The characteristics of indurated argillaceous formations, considered as potential host media for high level radioactive waste disposal, differ in many aspects from plastic clays or crystalline rocks. In order to develop generic research on the containment properties of such formations, the French Institute for Protection and Nuclear Safety (IPSN) selected the Tournemire site ]1]: a sub-horizontal claystone layer (250 m thick, argilites and marls from Toarcian and Domerian formations) located between two limestone aquifers. Water circulations seem to be of very minor importance in the claystones, with a main diffusive component for transport ]1]. However, since a few weak water inflows occur in fracturing networks induced by tectonics, one of the key issues in the site characterisation is to adequately identify the water flows and the related transport and retention features within these specific water-conducting features. It is the purpose of the authors to present some insights in such a complex topic, using informations acquired in the framework of the Tournemire Project.