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. Tracer tests at high altitude showed water circulations from the Helbronner peak (Mont-Blanc Massif, altitude of 3470 m) to the Mont-Blanc Tunnel. These tests revealed the discontinuous character of the permafrost and high velocities of groundwater infiltration. The observation of the spatial variation of all of the available physical and chemical parameters permitted a typological synthesis of the principal aquifer sub-types of the Mont-Blanc and the Aiguilles Rouges massifs. Hydraulic typology: The granitic aquifers showed high permeabilities, due to the influence of both micro and macro tectonic discontinuities. The gneissic aquifers are characterized essentially by homogeneous flow with low permeability, which is linked to systems of micro tectonic discontinuities. Hydrochemical typology: The analysis of major and trace elements showed a certain number of natural tracers for the three principal aquifer sub-types: The waters originating from the granitic aquifers of the Mont-Blanc Massif are very strongly marked by the presence of molybden, tungsten, fluoride, uranium and sodium. Uranium is clearly represented if related to the deep aquifers with rapid flow (recent water). The waters of the gneissic aquifers of the Mont-Blanc Massif show, overall, a small anomaly of arsenic while the gneissic aquifers of the Aiguilles Rouges Massif are marked by slightly higher concentrations of barium and magnesium. The markers of the principal sub-types were verified and confirmed by an analysis of principal components. The spring of Brocard (Martigny, VS, Switzerland), which was installed at the initial phase of the AQUITYP project for the characterization of the crystalline aquifers, showed itself to be representative of the characteristics of the gneissic aquifers of the Mont-Blanc and the Aiguilles Rouges massifs and non representative of the deep granitic aquifers of the Mont-Blanc. The comparison with other crystalline aquifers of the west alpine arc showed that the principal markers of the granitic aquifers of the Mont-Blanc (fluoride, uranium and sodium) were also found in the Aar and the Belledonne massifs. However, relatively small similarities were found with the deep aquifers of the crystalline basement in the north of Switzerland, which is the seat of deep and slow flow of thermomineral water. In contrary, the aquifers of the Mont-Blanc, Aar and Belledonne massifs are mostly characterized by rapidly renewed flows. The comparison with other aquifer groups of the AQUITYP network showed no significant hydrochemical similarities. The slightly mineralized water of the crystalline rocks, compared to the waters of the AQUITYP cross-section which contain globally much more dissolved minerals, revealed that lithium, fluoride, boron, arsenic, rubidium, tungsten and uranium are found in higher concentrations than in the reference network. These last elements may be considered as special markers for the crystalline, compared to the more mineralized groundwaters. This should be verified by the study of other springs which will be added in the AQUITYP network. The knowledge of the simple aquifer structures obtained in this study of the crystalline of the Mont-Blanc and the Aiguilles Rouges massifs allows a better comprehension of the complex hydrogeological conditions which often include mixture phenomena among several types of aquifers with different lithological characteristics. Such knowledge is one of the tools which permit the determination of the origin of water sources, particularly in underground construction projects.