Flash flood mitigation as a positive consequence of anthropogenic forcing on the groundwater resource in a karst catchment

The Mediterranean coastal region is prone to high-intensity rainfall events that are frequently associated with devastating flash floods. This paper discusses the role of a karst aquifer system in the flash floods of a Mediterranean river, the Lez river. Most of the Lez river watershed is located on karst terrains where interactions between surface water and groundwater take place. During extreme rainfall events, the presence of fractures and well-developed karst features in carbonate terrains enhances the infiltration processes and involves the concentration of the recharge into highly organized and permeable flow paths. The groundwater, therefore, quickly moves towards the natural outlets of the karst system. The influence of the Lez karst aquifer system on the associated river floods dynamics is analysed while considering the spatially distributed rainfall, as well as the time series of the groundwater level within the aquifer and of the Lez river discharge measured at various gauging stations. Special attention is given to the relative importance of the surface and underground processes involved in flash flood genesis. It is shown that the karst groundwater contributes to flash floods under certain conditions, while high-rate pumping within the karst aquifer, which generates significant drawdown, may mitigate flash floods under other conditions

Water storage and transfer in the epikarst of karstic systems during high flow periods

A monitoring of spring and rain waters in the South of France during two hydrological cycles is presented. Rain waters were sampled after each precipitation event at 3 rain-gauge stations. Four karstic springs located in the same area have also been daily (high discharge events) to monthly (low flow periods) sampled. This paper focuses on compositional changes in the Cl− and Br− ions and the oxygen-18 (δ18O) and hydrogen (δD) isotopes during the high discharge events. The responses of the different karstic systems are quite homogeneous and reflect the hydrological state of the system. The waters discharged during the major autumn and winter high discharge events originate from the epikarstic reservoir and show characteristics chemical variations related to residence in the unsaturated zone close to the surface. Their residence time in the order of 1–3 months. Correlations between the composition of the spring-water and the rainwater during three successive high discharge events during the summer of 1998 indicate that the water for high discharge event “n” is derived from water from precipitation event “n − 1” via a piston-type mechanism with residence time of 2 weeks. These results are interpreted as an indication of the major role of the epikarst reservoir in the karst recharge functioning. The similar behaviour of the four springs, although located in different geological contexts allows to think that the epikarst role could be more important than previously thought

Délimitation des périmètres de protection en milieu karstique : méthodologie basée sur la structure et le fonctionnement des systèmes karstiques ainsi que sur la cartographie multicritère de la vulnérabilité. Exemples d’application en Franche-Comté

Les systèmes karstiques sont considérés comme vulnérables aux pollutions par comparaison avec les hydrosytèmes des autres milieux, notamment ceux à porosité d’interstices. Cette sensibilité particulière aux pollutions est à mettre en relation directe avec la structure fortement hétérogène des aquifères karstiques et son fonctionnement. Les systèmes karstiques requièrent un plan de protection spécifique adapté à ses caractéristiques de structure et de fonctionnement. Face aux problèmes récurrents auxquels sont confrontés les bureaux d’étude et les hydrogéologues agréés pour élaborer les documents de base afin de délimiter des périmètres de protection pertinents et applicables en milieu karstique, la DIREN de Franche-Comté a été à l’initiative d’une étude menée par le BRGM qui a conduit à l’élaboration d’un guide méthodologique. La méthode RISK proposée est une méthode multicritère à index de cartographie de la vulnérabilité intrinsèque des aquifères karstiques basée sur la cartographie de 4 critères : (i) R pour la roche aquifère, (ii) I pour les conditions d’infiltration (diffuse, ponctuelle), (iii) S pour le sol et la couverture protectrice et (iv) K pour le degré de karstification. Chaque critère comprend 5 classes définies. Les 4 critères sont affectés d’un indice de pondération ; l’addition de l’ensemble des cartes permet d’obtenir par analyse multicritère une carte de vulnérabilité comprenant 5 classes. La carte de vulnérabilité croisée à une carte des risques constitue la base pour la délimitation des périmètres de protection selon la législation en cours

De la caractérisation géologique et de la mise en place de la karstification en contexte méditerranéen à l’étude hydrogéologique. Quel transfert possible pour l’étude des aquifères karstiques de la craie ?

La compréhension du développement de la karstification constitue une étape essentielle dans l’étude des régions karstiques en vue de l’évaluation et de la gestion de la ressource en eau souterraine des aquifères karstiques. L’élaboration des systèmes karstiques du pourtour méditerranéen a été fortement influencée par les variations du niveau de base, liées d’une part aux fluctuations du niveau marin et d’autre part aux mouvements tectoniques. Une méthodologie d’étude a été développée et testée sur un système karstique dans les Corbières Orientales (système du Bas-Agly). Cette méthodologie consiste à replacer le massif karstique dans un contexte de géodynamique de bassin sédimentaire. L’étude consiste (i) à effectuer une synthèse des périodes propices au développement de la karstification en considérant conjointement la géodynamique régionale et la charte eustatique mondiale et (ii) à reconnaître les morphologies d’aplanissement et l’enregistrement sédimentaire fossile des massifs karstiques. D’autre part, l’étude du fonctionnement hydrogéologique du système basée sur une approche globale à l’aide de décomposition d’hydrogrammes permet d’aboutir à un modèle conceptuel. Les interprétations hydrogéologiques sont cohérentes et mieux contraintes par les indicateurs géologiques. La mise en place de la karstification des aquifères de la Craie a été abordée jusqu’à ce jour par l’étude des réseaux de fracturation, par l’analyse morphologique spatialisée ou encore par l’étude sédimentologique des dépôts endo et exokarstiques. De nombreuses publications concernant l’évolution paléogéographique de la Craie existent par ailleurs. L’ensemble de ce matériel pourrait permettre un transfert de méthodologie et ouvrir peut être de nouvelles perspectives sur l’étude des systèmes aquifères de la Craie

Bacteriophages as surface and ground water tracers

International audienceBacteriophages are increasingly used as tracers for quantitative analysis in both hydrology and hydrogeology. The biological particles are neither toxic nor pathogenic for other living organisms as they penetrate only a specific bacterial host. They have many advantages over classical fluorescent tracers and offer the additional possibility of multi-point injection for tracer tests. Several years of research make them suitable for quantitative transport analysis and flow boundary delineation in both surface and ground waters, including karst, fractured and porous media aquifers. This article presents the effective application of bacteriophages based on their use in differing Swiss hydrological environments and compares their behaviour to conventional coloured dye or salt-type tracers. In surface water and karst aquifers, bacteriophages travel at about the same speed as the typically referenced fluorescent tracers (uranine, sulphurhodamine G extra). In aquifers of interstitial porosity, however, they appear to migrate more rapidly than fluorescent tracers, albeit with a significant reduction in their numbers within the porous media. This faster travel time implies that a modified rationale is needed for defining some ground water protection area boundaries. Further developments of other bacteriophages and their documentation as tracer methods should result in an accurate and efficient tracer tool that will be a proven alternative to conventional fluorescent dyes

Importance of a high resolution lithological and geometrical knowledge for Mediterranean coast sedimentary aquifers management. Application to the Roussillon basin, South of France

International audienceA good understanding of coastal aquifers is particularly important for their management and preservation,as well as for specific implementations (artificial recharge). A detailed knowledge of the lithology andgeometry of the aquitards and aquicludes is as important as that of the aquifers, particularly for seawaterencroachment dynamics. A new methodology, based on reservoir geology, is used to contribute to highresolutionlithological and geometrical knowledge of coastal sedimentary aquifers. The proposedmethodology is being applied on the Roussillon Basin, located along the southernmost part of the FrenchMediterranean coast, near the Spanish border. This basin covers an area of 700 km2 and comprises twoPliocene aquifers whose characteristics are linked to the Messinian event, and is overlain by the Quaternaryaquifer whose geometry is linked to eustatic variations. All these aquifers consist of alluvial and coastaldeposits. A detailed lithology and geometry model of this basin has been made mainly on the basis ofreservoir geology methods: (i) on-shore, through genetic stratigraphy and (ii) off-shore, through seismicstratigraphy. A detailed mapping of the elevation of the top and base of the aquifers andaquitards/aquicludes is thus available and will be used in a hydrogeological finite difference model. Thismodel will be used for various applications such as aquifer management, location of the outflow areas ofthe aquifers and sensitivity analysis on geometry (on-shore-off-shore)