Hydrogeology of a fractured shale (Opalinus Clay). Implications for radionuclide migration

As part of the Swiss high-level radioactive waste disposal programme, the Opalinus Clay, a 95 - 120 m thick Middle Jurassic shale (clay stone) formation is investigated with potential siting areas in Northern Switzerland. Observations in clay pits, and the results of a German research programme focusing on hazardous waste disposal aspects, have demonstrated that the near-surface Opalinus Clay is rather permeable. Hydraulic tests in deeper boreholes, yielded very low hydraulic conductivities (< 10-12 m/s) even though joints and faults were included in some of the test intervals. These measurements are consistent with hydrogeological information from Opalinus Clay sections of six railway tunnels and four motorway tunnels in the Folded Jura of Northern Switzerland. The hydrogeological dataset presented suggests that radionuclide transport through faults and joints is probably not a critical issue for assessing the suitability of Opalinus Clay as a host rock for a deep geological repository for nuclear waste. (author)