Diffusivity and porosity studies in rock matrix: The effect of salinity

The nature of diffusivity and porosity in rock was studied as a function of various parameters. The phenomena of main interest were dead-end porosity, ion-exclusion and sorption. The rock types studied were rapakivi granite, granite and gneiss, and tracer techniques with 36Cl-, 22Na+ and 3H (HTO) were used as a research method. An analytical solution for outdiffusion from a porous cylinder was developed by applying a corrected form of Fick's second law for a case where part of the pores are so- called dead-end pores. With this model the theoretical curve could be closely fitted to the measured values. It was found that the rock-capacity factor is an increasing function of the ionic concentration of the solution in the case of Cl- indicating ion-exclusion, while the opposite is true in the case of Na+ indicating ion-exchange type sorption. The effective diffusion coefficient was also found to vary as a function of the salinity in the case of 36Cl. In the case of 22Na, the effect was opposite and weaker. The diffusion of tritium through the rock samples was clearly higher than the diffusion of 36Cl. Part of the difference is explained by the smaller effective porosity of 36Cl. The rest can probably be explained by the steric effects on the chloride ion caused by the negatively charged pore surfaces in the narrow pores. Matrix diffusion was studied in the nature in the island of H stholmen on the coast of the Gulf of Finland, which due to postglacial land uplift started to rise from the Baltic Sea some 5000 years ago; the land uplift is some 3 mm per year. Rainwater has formed a layer of fresh groundwater floating on top of the saline layer. The coarse-grained granite bedrock of H stholmen has been investigated for suitability of an underground repository for reactor wastes. Non-fractured core sections of several metres were selected between adjacent water-conducting fractures, the water in the fractures is of low salinity. The sections were sampled at intervals of about one metre. The pore water from core samples was extracted by immersing the samples in ion-exchanged water. The contents of chloride and sulphate in the water were analyzed. Salinity of the groundwater is reflected in the anion concentration of the pore water. This study has shown that regions with gradually decreasing salinity can still be found indicating matrix diffusion in the scale of metres.