Modelling some aspects of ion migration in a compacted bentonitic clay

Abstract

This paper presents a study of the behaviour of soluble salts (major ions) in a highly compacted bentonite subjected to a thermo-hydraulic gradient. This is achieved via the application of a numerical model, COMPASS, which implements a fully coupled thermo/hydro/mechanical formulation for unsaturated soils, including certain aspects of reactive chemical transport. In this study, thermal transfer is considered to take place via conduction, convection and latent heat transfer; moisture transfer in both the liquid and vapour phases due to liquid and vapour gradients, and chemical transfer due to both advection and hydrodynamic dispersion are also considered. Chemical reactions considered include ion exchange reactions involving major cations (Na +, K +, Mg 2+ and Ca 2+) and precipitation–dissolution of trace minerals (calcite, dolomite, anhydrite and halite). As a first approximation an elastic constitutive deformation model is implemented in this study. For the simulation of the thermal and moisture field it is claimed that the models can represent the important mechanisms occurring in the soil. Considering the behaviour of major ions, the model is able to qualitatively simulate the experimental behaviour. Therefore, confidence in the coupling of the heat, moisture and chemical variables has been gained. Moreover, since the ions are also undergoing simultaneous geochemical reactions, with the main processes being ion exchange and precipitation/dissolution reactions, confidence in the coupling between the geochemical and the chemical transport model has also been gained. As regards the mechanical behaviour, where an elastic model was assumed as a first approximation, in the absence of experimental results, the model showed acceptable trends.