Evaluation of Noble Gas Extraction Methods for Inter- and Intragranular Porewater in Argillaceous Rocks

Abstract

The long-term, safe storage of nuclear waste within deep geological formations represents a critical focus of contemporary research and public interest. As nuclear energy continues to be an essential component of sustainable energy strategies – particularly within Canada – the use of noble gases as tracers of physico-chemical processes for the site characterisation of prospective Deep Geological Repositories (DGRs) is a promising yet technically challenging approach. This study develops and evaluates methodologies for extracting and quantifying isotopic noble gases from the porewater of the Opalinus Clay (OPA) formation in northwest Switzerland, a well-established analogue for argillaceous DGR settings (Bossart and Thury, 2008). The first study (Chapter 2) examines the traditional encapsulation of cores for porewater analysis within stainless-steel cylinders and evaluates three normalization techniques for estimating initial porewater concentrations from headspace noble gas measurements. A second study (Chapter 3) explores a more cost-effective encapsulation method involving core samples stored in vacuum-sealed PE/Al-foil bags, similar to Zuo et al. (2021). However, this method presented challenges due to limited degassing. Consequently, this required low-temperature baking to promote outgassing to form a headspace over a period of six weeks, although this process may have introduced isotopic fractionation associated with He, and elemental fractionation in Ne and Ar. Consequently, additional preparation, storage, and extraction refinements may be needed to realise the potential of this technique. Collectively, these findings highlight key methodological considerations in sampling and analysing argillaceous cores for noble gases, highlighting the need for further refinement towards establishing more comprehensive “best-practice” protocols.