Porewater Chemistry of Opalinus Clay: Methods, Data, Modelling & Buffering Capacity

Wersin, P.; Pekala, M.; Mazurek, M.; Gimmi, T.; Mäder, U.; Jenni, A.; Rufer, D.; Aschwanden, L. (2020). Porewater Chemistry of Opalinus Clay: Methods, Data, Modelling & Buffering Capacity (Technical Report, Nagra 18-01). NAGRA, National Cooperative for the Disposal of Radioactive Waste

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In the context of nuclear waste disposal, porewater chemistry is important to evaluate the stability of technical barriers and the mobility of radionuclides. The intimate association of porewater with the nanoporous clayrock, however, makes the characterisation of porewater chemistry a difficult task and virtually every method is prone to artefacts. During the last 25 years large progress has been made in characterising the porewater chemistry and unravelling the underlying geochemical processes in the Opalinus Clay and other clayrocks. The multi-national Mont Terri Project has played a pioneering role in this regard. For example, it has enabled to develop the appropriate methodology for characterising the porewater chemistry of the Opalinus Clay investigated through deep boreholes in northeastern Switzerland.
The information compiled in this report builds on the early synthesis of Pearson et al. (2003) and integrates newer data from the Mont Terri Rock Laboratory and from deep boreholes in northern Switzerland. In particular, pertinent data from the borehole at Schlattingen, where a suite of methods were applied, are included. The overall objective is to present the status of understanding porewater chemistry of the Opalinus Clay before starting the deep drilling programme of Sectoral Plan Stage 3. This is accomplished by (i) outlining current methods, (ii) analysing the acquired data and corresponding modelling developments, (iii) discussing the buffering capacity of the Opalinus Clay, and (iv) highlighting pertinent uncertainties.
A multi-method approach is required to obtain representative porewater data. Sampling of see-page waters from packed-off boreholes at Mont Terri has enabled to obtain a consistent dataset which has been complemented with data from other methods. High-pressure squeezing is a viable and efficient tool to sample porewater from drillcores. Complementary sampling methods are diffusively equilibrated borehole waters and advective displacement of porewater from drillcores. The methods of aqueous extraction and extraction of exchangeable cations via index cation displacement are well-established. This is also the case for the diffusive exchange method for determining water isotopes. Large progress has been made in the methodology for analysis of noble and reactive gases in porewaters.
A large database has been acquired in the Mont Terri Project, also including porewaters purposely affected by perturbations (e.g. addition of H2 or increase in salinity). The knowledge gained at the Mont Terri Rock Laboratory has helped to establish methods for the analysis of drillcores from deep boreholes. The derived porewater compositions in these boreholes are largely consistent with those obtained at Mont Terri, which is explained by the similar mineralogy at the different sites. Useful complementary porewater chemistry data has been acquired from the Mont Russelin tunnel and other deep boreholes in northern Switzerland.
On the basis of process understanding a robust chemical equilibrium modelling approach has been developed. The core of this model includes cation exchange reactions and equilibrium with carbonate minerals and celestite. Depending on the focus of the model, other mineral and surface reactions (e.g. clay minerals, surface protolysis) can be included. This model approach can also assess the effects of temperature and pressure changes on porewater chemistry. Calculations indicate that effects related to the extraction, cooling and sampling of drillcores are fairly small if appropriate measures are taken.
Sulphate released from aqueous extracts exhibits concentrations in excess of the expected porewater concentration, which points to an additional source of sulphate. This source has so far not been unequivocally identified. Dissolution of celestite during leaching may partly explain this "excess sulphate" in aqueous extracts but other sources, such as SO4 released from the carbonate fraction appear to contribute as well.
Due to its mineralogical-chemical and transport properties, the Opalinus Clay has a large buffering capacity which counteracts repository-induced or external perturbations. This is underlined by a number of experimental studies, observations from natural analogues and modelling exercises. Thus perturbations, such as ingress of O2, microbial sulphate reduction or contact with cementitious leachates are shown to be buffered efficiently and resulting changes in porewater chemistry are generally localised and small to moderate.

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