[en] The geochemical characteristics of the host rocks play a major role with regard to the mobility of radionuclides contained in wastes disposal in geological formations. Redox conditions in particular may control the potential migration of radionuclides within the geosphere. Geological and geochemical investigations were conducted in a quarry in the Tiber Valley, near Orte, with the aim of understanding the geochemical variations as well as their evolutive trend of local clay at the oxido-reduction front. The upper part of the local plio-pleistocenic series is made up of clay banks interbedded with sandy levels. Clay is dominant in the lower part of the series; sand constitutes on the contrary the final cover of the sequence. The different colors shown by the two different lithological components, grey for clay and yellow for sand, probably reflects a different geochemical condition, reduction for the grey, oxidation for the yellow one. Both clay and sand components were presumably originally characterized by reducing conditions; they were therefore grey colored. The preferential permeation of meteoric water within the permeable sands is at the origin of the oxidation of this component, once the whole series had been uplifted in the continental environment yellow color is a record of this event. The original geochemical content of U, Th and V of the clayey levels appear to be unaffected by the strong oxidizing environment of surficial water. This fact testifies the substantial impermeability of clay as well its capacity of maintaining the original reducing characters which should prevent the transuranic radionuclides of waste to be mobilized. Bacteria have been demonstrated to be at a latent state of life within clay. They may contribute in maintaining the original reducing character of clay