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Technical Report NTB 88-01

A case study in Northern Switzerland The copyright lies with Elsevier. The official version is digitally available at: http://www.sciencedirect.com/science/bookseries/01661116/43

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In 1980, Nagra, the Swiss National Cooperative for the Storage of Radioactive Waste, began a comprehensive field investigation programme to assess the feasibility and safety of a repository for the final disposal of high-level radioactive waste in northern Switzerland. The host rock of first priority was crystalline basement covered by a few hundred metres of sedimentary rocks. The various investigations cover an area of about 1200 km2 and include a regional geophysical programme, a regional hydrogeological programme comprising hydrodynamic modelling and hydrogeochemical investigations, a neotectonic programme and a deep drilling programme. Seven deep boreholes with depths between 1306 and 2482 m have been drilled to date. As the study programme for the seventh borehole was still in progress during the final preparation of this report, only incomplete results were available for inclusion in this report. The scientific programmes were designed by Nagra and its geology consultants and carried out under the project management of Nagra by more than 200 scientists from over 50 companies and universities from 8 countries.

An understanding of the deep groundwater flow regime is needed to predict flow paths, travel times and dilution effects of groundwater moving from a repository location to the biosphere. To assess the flow field, regional and local hydrodynamic models have been prepared. Currently, extensive improvements to the input data are being made and anisotropic flow properties are being included for several formations. Subsequently, validation of the hydrodynamic models with the results of the hydrogeochemical investigations will be attempted. The isotopic investigations presented in this report will probably be the most important contribution to this validation.

The hydrogeochemical programme started in 1981 with the collection of waters from about 140 springs and wells of different aquifers in northern Switzerland and adjacent areas; this was called the "regional programme" (Schmassmann et al., 1984). Detailed hydrochemical and isotopic analyses have been carried out on these samples. About 60 waters from the Nagra deep boreholes were also sampled and analysed, of which some 40 gave useful results. Additional geochemical and isotopic data on rock material are also available from these boreholes. A literature study yielded complementary hydrochemical data, the so-called "Fremde Analysen", both from points sampled during the regional programme and from additional points.

The purpose of this report is to present all the isotope data collected within the Nagra programme in northern Switzerland. The report also includes the first comprehensive synthesis and interpretation of these data. It is anticipated that special aspects of the data interpretation will be discussed elsewhere in the reviewed scientific literature.

Marc Thury

Department Head, Geology

Nagra, Baden, Switzerland

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