Redox front formation in fractured crystalline rock: an analogue of matrix diffusion in an oxidizing front along water-conducting fractures

Redox fronts may be formed in the geological environment surrounding a shallow geological repository for radioactive waste due to oxidized pore water infiltration. Such oxidation fronts can exert an influence on contaminant retardation during matrix diffusion via redox changes and direct fixation by accumulated oxides. However, the process of matrix diffusion in oxidizing fractured crystalline rocks has rarely been described. This study focuses on a redox front in Cretaceous granitic rocks from central Japan. Detailed observations reveal that ferric iron has accumulated at grain boundaries and formed micro-spherical Fe-oxyhydroxide aggregations. Geochemical analyses with X-ray fluorescence, inductively coupled plasma–mass spectrometry and electron probe microanalysis examination show that rare earth elements, Cs and U have migrated to the front. Uranium, in particular, has been enriched at the front edge and probably retained by Fe-oxyhydroxides and this relationship persists in the oxidizing zone, presumably due to the strong adsorption capacity of the oxyhydroxides. These findings suggest that matrix diffusion in the oxidizing zone also can be an effective contaminant retardation.