Porewater composition in clay rocks explored by advective displacement and squeezing experiments

Abstract

Argillaceous rocks are foreseen in many countries as the potential hosts for nuclear waste repositories. The knowledge of the chemical composition of the free porewater in these formations is required for the understanding of the paleo-hydrogeological evolution, for the assessment of radionuclide solubilities and migration parameters and for assessing the long-term stability of the technical barrier system. High pressure squeezing and advective displacement are two methods that aim at direct sampling of this porewater fraction while minimizing experimental artefacts. Within the framework of a recent deep drilling campaign in Switzerland, targeting the Opalinus Clay as the designated host rock, a substantial dataset of porewater compositions was obtained by these two methods. It included 51 squeezing and 30 advective displacement experiments on drillcore samples from the Opalinus Clay and confining units distributed over 8 boreholes in 3 study areas. Porewater compositions obtained by either method reflect the geochemical characteristics of each siting region, such as different salinities and water types as well as depth gradients informing on the diffusive exchange with bounding aquifers. An in depth comparison of the Opalinus Clay porewater compositions obtained by both methods shows a high degree of consistency with regard to the ion ratios or mineral equilibria. The pH/pCO2 system was found to be prone to experimental artefacts, but applying a correction, a fairly consistent dataset was obtained. Porewaters acquired by squeezing exhibit systematically lower salinities by 10–40% when compared to those from advective displacement. It is concluded that this is due to the mobilization of a higher fraction of an anion depleted porewater, either due to the higher mobilization of water from the diffuse layer or due to the expulsion of water from interlayer (-like) pores. The comparison with a geochemical model indicates that the experimental data from both methods can be considered as proxies for in-situ major-ion porewater compositions. It also confirms the robustness of the geochemical model predicting porewaters of the Opalinus Clay and confining units. Differences between in-situ, sample storage and extraction temperatures need to be taken into account when interpreting the porewaters obtained by either method and modelling in-situ porewater compositions. Combining different laboratory and analytical methods for porewater investigations in clay rocks provides an added value as it enables a detailed assessment of natural heterogeneities and experimental uncertainties.