Evaluation of tritiated water diffusion through the Toarcian clayey formation of the Tournemire experimental site (France)

Through-diffusion experiments with tritiated water were performed on argillaceous samples from various zones of the Tournemire test site. It was intended to evaluate the homogeneity of the transport property of unfracturated samples and the influence of the orientation and the nature of the samples (presence of an opened fracture or a pre-existing tectonic fracture filled with calcite and pyrite). Homogeneous values of the tritiated water (HTO) effective diffusion coefficients were deduced from experiments carried out when diffusion occurred parallel to the stratigraphic bedding, with an apparent sensitivity to experimental conditions. Anisotropy was significant, De(HTO) perpendicular to the bedding being 1/3 lower than that parallel to the bedding. The observed fractures of the samples created by mechanical stress and partial dehydration during sawing and the presence of a pre-existing opened fracture did not affect the effective diffusion coefficients of tritiated water, which is probably due to the healing ability of the clayey medium during the re-saturation phases of the equilibrium steps performed prior to the diffusion experiments. On the contrary, a significant decrease of this transport parameter was induced by the occurrence of a pre-existing tectonic fracture, which was assigned to the dense structure of the filling phases.