Isotope hydrology of the Mont Blanc Massif

Thanks to the construction of the Mont Blanc tunnel, which is about 12km long, it has been possible to collect water that has drained through cracks in the rock. The heavy isotope concentrations indicate that infiltration is essentially vertical and occurs over the whole topographical profile above the tunnel. Tritium concentrations show that the waters are recent (between a few months and a few years old), despite traversing about 100m of glacier and more than 2000m of rock, and confirm that there is only very slight dispersion through channels. The variable deuterium excess indicates that some waters have undergone a kinetic type of evaporation. The dissolved salt concentrations confirm that the waters pass through channels which are independent and essentially vertical. Comparison of the concentrations of heavy isotopes and salts indicates that the salt content of the granite waters is acquired in the layer of snow and ice and at the rock surface. An evaporation model experiment suggests that the phenomenon probably occurs in the presence of a large moisture deficit, which is attributed to a process of distillation affecting the small amount of water circulating in the unsaturated zone of the rock (evaporation in the cracks as a result of heating, and steam condensation in the upper ice zone by a cold wall mechanism). (author)