New insights into properties of the ground waters of Northern Switzerland

Since 1981, Nagra has conducted extensive regional studies into the hydrophysical and hydrochemical properties of deep groundwaters from northern Switzerland and adjacent areas. The understanding of the groundwaters in the Triassic and Permian aquifers as well as in the crystalline basement has been considerably enhanced by the deep drilling program of Nagra. This paper presents selected results of the investigations and discusses them with respect to deep groundwater flow systems in northern Switzerland. The composition of dissolved gases present in the groundwaters is very characteristic for each of the different aquifers. Oxygen is almost invariably absent and nitrogen is often the dominant gas. High nitrogen-argon-ratios, as typical of the Permian and Lower Triassic aquifers indicate that a part of the nitrogen is released from organic compounds. In the vicinity of the Rhine graben, ascending carbon dioxide results in a high concentration of this gas in the deep groundwaters of the crystalline basement. Radiogenic-produced helium is enriched in these waters due to the long subsurface retention times. South of the Jura mountains, high concentrations of hydrogen sulphide in waters of the Upper Muschelkalk aquifer are related to the occurrence of hydrocarbons. The Malm, Upper Muschelkalk and Permian-Lower Triassic aquifers and locally that of the crystalline basement are all separated by hydraulic barriers as demonstrated, for example, by the Weiach borehole. Waters of each of the different aquifers can be clearly distinguished from their hydrogeochemistry. The NaCl-character of the Malm aquifer water is attributed to marine formation waters that descended from the overlying Molasse sediments. Comparatively low mineralisation in the waters of the Upper Muschelkalk suggests that there is a significantly high flow rate in this aquifer. 3 refs., 3 figs., 5 tabs