Typologie des eaux souterraines du Flysch de la nappe tectonique du Niesen (Préalpes suisses)

The typology of the underground waters of the flyschs of the Niesen and Gurnigel tectonic nappes form part of the AQUITYP project which has been worked on by the geology laboratory of the Swiss Federal Institute of Technology in Lausanne since 1981. Its principal aim is the hydrogeological and geochemical characterisation of several types of aquifers of alpine origin. Amongst the alpine flyschs, that of Niesen (composed essentially of three subdivisions: Frutigen, Niesenkulm and Seron from the Maastrichtian age) with its important conglomerate and limestone facies, as well as the Gurnigel flysch (Maastrichtian-Eocene) with its sandy limestone facies, favour underground water movements. The Niesen nappe also includes, as a discontinuity (intra-nappe limit), an "ante-flysch" lower layer formed of disparate and discontinuous ground between the Triassic and Cretaceous ages. It is limited tectonically by the Ultrahelvétique region to the SE (periphery) and the submedian zone to the NW. Seventy-six underground flysch ground water outlets have been treated. They form the spatially divided observation network (N°1). Among these, there are several examples which are also influenced by the "ante-flysch" lower layer. Twenty springs are included in the time-space network (N°2) and six outlets were equipped and monitored for two years (time-based network N°3). To this network one can add the Lioson LLI spring which forms a link with the hydro-axis of AQUITYP project. Each sample underwent in-situ physical-chemical measurements and chemical analyses in the laboratory. Generally speaking the ICP-MS (Inductively Coupled Plasma Mass Spectrometry) technique provided the best results in the detection of cations, trace elements and micro-trace elements down to the threshold of 0.1 µg/l. However, some elements (Fe, B, Br, I et Sc) were more difficult to detect due to spectral interference and their detection threshold was about a factor of ten greater. The choice and validity of the elements were completed by a study of anthropological influences on some of the springs (salting roads, catchment pipes, and fertilizers), as well as the influence brought about by the soil. The most common atmospheric pollution forms based on Pb and Al were also detected by way of melt water analyses. Hydrogeological characteristics of aquifers Geomorphologically speaking, the Niesen nappe may be divided into three large compartments which can be distinguished by their catchment basins. The influence of flyschs on the NE sector, which has some mountain sited and some ridge born catchment areas, is well defined. The central sector, which is scored topographically (Simme, Louibach, Sarine), is fed by transversal flows influenced by extra-flysch ground, similar to the lower parts of the nappe. The continuity of the NE sector is regained in the SW sector (between the Sarine and the Grande Eau), but with larger basins towards the north. A more complex tectonic structure in this sector produce