The Alternative Buffer Material ABM5 experiment is an in situ medium-scale experiment performed at Äspö Hard Rock Laboratory (HRL) conducted by SKB in Sweden with the aim of analysing the long-term stability of bentonites used as an engineering barrier for a high-level radioactive waste repository (HLWR). In this work, four different ring-shaped Ca- and Na-bentonite blocks, which were piled around a carbon steel cylindrical heater, subjected to a maximum temperature of 250 °C and hydrated with saline Na-Ca-Cl Äspö groundwater (0.91 ionic strength), were characterized after dismantling. This work allowed us to identify the main geochemical processes involved, as well as the modifications in the physico-chemical properties and pore water composition after 4.4 years of treatment. No significant modifications in mineralogy were observed in samples close to the heater contact, except an increase in Fe content due to C-steel corrosion, carbonate dissolution/precipitation (mainly calcite and siderite) and Mg increase. No magnetite and a low amount of Fe(II) inside the clay mineral structure were detected. No modifications were observed in the smectite structure, except a slight increase in total and tetrahedral charge. A decrease in external surface area and cation exchange capacity (CEC) was found in all samples, with lower values being detected at the heater contact. As a consequence of the diffusion of the infiltrating groundwater, a modification of the composition at clay mineral exchange sites occurred. Ca-bentonites increased their Na content at exchange sites, whereas Na-bentonite increased their Ca content. Exchangeable Mg content decreased in all bentonites, except in MX-80 located at the bottom part of the package. A salinity gradient is observed through the bentonite blocks from the granite to the heater contact due to anions are controlled by diffusion and anion exclusion. The pore water chemistry of bentonites evolved as a function of the diffusion transport of the groundwater, the chemical equilibrium of cations at exchange sites and mineral dissolution/precipitation processes. These reactions are in turn dependent on temperature and water vapor fluxes.

Opalinus Clay is chosen as host rock for the deep geological disposal of nuclear waste in Switzerland and is also being considered in Germany. The underlying Staffelegg Formation comprises potentially water-bearing layers. To investigate the hydrogeological setting of these two formations at the Swiss Rock Laboratory in Mont Terri two bore holes have been drilled in the framework of the HS-Experiment (Hydrogeological Survey). The 58 m long BHS-1 starts in the Lower Shaly facies of the Opalinus Clay, crossects the entire Staffelegg formation and ends in the Triassic Klettgau formation. The shorter BHS-2 provides additional data from the carbonate-rich sandy facies and Lower Shaly facies of the Opalinus Clay. The presented data publication provides geochemical, mineralogical and petrophysical parameters of rocks, pore water and (dissolved) gases from these two drillings [1].[1] Bonitz, Marie et al. (2024): Hydrogeological characterisation of a Lower Jurassic rock unit at the Mont Terri - I: Data of the Opalinus Clay and the Staffelegg Formation. GFZ Data Services. https://doi.org/10.5880/GFZ.3.4.2024.