Typologie géochimique des eaux des aquifères carbonatés des chaînes alpines d'Europe centrale et méridionale

This study describes the spatial heterogeneousness of the chemical composition of underground waters, particularly the trace elements, and the carbon-13 isotope in several carbonate aquifers of the alpine belt in Europe. The research forms part of the AQUITYP project which have been worked on the last 14 years by the Geology laboratory of the Swiss Federal Institute of Technology in Lausanne (GEOLEP). The main aim of the project is to define the typology of the aquifers, based on the chemical characterisation of underground waters. The carbonate aquifers has been divided in several groups chosen after geological, geographical and hydrological criterias. The geological criterias are petrography and depositional environments of carbonates. The geographical criteria is the altitude of the catchment area. The criteria hydrology is the type of flow in karstic systems. The first stage of this research consisted of a selection of 87 carbonate aquifers divided in 13 regions of study in the alpine belt between France and Greece. Aquifers were selected in order to guarantee the absence of external contamination in groundwaters. Sampling were carried out in a period of base-flow, when groundwaters are equilibrated with rocks. The analyse of 112 samples of water provided the basis of the observations of the various chemical and isotopic composition of groundwaters of carbonate aquifers. Lixiviation of rocks forming aquifers provides information about geogenic origin of some trace elements. The geological and hydrogeological context of aquifers The aquifers studied are composed of platform and basin limestones, that form part of the paleogeographic domain of the Tethys. One example of Devonian limestone belongs to the complex of the Palaeozoic of Graz is also included. Each aquifer has homogenous petrographic composition, but from one aquifer to another the petrographic composition may differ. In this research are included pure limestones, marly limestones and dolomites. From a hydrogeological point of view the 87 aquifers show different degrees of karstification, they are placed either in regions of mountains (caracterised by an important hydraulic gradient), either in region of plateau. The surface of the catchment areas is variable between ten km2 (for example the karstic plateau in Slovenia) and a few km2 (for example the Parmelan massif from subalpine belt in France). Discharges of springs are also variable, between 1 and 1000 l/s. The chemical characterisation of water The study of the chemical typology of the waters of carbonate aquifers are carried out on non thermal water rich in oxygen (median values: temperature 8.8°C, dissolved oxygen 11.8 mg/l, Eh 0.4 V, pH 7.2). The total dissolved solids differs between 100 and 600 mg/l. Underground waters are calcium bicarbonate type. Within these facies in some cases, the Mg and S04 content can be superior, thus defining two sub-facies. The iodine content in the groundwaters of carbonate aquifers usually shows higher values than the other types of aquifers of the project AQUITYP. Concentration in the groundwaters of carbonate aquifers is variable between 10 and 60 µg/l. The origin of iodine is caused by the oxidation of fossil organic matter present in the carbonate rocks. Trace elements allows to distinguish groups of aquifers composed of pure limestones, dolomite or deep sea limestones : pure limestones. The aquifers composed of pure limestones are marked by a very small concentration of trace elements. This can be explained by the poorness of the detrital materials (clays, Fe-Mn oxides, heavy minerals) and the compact lithology that reduces the surface of water-rock interaction. dolomite. The typical elements of dolomite aquifers are Mg, U and Mo. The high primary porosity and the brittle deformation of this type of rock favorise a better water rock interaction compared to poor reef limestone aquifers. Examples of this group of aquifers have been selected in the Italian Dolomites and in the Zone of Karavanke in Slovenia deep sea limestones. The barium is a geogenic tracer in some aquifers composed of basin limestones. The aquifers of the Maiolica and of the Scaglia in the Apennine and the Malm aquifer (spring of Grandchamp) in the Prealpes in Switzerland have a high level of Ba in groundwater (between 60 and 220 µg/l). The limiting factor of Ba mobility in this carbonate environment is the concentration of SO42- in the water. The SO42- makes the Ba precipitate in shape of barytes. This process explains the presence of barytes in fractures found in the aquifer of the Calcare Massiccio in the Apennines. The content of carbonate-13 in groundwaters allows to make a distinction between waters infiltrated in catchment areas with or without vegetation and soils. This isotope is also interpreted as a tracer of the type of underground flow in Karstic system (rapid flow in Karstic channels or slow flow in little fractures of the rock). For the project an example has been taken in Tanneben karst, in Austria. This karst shows that water with a slow flow from blocks has a higher 13C content in comparison with fast flow channel water. Finally some analysis of waters from the bottom of soils on the Jura belt in Switzerland showed that during little rain periods the content of isotope is comparable to the one of slow flow waters of the unsaturated zone in the karst. This demonstrate that the isotopic exchange with rock is completed at low horizon in the soil. The vanadium is a geogenic tracer in the Malm aquifer of the Jura belt in Switzerland and in the Cretaceous aquifer in the region of Trieste, in Italy. The origin of vanadium differs in these two examples. In the Jura belt the vanadium comes probably from marly layers of limestones of the low Portlandien. In the region of Trieste the element vanadium is present in bituminous carbonate sediments from inner platform (Calcari bituminosi from Paleocene). In conclusion the importance and the limits of the chemical criterion for the typology of aquifers are shown. This method of distinction of groups of limestone aquifers is one of the tools which permit the determination of the origin of water sources, particularly in underground and borehole construction projects.