Applying the squeezing technique for pore water chemical characterization in Mont Terri for over 30 years

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 & Volckaert 1996; 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 & Coplen 1973) retarding the passage of charged anionic species but allowing the free movement of uncharged species and water (filtration effect). Anions are affected by the anionic exclusion due to the repulsion by negative electrical charge on the clay mineral surfaces and edges, being excluded from a portion of the pore space where three types of water are involved: interlayer water, diffuse double layer (DDL) water, and free pore water. Although pore water extraction from in-situ packed-off boreholes is considered a good proxy method for characterising the in-situ pore water, it is rarely feasible. Furthermore, in-situ collection may require long sampling times and is much more expensive than laboratory tests. In addition, seepage waters may be affected by artefacts on sampling: pH/alkalinity and redox changes due to oxidation, CO₂ outgassing, microbial growth, as well as effects of tunnelling and borehole construction on the formation properties, e.g., evaporation/drying of the rock and EDZ formation (Vinsot et al. 2008, 2013, 2014; 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 & Léon 2023; 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.