A considerable effort has been undertaken in the last 30 years to characterise the pore water chemistry in clayrocks using different approaches (e.g., NEA 2000; Fernández et al. 2014; Tournassat et al. 2015; Nagra 2020; Mazurek et al. 2025; Wersin et al. 2025): seepage water sampling, laboratory studies, and geochemical modelling. In the context of the Geochemical Task, the Mont Terri Project has been a pioneer in all of them. Since 1996, various in-situ and laboratory experiments have been conducted to obtain representative pore water samples from the Opalinus Clay formation. This is because the pore water chemical conditions and buffering capacity of the clayrock will control radionuclide concentrations and behaviour within the geological barrier over time (Horseman Altmann et al. 2012). Understanding pore water chemistry and water-rock interactions is important for the safety assessment of repository systems in clay-rich formations, and contributes to the paleohydrogeological interpretation of the system, needed to improve models predicting how the system might respond to future geological changes. However, obtaining reliable pore water composition in indurated clayrocks is difficult due to their low permeability, small pore size and low water content. Besides, compacted clay minerals may behave as semi-permeable membranes (Hanshaw Wersin et al. 2020, 2026). Squeezing is an ex situ water-collection method that has been used to characterise pore water chemistry at Mont Terri by BGS, CRIEPI and CIEMAT since 1996. In this work, a summary of tests conducted at CIEMAT up to 2026, including data from the FS experiment, and the HS experiment (Fernández Bonitz et al. 2025), is presented, supporting the validity and representativeness of the squeezing technique for obtaining reliable pore water samples from clayrocks, emphasising the conservative elements and isotopes results.
Ana María Fernández (Thu,) studied this question.