The mobility of radionuclides (sorption, solubility, diffusion) and the long-term stability of the engineered barriers strongly depend on the geochemical conditions in the host rock, notably the porewater chemical composition, pH and redox state. In addition, porewater chemistry can be used as independent evidence for the hydraulic barrier effect; this topic is addressed in Section 4.6.

There is a strong interaction between the mineral surfaces and the porewater in the nanometric pore space of clay-rich rocks. The section on solute distribution in the pore space (Section 5.4.2) introduces relevant concepts needed for the interpretation of porewater data and the pertinent dataset. Porewater cannot be directly sampled because of the very low hydraulic conductivities but requires laborious, dedicated experiments. The section on porewater chemistry (Section 5.4.3) presents the modelling approach used to integrate the available data and illustrates sensitivities of the model with respect to temperature and the choice of equilibrium phases.