Hydraulic conductivities of the Opalinus Clay and the surrounding rocks are very low. Accordingly, molecular diffusion is an important, or even the major, process by which radionuclides may be transported towards aquifers.
The modelling of the diffusive solute flux relies on appropriate diffusion coefficients. Diffusion coefficients for weakly- or non-sorbing species are typically determined in the laboratory using small, centimetre-sized samples and experimental setups that last from weeks to a few months. Field experiments in underground rock laboratories provide larger spatial scales (dm – m) and longer timescales (up to a few years) to validate laboratory parameters. Finally, profiles of natural tracers from deep boreholes provide a possibility to deduce transport processes at the formation scale (Section 4.6). This section presents and discusses diffusion parameters obtained in laboratory and field experiments and relates them to other rock properties.