Typologie des aquifères du cristallin : exemple des massifs des Aiguilles rouges et du Mont-blanc (France, Italie et Suisse)

This hydrogeological study of the crystalline aquifers of the Mont-Blanc and the Aiguilles Rouges massifs is part of the AQUITYP project, which has been carried out at the Geology Laboratory of the Swiss Federal Institute of Technology, Lausanne (GEOLEP). This study has encompassed the observation, sampling and systematic chemical analysis of more than 160 surface and groundwaters. Many of the springs are located within the numerous galleries and tunnels which cut the crystalline massifs of this region. The Mont-Blanc and the Aiguilles Rouges massifs, situated at the borders of France, Switzerland and Italy, are part of the "extreme crystalline" massifs of the western alpine arc. They are formed by a succession of metamorphic and complex eruptive rocks, mostly of the gneissic and granitic types of the Hercynien Age. The study of the regional groundwater systems necessitated the setting of an extensive observation network, with different investigative levels, with the aim of acquiring information on the time and space variations of the hydrogeological parameters. The groundwaters and surface waters (streams, snow, rain and soil wear), as well as the characteristics of the rocks, were the object of complete chemical analyses (both major and trace elements). Finally, laboratory tests of rock powder dissolution (lixivation) were carried out in order to simulate the mineralization of the groundwater and to decipher the origin of the dissolved ions. The chemical analyses of the major and trace elements of the surface waters (snow, rain and soil) showed that, with the exception of chloride, nitrate and barium, the dissolved solids revealed much lower concentrations than the least mineralized groundwater observed in all of the massifs. However, stream waters showed relatively similar characteristics to those of the groundwaters. More than 85% of the observed groundwaters were at a temperature less than 12°C, while the subthermal (between 12 and 20°C) and thermal water (>20°C) represented 5 and 10% of the springs, respectively. The springs observed at the surface as well as in the galleries could reach more than 4000 l/min. The total mineralization of the groundwaters studied varied h m 20 to 650 mg/l, with an average of 90 mg/l. The chemical classication of the groundwaters showed that more than 90% of the springs were of the calcium bicarbonate type, 8.5% of the calcium sulfate type and 1.5% of the sodium chloride type. Time variations studies showed large fluctuations of flow rate in all of the observed cases, with peak values generally corresponding to periods of important snow thaw at high altitude (May to July), while the lower values corresponded roughly to low air temperatures (as all of the precipitations were stored as snow). In Comparison to the large variations in flow rate, the temperatures showed a generally stable behavior pattern, while the mineralization of the water in the granitic aquifers showed small, but significant, variations. Trac