Porewater chemistry of the host rock is a major constraint on radionuclide mobility (speciation, sorption, solubility) and the stability of the engineered barriers. Furthermore, porewater composition affects the diffusion behaviour (Van Loon et al. 2023), with lower salinity (lower ionic strength) leading to lower anion and higher cation diffusion coefficients (Glaus et al. 2024; Section 5.8), the latter mainly for cations undergoing ion exchange reactions.
Porewater constituents can be subdivided into free constituents such as chloride and elements buffered by thermodynamic equilibria with host rock minerals. While the mineralogical composition can be considered constant over the period under consideration (Section 6.5.6), the free porewater constituents are affected by the diffusive interaction with bounding aquifers. The modelling of the tracer profiles (Section 4.6) shows that the Opalinus Clay porewater may slowly evolve towards lower salinity on a timescale of hundreds of thousands of years. Forward modelling of the Benken dataset suggests a decrease in chloride concentration of less than 1 g/L in the centre of the Opalinus Clay within 1 Myr (see Section 8.6.2 in Nagra 2002). The slowly changing porewater chemistry results in slightly decreasing diffusion coefficients for anions and a slight increase for cations that undergo mainly ion-exchange reactions.
The impact of decreasing porewater salinity on solubility is specific for each radionuclide and depends on the changing ion concentrations. A qualitative comparison of solubilities based on the NL and ZNO reference porewater with the one based on the dilute JO porewater yields only minor differences (Kulik & Miron 2024).