Diffusion pathways in crystalline rock—examples from Äspö-diorite and fine-grained granite

The influence of heterogeneous porosity, mineralogical composition and texture on the diffusion of alkali and alkaline earth metals in water-saturated crystalline rock was studied. Through-diffusion technique was used, and the concentration profiles in the rock samples were measured. The porosity distribution was studied by the 14C-polymethylmethacrylate (PMMA) impregnation method. The distribution of the tracers was observed from autoradiographs and was then compared to the mineralogical and structural composition of the rock. Radionuclides of the alkali and alkaline earth metals were used as tracers, i.e. 137Cs+, 45Ca2+, and 133Ba2+. A double porosity network of slow and fast migration pathways, with different sorption capabilities was used to describe the diffusion results.