Technischer Bericht NTB 14-02/VI

SGT Etappe 2: Vorschlag weiter zu untersuchender geologischer Standortgebiete mit zugehörigen Standortarealen für die OberflächenanlageGeologische GrundlagenDossier VI:Barriereneigenschaften der Wirt- und Rahmengesteine

Dossier VI: Barrier properties of the host rocks and confining units

Dossier VI on the barrier properties of the host rocks and confining units describes the parameters and concepts used for the dose calculations and the safety-based comparison of the siting regions as part of Stage 2 of the Sectoral Plan process. This forms the geoscientific basis for the qualitative and quantitative assessment of the numerous criteria set out in the Sectoral Plan together with the associated indicators, and for identifying any clear disadvantages.

The chapter on the barrier properties of the host rocks includes a characterisation of the host rocks with regard to the properties that are relevant for radionuclide transport. The definition of the lithofacial units is based on the information presented in Dossier II; they are divided into three categories based on experience regarding the connection between clay mineral content and hydraulic properties (< 20, 20 – 40 and > 40 wt.%):

  • The mineralogical composition of the Opalinus Clay lies in the range of claystones and sandy claystones. For the purpose of the provisional safety analyses, it can be considered as a single lithofacial unit with a clay mineral content > 40 wt.%.
  • The mineralogical analyses of the 'Brauner Dogger' show considerable variation in the Füchtbauer triangle. For the purpose of the provisional safety analyses, three lithofacial units are distinguished: the Clay-rich sequences generally with > 40 wt.% clay minerals comprise the host rock sensu stricto consisting of claystones and clay-rich marls of the Parkinsoni-Württembergica strata and the Variansmergel Formation. The Sandy-clay-rich sequences include the sandy claystones and marls of the Murchisonae-Oolite, Wedelsandstein and Humphriesioolite Formations, with an average clay mineral content between 20 – 40 wt.%. The term Sandy limestone sequences includes so-called 'hard beds' consisting mainly of sandy limestones and iron oolites with characteristic clay mineral contents < 20 wt.%.
  • The mineralogical analyses of the Effingen Member fall into the fields of (partly sandy) calcareous marls, clay-rich limestones and limestones within the Füchtbauer triangle. For the purpose of the provisional safety analyses, a distinction is made between the lithofacial units Calcareous marl sequences (clay mineral content 20 – 40 wt.%) and Limestone sequences (clay mineral content < 20 wt.%).
  • The mineralogical analyses of the Helvetic Marls lie typically in the area of calcareous-sandy claystones, sandy clay-rich and calcareous marls, clay-rich limestones to almost pure limestones in the Füchtbauer triangle and thus have a similar composition to the Effingen Member, but with more variability. The Helvetic Marls are so strongly imbricated and folded that they are divided into only two lithofacial units for the purpose of the provisional safety analyses. The Marls, which make up the main mass of the host rock, fall into the category with a clay mineral content of 20 – 40 wt.%. The Limestone sequences are often boudinated and sheared off and fall into the category with a clay mineral content of < 20 wt.%.

Investigations on drillcores from a number of deep boreholes in Northern Switzerland and Wellenberg have provided analytical results for a large range of parameters. The results for porosity in particular acquired using a range of different methods are presented and discussed.

The characterisation of the porewaters is based mainly on investigations on drillcores from the siting regions. Where available, analyses of groundwaters and, for the Opalinus Clay, results from the Mont Terri Rock Laboratory are also included. Possible regional differences and uncertainties are taken into account by considering variants of the reference porewater.

The characterisation of the hydraulic properties is based on in situ tests in boreholes, experiments on drillcores and observations in underground installations. A systematic comparison was also carried out of the structural inventory from drillcores and the results of in situ tests. The datasets describe the rocks on different scales. Laboratory tests on drillcores give a generally very low hydraulic conductivity for the rock matrix and thus indicate that the increased conductivities observed for a few host rocks must be due to discrete tectonic-structural elements.
 

  • For the lithologically relatively homogeneous Opalinus Clay, the good agreement between the very low hydraulic conductivities determined in laboratory experiments and in situ tests in boreholes (taking into account small faults and differing lithofacial sub-units) indicates that it is appropriate to model it as a homogeneous-porous medium with diffusion-dominated transport.
  • For the 'Brauner Dogger', in situ tests and investigations on drillcores both show a very low hydraulic conductivity of the undisturbed host rock. Only in the borehole Schlattingen-1, which is located close to the Hegau-Bodensee Graben, was an increased conductivity encountered in an interval with two potentially open structures in the Wedelsandstein and Humphriesioolite Formation. Efficient self-sealing is expected in the Clay-rich sequences, similarly to the Opalinus Clay.
  • For the Effingen Member, in situ tests in the undisturbed host rock and hydraulic tests on drillcores show very low hydraulic conductivities. Increased hydraulic conductivities are found in fractured zones, for example in the Gerstenhübel Beds in the Oftringen borehole. The limestones of this unit have an extremely low clay mineral content and no relevant self-sealing capacity is expected.
  • The Helvetic Marls are a fractured medium with a very low hydraulic conductivity matrix. Water flow is associated exclusively with brittle deformation structures and ductile structures that have undergone brittle overprinting (reactivation). Beneath a 500 – 600 m zone with increased hydraulic conductivities, the values are in the range of 10-13 to 10-10 m/s.

For all the host rocks for which faults or joints could be hydraulically active in at least some lithofacial units, the question arises to what extent individual boreholes can cover the expected bandwidth. To obtain a robust estimate of the transmissivities of tectonic-structural elements, experience from other lithologically similar rocks and conceptual considerations of self-sealing of faults are also taken into account.

The diffusion properties are based on measurements in the laboratory and on data acquired in field experiments in the Mont Terri Rock Laboratory. There is also an empirical correlation between the porosity and the diffusion coefficient (extended Archies law). This approach is based on an extensive international database and allows a robust estimate to be made solely on the basis of porosity. Porewater tracer profiles allow validation on the formation scale and over long timescales.

Independent evidence such as tracer profiles or anomalous hydraulic potentials in the host rock confirm the effective containment function of the host rocks over very long time periods and underpin – for example in the case of the Opalinus Clay – the dominance of diffusive over advective solute transport, as well as the fact that the hydraulic barrier function results in a marked groundwater storey structure.

In the chapter on the barrier properties of the confining units, the lithofacial units, rock parameters and transport parameters are derived and defined. The units in question are the upper confining units of the Opalinus Clay in the siting regions Jura Ost and Jura-Südfuss (Passwang Formation, Lower Acuminata strata) and the lower confining units of the Opalinus Clay in all the siting regions in Northern Switzerland. The lithofacial units of the rest of the rocks identified as confining units in the siting regions Südranden, Zürich Nordost and Nördlich Lägern (Opalinus Clay, 'Brauner Dogger' and Effingen Member) have already been characterised as host rocks. The Effingen Member in the Jura-Südfuss siting region and the Helvetic Marls in the Wellenberg siting region have no confining units.

The lithofacial unit Clay-rich Lias beneath the Opalinus Clay has typical clay mineral contents of 40 wt.% and very low hydraulic conductivities of ≤ 10-13 m/s. Only in the Jura-Südfuss siting region is the Lias beneath the Opalinus Clay comparatively sandy and calcareous and is therefore designated as Limestone-rich Lias. Observations of water and gas flow, particularly in the borehole Gösgen SB4, indicate that this unit has an increased hydraulic conductivity compared to the Clay-rich Lias.

The lithofacial unit 'Arietenkalk' consists of partly clayey and sandy limestones and showed low conductivities in the Benken, Weiach, Riniken and Schafisheim boreholes. On the other hand, evidence from near-surface zones and tunnels indicates that the 'Arietenkalk' can exhibit the characteristics of an aquifer when associated with strong deformation (faults, joints).

Between the 'Arietenkalk' and the Keuper aquifers clay-rich lithologies with low conductivity again occur in the lithofacial unit Clay-rich Keuper.

In the Upper Middle Keuper there are units that could exhibit aquifer characteristics depending on the local lithological composition. These are summarised together as the lithofacial unit Keuper aquifer.

Below the Keuper aquifer there are low conductivity rocks that are summarised together as the thick lithofacial unit 'Gipskeuper'.

The upper confining units of the Opalinus Clay in the siting regions Jura Ost and Jura-Südfuss include mainly the Passwang Formation. Similarly to the 'Brauner Dogger', this was divided into the lithofacial units Sandy-clay-rich sequences and Sandy limestone sequences. The Passwang Formation underwent hydraulic testing in the Riniken and Schafisheim boreholes, resulting in both cases in very low conductivities. With the Sissach Member in particular, it shows a locally pronounced limestone sequence over 10 m thick directly above the Opalinus Clay, which could show significantly higher hydraulic conductivities in areas with stronger jointing or small faults.

The chapter on site-specific conceptual models and parameters for radionuclide transport in the host rock and confining units presents the derivation of conceptual models and the geodatabase for the provisional safety analyses and the safety-based comparison in Stage 2 of the Sectoral Plan process. This is based on the information presented in the two preceding chapters and takes into account experience with similar rock types for the hydraulic parameters. The thicknesses presented for the lithofacial units are based on the information presented in Dossier II.