Diffusion of an alkaline fluid through clayey barriers and its effect on the diffusion properties of some chemical species

This work, which lies within the scope of the European project ECOCLAY II, focuses on the effects of an alkaline solution on the diffusion of solutes in clay barriers. The solids investigated here were a mudrock of the Bure site (Meuse/Haute-Marne, France) extracted from the Callovo-Oxfordian layer and a compacted MX-80 bentonite, as an example of a clay used for engineered barriers. The alkaline solution was a synthetic "evolved" cement water, controlled by portlandite dissolution (pHsimilar to12.5). Through-diffusion tests with HTO, K+ Ca2+, Na+, Cs+ and Cl- were performed in the presence of the alkaline solution. Diffusion properties of HTO, K+ and Ca2+ were also evaluated under site conditions (i.e., without cement water). Conclusions to date are as follows: The D-e values of calcium and potassium, which were the two major cations present in excess in cement water compared to site water, are higher when diffusion occurs with the alkaline fluid than when the migration of these species is determined under site conditions. The HTO effective diffusion coefficient decreases over a 12-month period when one face of the clay sample is in contact with cement water. Such a behaviour could be caused by clogging of voids. For a given clayey material, D-e values of the considered cations have the same order of magnitude, but D-e(Cl) is one to two orders of magnitude lower. Such a difference is attributed to anionic exclusion processes that occur in argillaceous media. It is thus concluded that assigning a single diffusion coefficient to all the dissolved species when modelling an alkaline plume spread in a clayey material is a debatable hypothesis. (C) 2004 Elsevier B.V. All rights reserved.