Technical Report NTB 93-07

The chemistry of groundwater in formations being considered as host rocks for nuclear waste repositories must be known to assess the performance of those repositories, and as media for laboratory experiments. Two potential repository siting areas in the crystalline basement of northern Switzerland are being assessed.

This report gives the chemistry of water in both areas for reference use in this assessment. The western area is in the region defined by the Kaisten, Leuggern, Böttstein, and Zurzach boreholes. The western reference water is an Na-SO₄-(Cl-HCO₃) type with total dissolved solids of 1.0 g/l. Its composition is based on samples from the Leuggern, Böttstein, and Zurzach boreholes. Kaisten water is of higher salinity (1.3 g/l). The concentration ranges of the reference water include Kaisten values, however. High quality samples and analyses, particularly from long term sampling at Zurzach and Leuggern, define the concentration ranges of many trace elements. The definition of this water assumes saturation with respect to calcite, baryte, fluorite, chalcedony, and kaolinite. The reference pe is based on the assumption that dissolved iron concentrations are controlled by the solubility of the mineral goethite, and is consistent with other redox indicators such as the measured Pt-­electrode potential and the ratio of dissolved As(V) to As(III).

The eastern area is characterized by the Siblingen borehole. The eastern reference water is an Na-HCO₃-SO₄-(Cl) type with a total dissolved solids content of about 0.5 g/l. Only three samples taken during borehole drilling are available to define this water, so it can be specified in less detail and with less precision than the western water. Its definition assumes saturation with respect to calcite, baryte, and fluorite. The samples permit only a broad definition of its oxidation potential and content of redox-sensitive metals such as Fe, As, Mn, and U. Trace element data for the most part are lacking.

Small quantities of much more saline water were found in two locations in the realm of the western reference waters. A sample of an Na-(Ca)-Cl-(SO₄) water was taken from 1326 m at Böttstein and of an Na-Ca-SO₄ water from 923 m at Leuggern. These waters have total dissolved solids contents of 13 and 5 g/l, respectively, and their chemistries are also given. They probably represent the extreme range of water compositions that might be encountered in parts of a repository in the crystalline basement of northern Switzerland.