Technischer Bericht NTB 91-22

Sondierbohrung Siblingen:Dokumentation der Wasserprobenentnahmen und Interpretation der hydrochemischen und isotopen-hydrologischen Analysen

As part of the regional investigation programme in Northern Switzerland aimed at identifying potential sites for disposal of high-level and long-lived intermediate-level radioactive waste, hydrochemical and isotope hydrology analyses were carried out on water samples from the Siblingen (SH) borehole. Samples were taken from the Muschelkalk, the Buntsandstein and different zones in the crystalline, and a wide range of hydrochemical analyses, as well as isotope and gas measurements, were carried out. The water sampling was documented, the quality of water samples evaluated and contamination by drilling fluid determined using tracer values and tritium; any necessary corrections were then made to the analytical results. Based on the results of geochemical model calculations, a thermodynamically consistent, corrected hydrochemical dataset was prepared for each sampled zone. These datasets formed the basis for the hydrochemical characterisation of the deep groundwaters of Siblingen. The results provide information on the hydrochemical conditions, as well as on the evolution, origin and residence time of the deep groundwaters. They are also used to validate a regional hydrodynamic model and to provide base data for radionuclide sorption investigations, which are necessary within the context of repository safety analysis.

The water from the Upper Muschelkalk (Trigonodus Dolomite), the uppermost zone sampled from an average drilling depth of 183.55 m, is a Ca-Mg-(Na)-SO4-HCO3 water with relatively low mineralisation. The hydrochemical modelling showed, compared with other Muschelkalk waters from Northern Switzerland, a marked undersaturation with respect to dolomite. This water is also undersaturated with respect to gypsum and anhydrite. The Muschelkalk water infiltrated under conditions similar to those prevailing today. Isotope measurements on dissolved sulphate gave values which do not lie within the range for typical Muschelkalk sulphates. Based on their carbonate isotope values, the calcites from open fractures and druses in the Muschelkalk are non-marine in character and are not in equilibrium with the Muschelkalk water as it occurs in Siblingen today. 14C measurements gave a model age of at least 16,000 years for the Siblingen Muschelkalk water. Based on hydrogeological and hydrochemical considerations, this 14C value for the Muschelkalk water appears to be too low and the model age too high, i.e. a younger "age" would have been the initial expectation. The 39Ar activity gives no indication of underground production and an 39Ar model age of at least 900 years. Based on the 39Ar/37Ar ratio, it can be stated that recoil is the dominant process for the escape of the Ar isotopes from mineral particles and fracture calcite into the groundwater. The tritium content can be traced back to around 2 % contamination with drilling fluid. Based on the 3H/85Kr ratio, it is concluded that the water sample from the Muschelkalk contains less than 2 to 3 % of young or recent water/drilling fluid. There are hydrogeological indications to the effect that the SE slopes of the Black Forest could represent the infiltration zone of the Muschelkalk water.

Both the waters from the Buntsandstein (fine- to coarse-grained sandstones with argillaceous intercalations), from an average depth of 341.05 m, and from the crystalline (cordierite-biotite-granite), from average depths of 478.95 m, 527.65 m, 1,158.95 m and 1,495.96 m, are Na-HCO3-SO4 waters with very low mineralisation. Based on the results of hydrochemical modelling, they are undersaturated with respect to dolomite, anhydrite and gypsum. The relatively light stable oxygen and hydrogen isotopes and the low noble gas recharge temperatures indicate that these waters probably infiltrated under ice-age conditions. Measurements of the sulphate isotopes indicate a mixture of sedimentary and crystalline sulphate. The results of carbonate isotope measurements imply that most of the fracture calcites from the deeper crystalline were probably formed under conditions similar to those prevailing today, and that they are more or less in equilibrium with the deepest waters from the crystalline. In contrast, the calcites from the upper section of crystalline are not in equilibrium with the water from the corresponding depth. A model age of more than 16,000 years was calculated for the water sample from the Buntsandstein, while the water samples from the crystalline had a somewhat higher 14C model age (> 17,000 to > 22,000 years). These data are consistent with infiltration of the waters under ice-age conditions. The 39Ar activity in the Buntsandstein water can be explained largely by underground production, and a model age cannot be estimated. It is assumed that a large component of the 39Ar activity originates from the crystalline. Probably 37Ar escapes into the groundwater mainly from calcite in fractures and druses as well as from fluorite. Based on their relatively high uranium content, the Buntsandstein and crystalline waters of Siblingen can be compared with the corresponding waters along the Rhine, which probably originate from the Black Forest. It can be concluded on the basis of the uranium-thorium activity ratios that there has been an intensive rock-water interaction within the last 106 years. The differing tritium contents in the waters from the Buntsandstein and the crystalline are not due to the presence of young groundwater but to contamination with drilling fluid. Based on the 3H/85Kr ratio in the Buntsandstein, it can be stated that this water sample contains less than 2 to 3 % of young or recent water/drilling fluid. The southern Black Forest is assumed to be the potential infiltration zone for the waters from the Buntsandstein and the crystalline.