Technischer Bericht NTB 91-30

Hydrochemische Synthese Nordschweiz:Buntsandstein-, Prem- und Kristallin-Aquifere

After the publication of the hydrochemical synthesis of the Tertiary and MaIm aquifers (NTB 88-07), this report presents the results of the Buntsandstein-Permian-Crystalline aquifer group. This investigation is mainly based on data from Nagra boreholes, from the Nagra regional programme as well as from external sources (= "Fremde Analysen"; boreholes for other purposes, Iiterature). Additionally, Buntsandstein, Permian and Crystalline waters from Germany and from present salt lakes were considered for comparison.

The hydrochemical analyses allowed the distinction of the following four main water types in northern Switzerland: alkali-earth bicarbonate, sodium bicarbonate, sodium sulfate and sodium chloride waters.

In the Lower Triassic (Buntsandstein) and the uppermost parts of the underlying formations, sodium sulfate and sodium chloride deep groundwaters were found. The sodium sulfate deep groundwaters are characterized by hydrochemical features specific to the formation and, therefore, are different from those waters of the same main water type observed in the Crystalline. The sodium chloride deep groundwaters exhibit affinities to the sodium sulfate deep groundwaters of the same formation. In Siblingen, no particular Buntsandstein water was encountered, because at this site, water from the crystalline basement ascends into Buntsandstein sediments.

Highly saline sodium chloride deep groundwaters with hydrochemical features specific to the formation were also found in the Rotliegend sediments of the Permo-Carboniferous trough of northern Switzerland. These waters are clearly different from waters of the same water type encountered in the Crystalline and the Buntsandstein, but show striking resemblances to some waters of present salt lakes. These waters most likely still contain relic connate waters (or dissolved connate salt precipitations) of continental Permian salt lakes or interstitial solutions which are mixed with waters of the hydrologic cycle, but are not yet completely displaced by them in the course of the last 250 Ma. From this, a very small water circulation in the Permian sediments can be concluded. An impact to the original water chemistry by water-rock interaction is likely. A similar water was found as well in the Rotliegend sediments of the northern trough shoulder of the Permo-Carboniferous trough.

In the crystalline basement of the southern Black Forest and north of the Permo-Carboniferous trough, a regional flow system could be recognized, based on both a hydrochemical evolution sequence and isotope data. This evolution sequence starts with young alkali-earth bicarbonate groundwaters of the southeastern slope of the Black Forest, continues with higher evolved sodium bicarbonate deep groundwaters as found e.g. in Siblingen and ends with sodium sulfate deep groundwaters as confirmed in the area between Zurzach and Säckingen, where the waters attain underground residence times in the order of 10 to several 10 ka.

Sodium chloride deep groundwaters which probably represent an influence of the realm of the Permo-Carboniferous trough were observed directly underneath and beside the trough. By this, a dilution or mixing sequence from sodium chloride waters of the Rotliegend to those of the Crystalline to the sodium sulfate waters of the Crystalline can be recognized. The underground residence time of these crystalline waters is assumed to be very long, but less long than that of the waters in the trough itself.

In Säckingen, a particular case of a mixing sequence could be recognized. A highly evolved crystalline deep groundwater is diluted with a young alkali-earth bicarbonate water of the type observed in the Black Forest.

The sodium bicarbonate waters with magmatic carbon dioxide must be considered as a particularity. These waters have the same origin of the components as regularly evolved crystalline deep groundwaters, but their water chemistry is additionally influenced by the high amounts of carbon dioxide.