typologie des eaux souterraines de la molasse entre chambéry et linz

ABSTRACT
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Abstract
The geochemical typology of natural subsurface waters of the Molasse basin is part of the AQUITYP project,
initiated by the Geology Laboratory of the Swiss Federal Institute of Technology in Lausanne. 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 silicon, and High
Resolution Inductively Coupled Plasma Source 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The water withdrawn can be generally classified as a calcium-magnesium-hydrogenocarbonate (Ca-Mg-HCO 3)type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"). It's 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.