Deuterium distribution in the pore water of Tournemire clay-stone. First modelling attempts

As part of the IPSN/CEA (France) research program for determining the hydrogeological behaviour of an indurated clay-stone formation (Tournemire, France), an isotopic investigation of pore-water has been performed by LHGI/Universite de Paris Sud. The isotope characterisation of pore-water from the non fractured zone of the argillite formation gave indications about the meteoric origin of this water. Isotopic data are situated in the δD vs. δ18O diagram around the global meteoric water line but below present day karst water. This suggests that pore-water has been 'recharged' under cooler meteoric condition than present. The distribution profile of deuterium in pore-water, symmetrical with respect to the center of the formation suggests a diffusive mixing in variable proportions between two kinds of water. In order to confirm this hypothesis, attempts to model the diffusion process in the argillite formation have been performed. The reasonable agreement between the observed data and the model shows that a pure diffusion process may be at the origin of the stable isotope distribution. (author)