Engineered barrier system (EBS) plays a critical role in the safety of geological disposal concepts. Bentonite is a candidate buffer/backfill material, which provides key functions within EBS. The groundwater composition undergoes significant changes during infiltration in the reactive and porous system of compacted bentonite buffer. Re-saturation phase of bentonite buffer/backfill is a critical stage of EBS evolution in which combined impacts of groundwater filtration and elevated temperature exist. Understanding the hydro-geochemical changes in clay buffer is critical as they have significant impacts on various aspects of EBS performance (e.g. re-saturation time, swelling pressure, hydraulic conductivity, and retardation of corrosive species). A number of experimental studies and numerical investigations on hydro-geochemical evolution of clay buffer have been presented in the last 15 years. However, there are still a number of major challenges, which limit the applicability of the experimental works and the accuracy of predictive models for assessing the future evolution of groundwater within the EBS. In this project, the hydro-geochemical modelling of pore water evolution of compacted bentonite will be fundamentally revisited and tested against a new set of experimental data, which will be generated to describe more accurately the evolution of saline groundwater in the EBS for a generic GDF in lower strength sedimentary rock of the UK (LSSR).