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Arbeitsbericht NAB 20-13

SGT-E3 deep drilling campaign (TBO): Experiment Procedures and Analytical Methods at RWI, University of Bern (Version 1.0, April 2020)

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Within phase 3 of the sectoral plan for deep geological repositories (SGT-E3), a deep drilling project (TBO) is carried out by Nagra. A major focus will be the characterisation of porewater in the rock formations of low-permeability and the distribution of natural tracers across these formations and extending into the bounding aquifer horizons.

Porewater residing in low-permeability rock cannot be sampled by conventional groundwater sampling techniques and has to be characterised by indirect methods based on originally saturated rock material obtained from boreholes. Collection of drillcores from below the surface will inevitably change intrinsic porewater parameters such as temperature, pressure and O2- and CO2-fugacity from those existing under in situ conditions. The drilling process, the on-site sample conditioning and the various preparation steps of the core material after reception in the lab will induce additional modification of specific parameters of the in situ porewater. Induced modifications of the in situ porewater of sedimentary environments are specific to the rock type under investigation and differ based on the reactivity of the rock mineralogy in terms of the carbonate and sulphur systems, ion-exchange and redox conditions.

A consequence of indirect methods for porewater characterisation is the fact that such methods can only deliver partial information about the complete chemical composition, pH and redox state of the in situ porewater. Therefore, the methods have to be applied complementary if aimed at the chemical composition of the porewater. Depending on the degree of detail required about the porewater composition available indirect methods are of largely different complexity with respect to sample conditioning, experimental set-up and analytical efforts. Natural tracer profiles, for instance, focus on chemically conservative compounds in the porewater and the extraction of such compounds is rather straightforward. Nevertheless, substantial differences in conditioning and extraction exist in case of dissolved tracer (e.g. Cl, Br), dissolved gases (e.g. He) or the stable isotope composition of the porewater molecule itself (δ18O, δ2H). Aiming at the chemical composition, pH and redox state of the in situ porewater by indirect extraction techniques based on drillcore material is ambitious and has inevitably to be accompanied by geochemical model calculations.

Depending on the focus of the porewater investigations such characterisation requires specific treatments and designs for the on-site sampling of drillcore material, the preparation of the rock material for individual extraction techniques and the adaption of analytical techniques.

This document describes the workflow, the preparation of the rock material for individual extraction techniques and the analytical techniques after reception of the on-site conditioned drillcore samples in the RWI-Laboratories of the Institute of Geological Sciences, University of Bern. It adopts the sample labelling given by Rufer (2019) who also gives a detailed description of sample collection and sample conditioning in the field.

Chapter 2 of the present document gives a short description of the different types of porewater samples, their designation and analytical work to be carried out as already documented in Waber (2018). Chapter 3 describes the workflow for each porewater sample type. Chapters 4 to 12 give detailed descriptions of sample preparation and analytical methods applied to solid material and extract solutions obtained from the various techniques, respectively. Chapter 13, finally, gives a description of the analytical techniques applied to experiment solution of the various experiments.

The standard workflow and details about extraction techniques given here are those for routine porewater investigations on a large number of samples within a restricted time frame. In some aspects it thus represents a compromise between efficient sample handling, infrastructure and personnel, and the technical/scientific ambition for a highest possible characterisation of the porewater and concentration profiles of natural tracers.

This document describes the status of April 2020. According to gained experience and – most importantly – to the specific rock type under investigations, the standard workflow, detailed procedure of extractions and subsequent analytical programme are subject to change. Furthermore, to resolve specific questions (e.g. redox, pH, ion-accessible porosity, diffusion parameters) that require more time and infrastructure-intensive investigations, the standard workflow and extraction techniques will be modified and adapted for specific samples.

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