Estimation of Chloride Diffusion Coefficient and Tortuosity Factor for Mudstone

An experimental estimation of the chloride diffusion coefficient and the corresponding tortuosity factor for a saturated, intact mudstone is described. Laboratory tests simulating one‐dimensional diffusive transport are performed by placing distilled water directly above Bison mudstone specimens having high chloride porewater concentrations. Chloride and other species naturally occurring in the pore water are then permitted to diffuse out of the sample and into the distilled water reservoir for a period of up to 34 days. At the end of the test, the sample is sectioned, and the chloride pore water concentration profile is measured. Fickian diffusion theory is then used to deduce the chloride diffusion coefficient. The diffusion coefficient at a temperature of 10° C ranges from 1.5 to $2.0\times {10}^{-6}{\mathrm{cm}}^2/s\text{,}$ from which a tortuosity factor ranging from 0.15 to 0.20 can be calculated. For comparison, an attempt is made to obtain the diffusion coefficient for bromide diffusing into the sample simultaneous with chloride diffusing out. It is found, however, that the concentration profile obtained for bromide cannot be fitted by the diffusion theory, due to interactions between bromide and other species naturally occurring in the rock sample.