeological basin to which it belongs. If the infiltration and emergence zones are adjoining, then modifications in their hydraulic, geochemical and thermal behaviours may occur (mixing of two types of waters with different mineralization and temperature). On the other hand, if the zones are not adjoining, the fluctuations are small. The joining models were observed in the Ultrahelvetic, while the non-joining ones characterize the hydrogeological basins of the Median Prealps, the Helvetic and the Penninic. The various tracing studies carried out in the evaporitic karst permitted the determination of rather weak velocities, between 10 and 1000 m/day. These are clearly inferior to those observed and the karstic conduits of Jura. The moderate speeds are linked to the hydrogeological basin model; the water movement is more rapid when the infiltration and emergence zones are adjoining. The study of spatial variations of the physical-chemical parameters of the water showed a rather weak heterogeneity at the scale of Rhone Basin. This was also observed in the composition of the gypsum. The maximum measured flows were approximately 5000 l/min. The average temperature was 8.7°C. A detailed study of the geochemical characteristics of the aquifers showed that those of the Penninic, and to a slight degree, those of the Ultrahelvetic, were different. Generally, the waters from these aquifers are more mineralized than those of the Median Prealps and the Helvetic. Particularly high levels of uranium were observed in the selenitic waters of the Penninic. The maximum values were on the order of 70 mg/l. They are due to the presence of extra-Triassic formations in Triassic rocks, particularly the permo-carboniferous black shales of the Penninic. This geological structure does not exist in the other three cases. Generally speaking, the evaporitic aquifers of the Penninic have hydraulic and geochemical characteristics which are more exotic than the other three secondary types. A comparison of the data from this study with those of waters from the same lithologic and stratigraphic origin in the northern part of Switzerland show a strong homogeneity. However, this is not the case for certain aquifers of Trias in Northern Italy. Globally, the evaporitic aquifers have a chemical composition which is very different from those which characterize other aquifer types, especially those studied in the AQUITYP project. They are especially more mineralized, and the minimum levels of SO4, Ca, Mg, Sr and Rb observed in the selenitic waters are superior to the former in many cases. The aims of this study were more than achieved. The geochemical characteristics of the four secondary types of evaporitic aquifers were observed and distinguished in complex hydrogeological systems.