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.

Typology of recent groundwaters from different aquifer environments based on geogenic tracer elements

A key problem in environmental science and in engineering geology is the often incomplete understanding of the origin of dissolved components in groundwater. The dissolved contents of trace elements in groundwater are of special importance for groundwater quality control. The AQUITYP project aims to establish a detailed typology of recent groundwaters based on their geogenic trace element compositions, and to derive a so-called "geo-reference" for groundwaters from five principal aquifer lithologies in the Alpine belt. This geo-reference provides a database for investigations related to groundwater contamination, groundwater resources management and engineering geology. Groundwaters from crystalline, carbonate, and evaporite rocks, as well as molasse and flysch sediments in Switzerland and in neighbouring countries were sampled and documented by previous researchers (Dubois, 1993; Dematteis, 1995; Mandia, 1993; Hesske, 1995; Basabe, 1993). Based on a statistical analysis of the data and examination of the relationship between aquifer lithology and chemical groundwater composition these researchers identified a number of characteristic tracer elements (geogenic tracers). The present study includes (1) a synthesis of the hydrogeology and the hydrochemistry of recent groundwaters in these five aquifer types based on groundwater data acquired within the AQUITYP project, and (2) a hydro-geochemical investigation of the origin and chemical behaviour of the geogenic tracer chromium, based on a comprehensive field and laboratory study. To enable a comparison of the chemical groundwater data gathered and analysed over a time span of 20 years, a rigorous quality control of the entire database was made. An assessment was made of the sampling techniques, the sample treatment and the analytical methods applied in the AQUITYP project since 1981. Different tests were carried out to evaluate the quality and comparability of the data, including geochemical model calculations, comparison of different analytical techniques, and tests to estimate the influence of the filtration procedure. In order to make this large number of quality controlled data accessible also for future investigations, a groundwater data storage system was developed (AQUITYP-DataBase). 1.) Typology of recent groundwaters In the synthesis of groundwater hydrochemistry, the emphasis was laid on the hydro-geochemical evolution leading to the characteristic groundwater composition in each of the five aquifer types. Chemical characteristics and differences between the groundwaters from the different aquifer types were identified and natural concentration ranges for each aquifer type derived. The proposed geogenic tracers were evaluated and the potential sources of these tracers identified. The dominant processes leading to the typical mineralisation of recent groundwaters were investigated using geochemical modelling strategies. Finally, the concentrations of chemical elements in the groundwaters from the different aquifer types were compared to the Swiss drinking water standards in order to assess the quality of the investigated groundwaters. It has been found that each rock type contributes in a characteristic way to the major and trace element composition of the corresponding groundwater: The groundwaters derived from the crystalline Mont-Blanc and Aiguilles-Rouges massifs are characterised by a low total mineralisation (TDS 22 to 158 mg/L) dominated by Ca2+, Na+, Mg2+, alkalinity, SO42-, and F- (Ca-Na-HCO3,-SO4, waters). Elevated amounts of Mo, U, W, and As occur. These groundwaters derive their mineralisation mainly from the interaction with hydrothermal minerals present along fractures. Fractures act as major groundwater flow paths. Minerals relevant for groundwater mineralisation include carbonates (Ca2+, Mg2+, HCO3-), clay minerals (Ca-Na ion exchange), fluorite (F-, Ca2+), Fe-, As-, and Mo-sulphides (SO42-, As, Mo), and U- and W-minerals (U, W). In these crystalline groundwaters the natural concentrations of F- (23% of the investigated springs) and As (7%) exceed the Swiss limits for drinking water. In addition, the WHO guideline values for U are exceeded in 65% of the cases and for Mo in 15% of the cases. The carbonate karst groundwaters obtain their low to intermediate mineralisation (TDS 161 to 547 mg/L) from the dissolution of calcite (Ca-HCO3, waters), as well as in certain regions dolomite (Ca-Mg-HCO3, waters) and gypsum (Ca-Mg-HCO3,-SO4, waters). Together with their very short residence times, the carbonate karst groundwaters generally contain very low trace element contents. Nevertheless, geogenic trace elements occur in specific regions in relation with fossil organic matter (I, V) and accessory minerals such as barite in deep sea limestones (Ba), evaporite minerals (gypsum, celestite: Sr, Li), clay- and Fe-minerals (V), and Mo-sulphides and U-minerals in dolomitic limestones (Mo, U). In 18% of the carbonate karst springs atmospheric derived Pb exceeds the Swiss drinking water quality target value. The groundwaters from Triassic evaporites in the Swiss Rhone basin are characterised by a high total mineralisation (TDS 760 to 2788 mg/L) expressed by elevated amounts of Ca2+, Mg2+, Sr2+, SO42-, and alkalinity (Ca-Mg-SO4-HCO3 waters). Elevated amounts of the trace elements Mn, Ni, Cu, Li, Rb, Y, and Cd occur. The hydrochemical evolution of these groundwaters is governed by incipient dedolomitisation, involving dissolution of gypsum, celestite and dolomite and simultaneous precipitation of calcite. The Na+ and K+ contents probably controlled by ion-exchange reactions on clay minerals. Characteristic trace elements originate mainly from the dissolution of dolomite (Mn, Ni) and small amounts of apatite (Y, Cd), and from the oxidation of sulphide minerals (Cu, Ni, Cd). The elevated concentrations of the highly soluble Li and Rb may be related to brine inclusions in evaporite minerals and eventually to clay minerals. These evaporite groundwaters contain SO42- concentrations exceeding the Swiss quality target for drinking water in all springs, and the concentrations of U and Ni exceed the WHO guideline values in respectively 58% and 2% of the cases. In addition, Mn, Cd, and As concentrations exceed the Swiss quality targets in respectively 11%, 10%, and 7% of the investigated springs. Recent groundwaters circulating in the porous and fissured molasse sandstones and conglomerates acquire their intermediate mineralisation (TDS 48 to 714 mg/L) primarily by dissolution of calcite and minor dolomite (Ca-Mg-HCO3 waters). The particular mineralogy of certain Molasse formations is reflected in specific trace element compositions of corresponding groundwaters: ophiolite detritus in OMM sandstones in western Switzerland (Cr), barite fracture mineralisations in subalpine and folded Molasse units (Ba), granitic detritus containing sulphides (Mo), U-minerals (U) and abundant mica (Li) in the "Glimmersand" (OSM, Ca-Mg-HCO3-SO4 waters), and evaporite minerals (Li, Sr), sulphides (Mo), and U-minerals (U) in the "Gypsum-bearing Molasse" (USM, highly mineralised Ca-Mg-SO4-HCO3 waters, TDS 984 to 1346 mg/L) . In these molasse groundwaters the Swiss quality target values for drinking water of Cr and Pb are exceeded in 36% and 6% of the springs, respectively. The U concentrations of 14% of the molasse groundwaters ("Gypsum-bearing Molasse" and "Glimmersand"), exceed the WHO guideline value. The groundwaters from the "Gypsum-bearing Molasse" display similar quality problems as the evaporite groundwaters. Groundwaters derived from the shallow flysch aquifers in the Niesen and Gurnigel nappes are poorly evolved Ca-(Mg)-HCO3 waters. Their low to intermediate total mineralisation (TDS 160 to 459 mg/L) is acquired primarily by dissolution of calcite and to a lesser degree dolomite. The low trace element content is dominated by Ba originating from barite fracture mineralisations. The poor chemical evolution of recent flysch groundwaters results from (1) their short residence time in the fractured flysch rocks, and (2) the absence of readily dissolving minerals except carbonates and barite. In 32% of the flysch springs Pb derived from atmospheric sources exceeds the Swiss quality target value. 2.) Case study on the chemical weathering of molasse sandstone: Sources and chemical behaviour of Cr For the characterisation of the potentially toxic tracer element Cr a comprehensive field study was carried out on a selected catchment (Lutry spring catchment near Lausanne) situated in a molasse sandstone (OMM, Burdigalian). The investigation of the processes controlling the dissolved Cr content in these groundwaters was based on the groundwater chemical data, as well as on mineralogical, geochemical, and hydrological data. Relic Cr-bearing spinel and pyroxene in the sandstone were identified to be the primary sources of Cr. An electron microscope study showed that in the Burdigalian sandstone and overlying soil these minerals are strongly weathered. The slow weathering of these minerals is the major Cr releasing process. Under the oxidising conditions reigning in the investigated groundwaters, Cr prevails in solution in its highly soluble and toxic hexavalent state (CrO42-). In this case, retention by secondary Cr-hydroxide phases does not occur, as can be shown by geochemical model calculations. Laboratory leaching experiments were carried out with Burdigalian molasse sandstone from the field site, in order to support the field study findings and to quantify the processes responsible for the Cr mineralisation observed in the Lutry groundwater. Two experiments with mountain-wet oxidised and reduced sandstone were carried out over a time span of 2 months each, to obtain information about the influence of the oxidation state of the substratum on the mobilisation of Cr. The experiments clearly showed that the Cr-releasing processes are fast enough to explain the Cr contents found in the groundwater, and that the release of Cr into the groundwater depends on the weathering state of the sandstone. In the oxidised, Fe-hydroxide-coated sandstone, Cr is faster released into solution than in the less altered reduced sandstone. This indicates that the Cr contained in the reduced sandstone is in a more stable state, i.e. mainly incorporated in detrital minerals, while in the oxidised sandstone, the Cr is partly in an unstable state, i.e. adsorbed on the surface of secondary Fe-hydroxides, from where it is more easily leached.

EPFL2001

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.

EPFL1995

Méthodes d'analyse du fonctionnement hydrogéologique des versants instables

Pierre Tullen

Significant progress was made these last decades in the development of hydrogeological methods. However, these new methods are still very little used in the problematics of unstable slopes, mainly because of the hydrogeological complexity, which characterizes them (thin aquifers, discontinuous media, succession of saturated and unsaturated zones, lower permeability, higher hydraulic gradients, evolution of the permeability in time in relation to slope movements, etc.). Consequently, the approach usually developed to study slope instabilities is based on the setting-up of relations between hydroclimatologic parameters and slope movements. This approach exclusively concentrates on causes and effects, without developing the processes induced by these causes and responsible for these effects. This "black-box" type of relation is in some cases satisfactory, but in others, the analysis of the induced hydrogeological processes becomes essential to understand the functioning of the instability phenomenon and to predict the movements. Our project aims at introducing this missing link to allow a better understanding of the hydrogeological processes caused by particular hydroclimatologic conditions. It is a matter of determining the relation between hydroclimatologic phenomena and a critical hydrogeological state, which can lead a phenomenon of instability to reactivation or to a state of rupture. Unfortunately, this relation cannot be approached in the same way for all instabilities throughout the world. Owing to the specificity of each slope, the relation must be established individually, which may lead to a hydrogeological typology of unstable slopes. Accordingly, we propose a methodology comprising a succession of stages that are useful for the understanding of the hydrogeological processes underlying slopes. This approach aims at establishing a hydrogeological assessment on the scale of the slope or, if necessary, on a regional scale, in order to determine groundwater infiltration zones, trajectory and velocity of groundwater run-off as well as outlet areas. The usual ways of doing this are generally the measurement of the river flow rate, a pluviometric follow-up or direct hydrogeological measurements via piezometers. These traditional approaches generally remain limited by problems of space representativeness or life span; on top of this come problems of cost due to the often chaotic morphology of the slope instabilities and the fact that they are not easily accessible. To make up for these various problems, we propose a complete methodological approach to improve the understanding of the groundwater run-off systems for the unstable slopes: A characterization of the geological domain based on geophysical measurements in addition to the borehole data. This approach, original in the specific context of unstable slopes, makes it possible to obtain a three-dimensional representation of the heterogeneity which composes the subsurface grounds in a fast, efficient and economic way; A spatialization of the climatologic observations in order to take into account the distribution of the precipitations while considering the various states (rain and/or snow) that constitute the water contribution to a slope in relation to altitude; A hydrogeochemical characterization of the outlets, which makes it possible to assess the general groundwater circulation system within the slope, and to come to a decision about one or several possible trajectories of groundwater run-off; An isotopic characterization of the outlets, which allows estimating the altitude of infiltration of the groundwater. This characterization can also give some information on the infiltration zones. These methods also make it possible to characterize the general groundwater run-off system. In some cases, it is also possible to come to a conclusion, quantitatively speaking, concerning the mixing between several reservoirs and to estimate the time of residence of the water in the aquiferous system; A qualitative validation of the assumptions on the basis of artificial tracing tests which will attest the existence of hydrogeological connections between a catchment area and an outlet. This approach will also inform about the time of residence of water; A validation of the conceptual model on the basis of a bi- or three-dimensional numerical simulation of the groundwater run-off in order to validate the hydrological and hydrogeological relations. The calibration of these numerical models is based on a set of hydrogeological data, in particular the outlet discharge, the isotopic composition of water and/or their hydrochemical composition. These strictly hydrogeological numerical simulations can then be translated into a mechanical state to evaluate the stability of the slope. The whole of the points suggested by this methodological approach is not always essential to determine the hydrogeological processes that govern each slope. According to the characteristics of each study site and the means at disposal, some could be treated only partly, or not at all, depending on the established objectives. In the end, a hydrogeological typology of the unstable slopes should make it possible to propose a standard methodological approach and specific tools for analysis in direct relationship to the hydrogeological context which characterizes each slope. The methodological approach suggested in this PhD thesis was tested and validated on three representative sites through Switzerland and the alpine arc that present quite distinct characteristics of hydrogeological functioning. They are the slopes of Hohberg in the canton of Fribourg, Triesenberg in the Principality of Liechtenstein and Peney in the canton of Geneva. This validation phase allowed to assess the hydrogeological functioning of the three studied slopes and to evaluate the relation between the hydrogeological processes and the phenomena of instability. An approach such as that presented in this study opens up prospects to carry out means of sustainable stabilization with an aim of solving problems of slope instabilities.

EPFL2002