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The hydrogeochemical transport model PHREEQC was extended with options to calculate multicomponent diffusion in free pores and in the diffuse double layer (DDL). Each solute species can be given its own tracer diffusion coefficient. The composition of the DDL is calculated with the Donnan approximation. With these options, solute species can be transported in coexisting charged and uncharged regions as may exist in clays and membranes. The model was developed to simulate in-situ tracer diffusion experiments in Opalinus Clay with tritium, iodide, and sodium. Tritium gives the formation's tortuosity factor, which applies in principle for all the neutral species. Half of the porosity is not accessible for iodide due to anion exclusion, and assumed equal to the amount of DDL-water. With this assumption, the tortuosity factor for iodide is 1.4 times higher than that for tritium. The sodium data can be matched by reducing the tortuosity factor 1.6 times relative to tritium, and by distributing the cation exchange capacity over the DDL and fixed sites that are spread heterogeneously over the model domain. The physical origin of the variable tortuosity for differently charged species is discussed.

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