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 produces overlapping flysch units. Underground water flows correspond to an aquifer environment with discontinuity porosity (including micro-karst) in the Niesen flysch. Alternating mudstones and fines turbites, as well as the folded tectonic structure, provoke aquicludes, which limit regional flows. Flow rates of the order of 1000-1500 l/mn at low water were observed in the most aquiferous sectors. Time flow variations are on the order of a factor of 3 for the Neissen groundwater, and 2.5 for the Gurnigel groundwater, which differentiates them from typical karstic flows. Geological and chemical characterisarion of subterranean waters The Niesen flysch water is cold (average = 6°C). It has a total hardness of 350 mg/l. The waters of the Gurnigel flysch are slightly warmer and harder. The temperature is dependent on altitude as geothermal effects are negligible. The statistical study and the regional distribution of the parameters produced the range of variation in concentration for the waters of the Niesen flysch as well as their differences in comparison with water which had extra-flysch influences on its make-up (c.f. Table 6.1). The studies of time dependant changes indicate a general stability of the majority of components. The flow rates of the most productive outlets can fluctuate between 50 and 500 l/m. The subterranean waters of the Niesen nappe belong to six principal hydtochemical facies (JÄCKLI'S type): 1) Ca-HCO3, 2) Ca-(Mg)-HCO3, 3) Ca-Mg-(Na)-HCO3, 4) Ca-HCO3-(SO4), 5) Ca-(Mg)-HCO3-(SO4 ) and 6) Na-HCO3-(SO4). Within these facies, the Mg and SO4 content can be superior, thus defining the sub-facies. A study of the reservoirs geologically characterises each outlet. Hence, facies 1 and 2 correspond, above all, to the Niesen (Frutigen, Niesenkulm and Seron) and Gurnigel flyschs, of which the outlets are situated at the surface. Deeper and slower flows in the flyschs are characterised by sub-facies 2B (with Mg between 20 and 50 %), facies 3 and a generally higher content of all the other components (including the trace elements). Facies 4 and 5 correspond to outlets of varying nature from extra-flysch ground (intra-nappe) or from Triassic evaporites (periphery). Facies 6 correspond to a source situated in the extreme NE; it defines another type of extra-flysch influence with very great anomalies in Fe, B. Li and I (from 277 to 950 µg/l). Incidentally, the most abundant trace components of flysch waters are Ba, Fe and B with averages between 13 and 35 µg/l. Li, Br and Zn are present in quantities ranging from 2 to 7 µg/l. Micro-trace components (Rb, Al, Ni, I, Mn, Cu, U, Cr, V, Co, and La) were also detected with mean concentrations ≤ 1.5 µg/l. A multi-variable analysis of the principal components (factors) was carried out with the space-time network parameters. The principal hydrochemical facies remain staggered despite the time variations. Lixiviation tests of different flysch rocks and their analyses allowed comparison of the hydrochemical components of the soluble portion of the rocks with those of subterranean waters. In fact the lixiviats contain virtually all of the components found in the waters in generally superior concentrations, which confirms the geological origin of these components in subterranean waters. The base axis of the project AQUITYP includes five groups of aquifer types. The flysch waters are included in group 2 : aquifers of sedimentary rocks with fissures and interstices. Amongst the outlets, the Cornalle and Alliaz springs present anomalies in Cl, Na Ba, B, Li, Br, I and Cr in comparison with the other outlets in this group, including those of the flyschs. Their chemical composition resembles that of the outlets with extra-flysch (evaporitic) influences. In comparison to the rest of the aquifers of the AQUITYP network, the chemical composition of flysch outlets corresponds quite well with that of carbonated karstic rocks (group 3). It is also similar to that of loose quaternary soils (group 1) but with concentrations which are inferior in certain minerals, particularly in HCO3, NO3, Cl, Br and I. However, flysch waters are distinguishable from those of karstic evaporitic aquifers (group 4) which have higher concentrations in SO4, Ca, Mg, Sr, Si, Fe, Li, I and Cu. They also differ from crystalline aquifer waters (group 5) whose mineral concentrations are generally lower, with the exception of the As, Mo, W and U which are characteristic of this water. This work has allowed us to describe the hydrochemical and hydrogeological characteristics unique to the aquifers of the Niesen and Gurnigel which distinguishes them from the extra-flysch ground of the same region (mainly evaporites). A distinction has also been obtained when deeper and slower flows in the flyschs have been considered. As far as AQUITYP is concerned, the flysch aquifers have been found to form a distinct type at the heart of this project and can be said to have been found to be clearly distinguishable from other alpine aquifer types.