Contaminant retardation in fractured shales: Matrix diffusion and redox front entrapment

Diffusion of contaminants from fractures into the interconnected wall-rock porosity is an important retardation process in geosphere transport. Contaminant immobilization can also be achieved at a redox front, owing to contrasting solubilities of reduced and oxidized species along with sorption and coprecipitation on secondary phases produced at the front. Here, a natural analogue study is presented that demonstrates the applicability of these concepts to potential sedimentary host formations for a high-level radioactive waste repository in Switzerland. The existence of matrix diffusion to at least 0.08 m into the wall rock (the full length of the profile examined) is demonstrated on the basis of mineralogical and porosimetric evidence, pore-space geometry and disequilibria in the U-238-and Th-232-decay series. Radioactive disequilibria are due to at least two distinct processes, namely pervasive U-234 and Ra-228 leaching throughout the rock along with U deposition around the redox fronts.