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Technischer Bericht NTB 18-01

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

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Abstract

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 see­page 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.

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