Avoiding Unrealistic Behaviour in Coupled Reactive-Transport Simulations of Cation Exchange and Mineral Kinetics in Clays

Bentonite clay is often included as a buffer, backfill and/or sealing material in designs for deep geological repositories for radioactive wastes. It is expected that bentonite materials may undergo some mineralogical alteration as they interact with in situ groundwaters over long timescales on the order of thousands to millions of years. Long-term modelling of these materials is therefore important in order to demonstrate confidence that the engineered designs will continue to perform as required over their intended lifetimes (required assessment timescales can be up to 1 million years). The key geochemical processes that must be considered in such modelling are mineral dissolution and precipitation and cation exchange. These processes are expected to occur simultaneously and so modelling of their coupled effects and their rates (kinetics) is necessary. Illustrative reactive-transport models of the geochemical alteration of montmorillonite (the primary mineral in bentonite exhibiting cation exchange) are presented which demonstrate that one possible approach to fully coupling cation exchange and clay mineral dissolution kinetics, referred to here as the 'all-component coupling' approach, may lead to unrealistic behaviour due to feedback that may occur in the formulation. This feedback can be avoided if a 'common-component' conceptual model for the dissolution of exchanger end members is adopted, where only the saturation of the exchanger 'structural unit' is considered when evaluating the potential for dissolution of the mineral. Such considerations have been proposed historically in stability analyses for montmorillonite, but have not been explored widely in the modelling literature.