Reference porewaters constitute an abstraction of the ensemble of the experimentally derived data, constraints from other data (e.g. mineralogy, aqueous extraction, cation exchange experi­ments) and modelling (Mäder & Wersin 2023).

A consistent thermodynamic porewater model is an important component in the derivation of the reference porewaters (Mäder & Wersin 2023) and, generally speaking, in the derivation of the in-situ porewater compositions of the Opalinus Clay and its confining units in the different siting regions (Kiczka et al. 2023). The model takes into account the "free" solutes (conservative species, such as Cl, not affected by mineral reactions), dissolved gases, cation exchange reactions and equilibria with relevant rock-forming minerals (Kiczka et al. 2023, Mäder & Wersin 2023). Such calculations, commonly carried out with a geochemical code like PHREEQC, have been shown to yield consistent results with regard to squeezing and advective displacement data (Wersin et al. 2016, Mäder & Wersin 2023, Kiczka et al. 2023), thus providing an independent measure for the robustness of the experimentally obtained data. Uncertainties with respect to key parameters (e.g. pH – pCO₂ – alkalinity, or sulphate) were addressed by defining model variants for the reference porewater that are also thermodynamically consistent in the same way as the reference porewater (Mäder & Wersin 2023).

An integral part of the model is the phase rule in which – at constant temperature and pressure – each component is constrained by fixed concentrations, mineral or cation exchange equilibria, respectively. The constraints used for the calculation of the reference porewater and for an alternative model approach are illustrated in Tab. 5‑1. The purpose of the alternative model is related to the uncertainty in pCO₂ and pH. In fact, the experimental determination of pH and partial pressure of CO₂ is generally difficult and prone to experimental artefacts and changes between in-situ and laboratory conditions, such as temperature and pressure (Kiczka et al. 2023) (outlined below). The two models constrain pH and the partial pressure of CO₂ (pCO₂) in different ways, i.e. (i) fixing pCO₂ according to expert judgement based on earlier work or (ii) assuming a pair of equilibria of clay minerals (Kiczka et al. 2023, Mäder & Wersin 2023). The latter approach does not require an a priori assumption on pCO₂ but requires a judgement on the selected mineral pair and the thermodynamic data for such phases (illite, illite/smectite, chlorite) that are still uncertain as e.g. discussed in Wersin et al. (2020). The two models show similar results for calculations at 25 °C for most underlying mineral pairs and databases (Fig. 5‑20).

Tab. 5‑1: Constraints on major component concentrations used for modelling of reference porewaters (Mäder & Wersin 2023)

^a) best estimate expert judgement based on available data

^b) based on data obtained from squeezing and advective displacement experiments

^c) based on data obtained from cation exchange experiments

^d) only initial estimate, later adjusted slightly by model simultaneously to mineral equilibria

^e) S(VI)/pyrite redox couple assumed to be active

^f) kaolinite/chlorite or kaolinite/illite or illite/chlorite equilibrium