Sorbing tracer experiments in a crystalline rock fracture at Äspö (Sweden): 1. Experimental setup and microscale characterization of retention properties

Abstract

Mineralogical and retardation properties of rock materials responsible for water‐rock interaction in in situ migration experiments with sorbing radioactive tracers were studied in laboratory experiments. The porosity was studied by water saturation measurements and the PMMA method was used for detailed porosity characterization of heterogeneity distributions and porosity profiles toward the fracture surface. Mylonite and altered diorite sampled in the rim zone of the fracture and representative bulk rock types were investigated by batch sorption measurements with crushed materials and through‐diffusion and in‐diffusion experiments in intact rock pieces. Autoradiography was used for visualization of in‐diffusion profiles of sorbing tracers. The use of detailed porosity information and quantitative data on heterogeneity in porosity is shown to significantly improve the interpretation and evaluation of laboratory‐scale diffusion experiments. We show through the combined approach of detailed porosity characterization and laboratory sorption and diffusion investigations that we can distinguish retention properties of bulk rock and altered rock and provide qualitative and quantitative data of heterogeneous rock properties that expand the possibility for including relevant processes in the interpretation of the results of in situ tracer tests.