Au sein des études menées par l'ANDRA pour caractériser le Callovo-Oxfordien, la chimie de l'eau interstitielle constitue une thématique clé car elle détermine le devenir dans le temps des matériaux introduit sur les sites de stockage (bentonite, béton, métaux, colis de verre). Elle détermine aussi la spéciation (et donc la mobilité) des radionucléides. La méthode développée dans le cadre du projet THERMOAR permet l'acquisition d'un jeu complet de données pour modéliser la chimie des eaux à partir de carottes de roche. Elle nécessite une étude minéralogique approfondie, un modèle de répartition eau libre/eau liée, des expériences de lessivage, des mesures des ions adsorbés, l'acquisition de constantes d'échange d'ions, la mesure des pressions partielles en CO 2. L'ensemble de ces expériences et mesures a été appliqué à des échantillons provenant du site du laboratoire Meuse/Haute-Marne et des forages régionaux de l'ANDRA. On observe ainsi la stabilité régionale d'un grand nombre de paramètres à l'exception d'une diminution de teneur en Na et Cl suivant un axe nord-est / sud-ouest passant par le laboratoire. Le modèle d'équilibre eau/roche permet de calculer la composition chimique des eaux interstitielle de la formation.

Sodium‐K, Na‐Ca, Na‐Mg, and Na‐Sr exchange isotherms were performed at a total Cl concentration of about 0.005 mol L−1 on the fine fraction (<2 μm) of MX80 montmorillonite, mixed‐layer illite smectite minerals (IS), and clayey stones from European underground research laboratories. The derived selectivity coefficients for Na‐K, Na‐Ca, Na‐Mg, and Na‐Sr were found to vary very significantly as a function of the exchanger composition. A model was built to reproduce the data, taking into account the change of selectivity coefficient as a function of the exchanger composition of smectite. Model parameters were fitted using data from the present study and data from the literature obtained on smectite minerals. The addition of parameters for cation–anion pairs was found to improve the fitting results. The model was then successfully tested to reproduce published cation‐exchange features on natural clayey materials and soils suspended in fresh or saline water. Finally, it is shown that the simple combination of this smectite exchange model with a previously published illite exchange model is quite successful although perfectible in reproducing exchange data on IS.

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Na/K, Na/Ca and Na/Mg exchange isotherms were performed on the fine fraction (<2 lm) of Imt-2 illite samples at a total normality of about 0.005 mol/L in anionic chloride medium. The derived selectivity coefficients for Na/K, Na/Ca and Na/Mg were found to vary as a function of the exchanger composition and compared well with the data collected in the literature for similar experimental conditions. Two models were built to reproduce the data: the first was a multi(2)-site model with constant Gaines and Thomas selectivity coefficients; the second was a one-site model taking into account surface species activity coefficients. The results of the models were in rather good agreement with both our data and literature data. The multi-site model proved to be efficient in predicting the exchanger composition as a function of the Na/Ca/Mg/K concentrations in solution, whereas the one-site model proved to be a better approach to derive the Na/ Ca/Mg/K concentrations in solution based on the knowledge of the exchanger composition and the total normality of the solution. The interest of this approach is illustrated by the need for major cation solute concentration predictions in compacted clay for the characterization of nuclear deep disposal host rock repositories.

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