Arbeitsbericht NAB 09-20

Borehole Gösgen KB5a: Mineralogy, Porosimetry and Pore Water Chemistry of the Effingen Member

The Effingen Member (synonymous with Effinger Schichten) is one of the potential host rocks for the geological disposal for radioactive waste. First investigations of the Effingen Member date back to the Sediment Study performed by Nagra 25 years ago (Nagra 1988). More recently, the Effingen Member and surrounding lithologies have been investigated in more detail and applying most recent technologies in the EWS-borehole Oftringen (Waber 2008a).

Between October 2008 and January 2009 six cored (KB1, KB2, KB3, KB4, KB5 and KB5a) and one percussion-drilled (SB2) boreholes were drilled by KKG & Atel for various investiga.tions on the site of the nuclear power plant Gösgen. The geological and stratigraphic description of the rock material recovered from these boreholes is given by Albert et al. (2009). In addition, mineralogical, chemical and physical properties of the encountered limestones to calcareous marls and their pore water could be performed on the core material of borehole KB5a. At the Gösgen site the rocks of the Effingen Member (Oxfordian) occur between about 66 and 307 m depth, are overlain by the Geissberg and Olten Members and underlain by the Birmenstorf Member, all of Oxfordian age. The Effingen Member occurs thus at shallower depth below the surface and at somewhat greater thickness compared to Oftringen (420 – 642 m depth) located some 8 km southwest of the Gösgen site.

Pore-water from low-permeability rocks cannot be sampled by conventional groundwater tech.niques and has to be characterised indirectly based on originally saturated rock material. In case of the Gösgen borehole 2 samples could be conditioned on-site immediately after recovery in order to minimise artefacts induced by exposure to the atmosphere (e.g. evaporation, oxidation), and this constitutes a pre-requisite for a complete pore water characterisation. Further 10 samples were treated conventionally following about 4 months of storage under atmospheric conditions, which limited the pore water investigations for these samples. Indirect extraction methods deliver only partial information about the pore water composition and need to be supported by mineralogical and petrophysical data. By combining the results of different experimental and modelling techniques a pore-water composition might then be derived. The plausibility of the derived composition is assessed by comparing the spatial distribution of pore.water compositions with that of groundwater in over- and underlying aquifers and the palaeo.hydrogeological evolution of the area.

The present report describes the mineralogical, petrophysical and geochemical investigations performed on drillcore material obtained from borehole KB5a drilled at the Gösgen site. It should be noted that borehole KB5a was drilled only to about 156 m depth and samples for porewater investigations are restricted to the upper half of the Effingen Member rocks. Supporting data such as the geological and hydrogeological description are given in Albert et al. (2009) and Enachescu et al. (2010).