Typologie des eaux souterraines de la molasse entre Chambéry et Linz (France, Suisse, Allemagne, Autriche)

The geochemical typology of natural subsurface waters of the Molasse basin is part of the AQUITYP project, initiated by the Geology Laboratory (GEOLEP) of the Swiss Federal Institute of Technology in Lausanne (EPFL). This study provides an overview of the variation in space of the hydrochemical parameters, especially of trace elements, in order to determine the geological origin of their aquifers. Our data base is mainly made up of two observation networks: a subsurface water network and a deep groundwater network. Complete analyses of all samples have been executed using standard methods for anions and silicium, and high resolution inductively coupled mass spectrometry (HR-ICP-MS) for major cations and trace elements down to the detection limit of about 0.2 µg/l. Subsurface water network It is the main observation network of this investigation and comprises 112 water samples from different springs, collected all over the Molasse basin between Chambéry (France) and Linz (Austria), but mainly in the Swiss Molasse basin. For this region, four groups are generally distinguished, according to the sediment deposition in a continental or marine environment, in ascending stratigraphic order: (1) the Lower Marine Molasse (UMM and North Helvetic "flysch" p.p.), (2) the Lower Freshwater Molasse (USM), (3) the Upper Marine Molasse (OMM) and (4) the Upper Freshwater Molasse (OSM). Our water samples originate from typical sedimentary deposits which we divided in about 30 different sub-type aquifers according to their geological origin and their depositional environment. They are defined on the basis of lithological and structural homogeneity. Therefore, only catchments that completely drain a single, lithologically homogenous, Molasse terrain unit are considered here. Spring catchments selected were generally not affected by runoff from towns, or paved roads. Agricultural activities have been avoided as well as possible. Such spring water should be free of major human influences and only in few cases contain a certain amount of fertilizer products (p.ex: nitrates). Leachate tests of crushed rocks and comparison with snow and soil water compositions allowed to decipher the geogene origin of the dissolved ions in natural groundwaters. The water withdrawn can be generally classified as a calcium-magnesium-hydrogenocarbonate (Ca-Mg-HCO3)-type water. The trace analyses allowed to distinguish several subtypes within this homogenous water type group. The general spacial distribution of the trace elements show, that spring waters in the Bavarian Molasse basin are caracterized by elements as lithium and uranium linked to acid rocks, while those of the Swiss Molasse basin are preferentially influenced by elements derived from mafic or ultramafic rocks, such as chromium and cobalt. The following elements show specific links between aquifer rock type and groundwater. Sulfates: Total mineralisation of spring waters issued from the gypse-bearing Molasse (USM) and the "Glimmersande" (OSM) are highly influenced by high sulfate contents. It can be demonstrated that sulfate ions are produced from two different sources within this two sub-type aquifers: from the dissolution of gypsum in water, producing high calcium contents as well, and from the oxydation of sulfurous ore minerals, especially pyrite, which are abundant in the "Glimmersande". The "Glimmersand" aquifers in NE Switzerland are furtheron caracterized by high lithium, molybdenum and uranium concentrations, reflecting the granitic composition of this sandstones. High barium contents (>150 µg/l) have been found in spring waters issued from aquifers with a dominant fissure permeability ("Grès de la Cornalle", "Gibloux-Delta" and "Rigi-Schüttung"). Its origin is supposed to be the mineral barite, which tends to precipitate in fractures. Therefore, high barium concentrations may indicate the presence of fracture flow within the aquifer. Considerably high silicon contents are linked to the quartzitic and fine grained aquifers of the brackish marine molasse in Bavaria. Both, petrographical composition and slow flow rate are favouring the water-rock interaction. Caracteristic lithium footprints are found in several waters, as in the aquifers composed of the "Glimmersande", the gypse-bearing molasse, the Rigi-conglomerates, the brackish and the estuarine marine molasse. Chromium strongly marks the waters originating from the Swiss marine Molasse aquifers. It is probably related to ophiolithic detritus derived from the flysch of the Prealpes. A couple of waters show weak anomalies of cobalt, which are nevertheless intimately linked to the presence of ophiolithic components in the sediments. Cobalt might be used as a hydrostratigraphical tool to identify greenstone inputs. Analysed colloidal aluminum may serve as a hydrodynamical parameter, indicating microturbidity within the aquifer. High amounts of aluminum varying in time, have been found in estuarin deposits of the OMM. Generally speaking, we propose three main origins of trace elements in the spring water of the Molasse basin: presence of certain petrographical components within the sediments such as basic and ultrabasic rocks, heavy minerals and evaporites. a small flow rate through interstitial porosity of fine grained sediments, favouring water-rock interaction. the abundance of colloidal material in the waters which depends on the petrographical composition of the aquifer and on sample treatment, e.g. filtration techniques. Deep groundwater network A second observation network composed of 21 borehole waters has been added for comparison. Compared to the subsurface waters, deep water chemistry might differ. The following reasons seem to be responsible: Long residence times of water in the underground favours slow reaction kinetics. Chemistry of very deep water can be influenced by mixing with older formation waters. Most of the deep boreholes in the Swiss Molasse basin are situated in the OMM and therefore represent a likely homogenous lithology. Borehole waters, because of different aquifer rock types and unknown surface and borehole influences, are least likely to represent typical Molasse water composition.