The transient diffusion of cationic and anionic tracers through clay-rocks
is usually modeled with parameters like porosity, tortuosity (and/or
constrictivity), sorption coefficients, and anionic exclusion. Recently,
a new pore scale model has been developed by Revil and Linde [Revil
A. and Linde N. (2006) Chemico-electromechanical coupling in microporous
media. J Colloid Interface Sci. 302, 682-694]. This model is based
on a volume-averaging approach of the Nernst-Planck equation. The
influence of the electrical diffuse layer is accounted for by a generalized
Donnan equilibrium model through the whole connected pore space that
is valid for a multicomponent electrolyte. This new model can be
used to determine the composition of the pore water of the Callovo-Oxfordian
clay-rock, the osmotic efficiency of bentonite as a function of salinity,
the osmotic pressure, and the streaming potential coupling coefficient
of clay-rocks. This pore scale model is used here to model the transient
diffusion of ionic tracers ((22)Na(+), (36)Cl(-), and (35)SO(4)(2-))
through the Callovo-Oxfordian clay-rock. Speciation of SO(4)(2-)
shows that similar to 1/3 of the SO(4) is tied-up in different complexes.
Some of these complexes are neutral and are therefore only influence
by the tortuosity of the pore space. Using experimental data from
the literature, we show that all the parameters required to model
the flux of ionic tracers (especially the mean electrical potential
of the pore space and the formation factor) are in agreement with
independent evaluations of these parameters using the osmotic pressure
determined from in situ pressure measurements and HTO diffusion experiments.