Technischer Bericht NTB 88-07

Hydrochemische Synthese Nordschweiz:Tertiär- und Malm-Aquifere

This Tertiary-MaIm synthesis represents the first part of an overall hydrochemical synthesis of deep groundwaters in Northern Switzerland and adjacent areas. The interpretation of data from the underlying aquifer groups (Dogger, Lias, Keuper; Muschelkalk; Buntsandstein-Permian-Crystalline) is still in progress. This investigation is mainly based on data from Nagra deep boreholes, from Nagra regional programme as well as from external sources (= "Fremde Analysen"; boreholes for other purposes, literature).

The first part of this report NTB 88-07 provides a hydrogeological overview including a short description of the aquifer rocks. Additionally, a compilation of all existing hydraulic potential data is given and the distribution of hydraulic potentials is discussed for the regions of Northern Switzerland, the Bodensee area and the western Swabian Alb.

In the Molasse Basin, hydrochemical and isotope analyses allowed a distinction of three main water types which are positioned one upon another: calcium-magnesium-bicarbonate groundwaters, sodium-bicarbonate and sodium-chloride deep groundwaters. Older waters are generally situated at greater depth. Hydrochemical and isotope-geochemical details of these three water types comprise the major part of this report.

Unlike the other two water types, the calcium-magnesium-bicarbonate groundwaters also include shallow waters with considerable tritium activities, indicating a mean residence time of less than 35 years. These shallow groundwaters were locally found only to depth of about 25 m, but at other localities they can also slightly exceed a depth of about 100 m. The calcium-magnesium-bicarbonate groundwaters infiltrated during similar climatic conditions as today. In contrast, most of the sodium-bicarbonate groundwaters represent recharge during an earlier, colder climatic period, as during glaciation. The sodium-chloride groundwaters are probably of marine or brackish origin, now diluted with infiltrated waters.

Finally, the spacial distribution of these three water types is demonstrated and different secular flow models in the Tertiary-MaIm aquifer group are discussed.