Key points:

• In the Opalinus Clay, the diffusion coefficient for the water tracer HTO shows minor variation and is well comparable among the different sites. Owing to differences in porewater ionic strength, the effective diffusion coefficient for chloride shows site-specific differences with lowest values in JO (more pronounced anion exclusion).
• The validity of diffusion properties is established at different scales of space and time: Consistent diffusion properties are observed in small-scale lab experiments and in-situ experiments in the Mont Terri rock laboratory.
• The laboratory diffusion coefficients contribute to plausible modelling of the formation-scale profiles of natural tracers in porewater that evolved over geological timescales.
• Diffusion properties can be estimated from clay-mineral content or porosity.