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The Bajocian-Bathonian aquifier in the Caen region is largely exploited for drinking water and agriculture. When this coastal carbonated aquifer becomes confined, groundwater presents an increasing TDS both vertically and along the flow path. This is explained by water-carbonate interaction and by a mixing with marine water (up to 4 %). Radiocarbon dating, after correction of the initial activity according to different models, shows a wide range of groundwater ‘ages’, from 40 000 years B.P. to the present. The marine transgression has thus been estimated as having occurred during the Early Holocene period.

International audience; An inter-laboratory comparison exercise involving the environmental tracers used for groundwater dating was organized in 2012 in France. This paper focuses on SF6 and CFC tracers. Sampling and analytical protocols were compared in three different exercises using: (1) groundwater from a homogeneous aquifer, (2) groundwater from a fractured heterogeneous aquifer and (3) an air standard. The results show good agreement between laboratories (except some outliers) for the aquifers and air standard. Variation in the SF6 results on the air standard was low (RSD = 2%) compared to CFCs (RSD 3–7%), even though its concentration was two orders of magnitude lower. Results obtained on recent groundwater (recharged post 1980) show that the inter-laboratory uncertainty for groundwater dating with SF6 is 3–4 years. This large uncertainty is mainly due to sampling and/or analytical problems. For CFCs, the uncertainties obtained from all laboratories were less than 2 years for groundwater recharged between 1965 and 1996. A higher RSD was observed for CFCs with the air standard analysis, as compared to groundwater measurement, which could indicate an additional uncertainty due to inadequate standardization. Different sampling protocols were used by the laboratories (glass bottles, steel cylinders, etc.) but no systematic effect on measurement uncertainty could be attributed to any of these protocols for either CFCs or SF6. The better precisions obtained through these exercises indicate that, except for CFC-11 and SF6, the corresponding uncertainty in recharge date determination on recent groundwater (1993 – present) is higher than the commonly assumed 1–2 years (i.e., 7 years for CFC-12 and 4 years for CFC-113). These results confirm the need for regular inter-laboratory comparison exercises to improve the analytical and sampling procedures used in groundwater dating.

International audience; We study the temporal variability of water transfer through the infiltration zone of a karst aquifer by estimating the impulse response of the system using cross-correlogram analyses between rainfall and piezometric level time series. We apply a sliding-window cross-correlation method, which calculates cross-correlograms on partially superposed short time series windows. We apply this method for rainfall and piezometric level time series at six boreholes in a fractured karstic aquifer located in Burgundy, France. Based on cross-correlogram functions, we obtain a time series of response time. At most of the boreholes, the cross-correlation functions change over time, and the response times vary seasonally, being shorter during the summer. This unusual structure can be partly explained by the seasonal variability in rainfall intensity, which is higher during the summer (May–September), inducing the seasonal behaviour of the epikarst. During the summer, when rainfall intensity is higher, the epikarst is more easily and quickly saturated. This induces an increase in lateral water transfer within the epikarst and an increase in concentrated fast flows. We also show that the response time seems to tend towards a limit which represents the maximum saturation of the epikarst.

Karst aquifers are known for their wide distribution of water transfer velocities. From this observation, a multiple geochemical tracer approach seems to be particularly well suited to provide a significant assessment of groundwater flows, but the choice of adapted tracers is essential. In this study, several common tracers in karst aquifers such as physicochemical parameters, major ions, stable isotopes, and δ13C to more specific tracers such as dating tracers – 14C, 3H, 3H–3He, CFC-12, SF6 and 85Kr, and 39Ar – were used, in a fractured karstic carbonated aquifer located in Burgundy (France). The information carried by each tracer and the best sampling strategy are compared on the basis of geochemical monitoring done during several recharge events and over longer time periods (months to years). This study’s results demonstrate that at the seasonal and recharge event time scale, the variability of concentrations is low for most tracers due to the broad spectrum of groundwater mixings. The tracers used traditionally for the study of karst aquifers, i.e., physicochemical parameters and major ions, efficiently describe hydrological processes such as the direct and differed recharge, but require being monitored at short time steps during recharge events to be maximized. From stable isotopes, tritium, and Cl− contents, the proportion of the fast direct recharge by the largest porosity was estimated using a binary mixing model. The use of tracers such as CFC-12, SF6, and 85Kr in karst aquifers provides additional information, notably an estimation of apparent age, but they require good preliminary knowledge of the karst system to interpret the results suitably. The CFC-12 and SF6 methods efficiently determine the apparent age of baseflow, but it is preferable to sample the groundwater during the recharge event. Furthermore, these methods are based on different assumptions such as regional enrichment in atmospheric SF6, excess air, and flow models among others. 85Kr and 39Ar concentrations can potentially provide a more direct estimation of groundwater residence time. Conversely, the 3H–3He method is inefficient in the karst aquifer for dating due to 3He degassing.

International audience; Cold seeps, located on sea floors and frequently fueled with methane, have been much studied for the formation of carbonate nodules, crusts or masses resulting from the activity og communities of anaerobic bacteria. In this paper, we examine the impact of cold seepage on their caging sediments, as far as their geochemical composition and microfaunal content are concerned. To this end, two fossil cold seeps of the Jurassic Beauvoisin site (Vocontian Trough, SE-France) are studied, focusing on the redox-sensitive and/or sulfide-forming trace metals. We observe that: 1) the carbonate edifices of the cold seeps are enriched in specific elements, namely, Sr, Mo As and Sb, compared to their caging marlstones, and 2) the benthic foraminifer populations were present close to the edifices. The Sr enrichment is ascribed to a contamination of the seeping fluids at the contact with underlying evaporites; the Mo-As-Sb enrichment may have several, non-exclusive, origins. We retain two scenarios; in the first one, the enrichment is due to the ascending fluids that could collect these elements when passing through thick shaly formations; in the second scenario, Mo, As and Sb were supplied to the sites of carbonate precipitation together with Fe- and Mn-oxy-hydroxides (the so-called particulate shuttle process). We favor the second scenario (possibly coupled to the first one) because the rather uncommon Mo-As-Sb association is also observed for reducing sediments deposited under the influence of Fe-Mn-shuttling.

[1] Temperature reconstructions for recent centuries are the basis of estimations of the natural variability in the climate system before and during the onset of anthropogenic perturbation. Here we present, for the first time, an independent and physically based reconstruction of mean annual temperature over the past half millennium obtained from groundwater in France. The reconstructed noble gas temperature (NGT) record suggests cooler than present climate conditions throughout the 16th–19th centuries. Periods of warming occur in the 17th–18th and 20th century, while cooling is reconstructed in the 19th century. A noticeable coincidence with other temperature records is demonstrated. Deuterium excess varies in parallel with the NGT, and indicates variation in the seasonality of the aquifer recharge; whereas high excess air in groundwater indicates periods with high oscillations of the water table.

A tritium (H-3) profile was constructed in a long-screened well (LSW) of the Fontainebleau Sands Aquifer (France), and the data were combined with temperature logs to gain insight into the potential effects of the ambient vertical flow (AVF) of water through the well on the natural aquifer stratification. AVF is commonly taken into account in wells located in fracture aquifers or intercepting two different aquifers with distinct hydraulic heads. However, due to the vertical hydraulic gradient of the flow lines intercepted by wells, AVF of groundwater is a common process within any type of aquifer. The detection of 3H in the deeper parts of the studied well ( approximate depth 50m), where H-3-free groundwater is expected, indicates that shallow young water is being transported downwards through the well itself. The temperature logs show a nearly zero gradient with depth, far below the mean geothermal gradient in sedimentary basins. The results show that the age distribution of groundwater samples might be biased in relation to the age distribution in the surroundings of the well. The use of environmental tracers to investigate aquifer properties, particularly in LSWs, is then limited by the effects of the AVF of water that naturally occurs through the well.

[1] A multitracer (3H/3He, 85Kr, 39Ar, and 14C) approach is used to investigate the age structure of groundwater in the semiconfined Fontainebleau Sands Aquifer that is located in the shallower part of the Paris Basin (France). The hydrogeological situation, which is characterized by spatially extended recharge, large screen intervals, and possible leakage from deeper aquifers, led us to expect a wide range of residence times and pronounced mixing of different water components. Consequently, a large set of tracers with corresponding dating ranges was adopted. Commonly used tracers for young groundwater (3H, 3He, and 85Kr) can identify only those components with ages below 50 years. This approach is reliable if a large fraction of the water recharge occurs within this period. However, if a considerable fraction is older than 50 years, a tracer that covers intermediate age ranges below 1000 years is needed. We examine the use of 39Ar, a noble gas radioisotope with a half-life of 269 years, to constrain the age distribution of groundwater in this timescale range. Recharge rate, depth of water table, and the age structure of the groundwater are estimated by inverse modeling. The obtained recharge rates of 100–150 mm/yr are comparable to estimations using hydrograph data. Best agreement between the modeled and measured tracer concentrations was achieved for a thickness of the unsaturated soil zone of 30–40 m, coinciding well with the observed thicknesses of the unsaturated zone in the area. Transport times of water and gas from the soil surface to the water table range between 10 and 40 and 1 and 6 years, respectively. Reconstructed concentrations of 85Kr and 3H at the water table were used for saturated flow modeling. The exponential box model was found to reproduce the field data best. Conceptionally, this finding agrees well with the spatially extended recharge and large screened intervals in the project area. Best fits between model and field results were obtained for mean residence times of 1–129 years. The 39Ar measurements as well as the box model approach indicate the presence of older waters (3H and 85Kr free). Using 39Ar to date this old component resulted in residence times of the old water components on the order of about 100–400 years. The 14C measurements provide additional evidence for the correctness of the proposed age structure.

International audience; The latest Jurassic of the Boulonnais cliffs (N-France, Strait of Dover) corresponds to a homoclinal ramp-type depositional environment, where sedimentation was clastic-dominated (marlstones-sandstones). The Tithonian marlstone formations exhibit two or three carbonate beds in the vicinity of Wimereux city (3 beds at Wimereux-North and 2 beds at Wimereux-South). The beds are made up with quite pure, fine-grained (microspar) carbonate in sharp contrast with the background sedimentation. This paper presents a sedimentological and geochemical study of these limestone beds, aiming to determine whether they are of diagenetic origin and hence whether they could have acted as permeability/migration barriers during hydrocarbon maturation/migration. Mineralogical, chemical, and C and O stable isotope data allow us to infer that the beds formed during synsedimentary early diagenesis at the sediment-water interface (or close to it) as the result of a rise of alkalinity induced by bacterial sulfate reduction. The rise of alkalinity was not counter-balanced by the accumulation of H2S released by sulfate reduction and carbonate ion supersaturation was rapidly reached, causing the formation of laterally-continuous limestone beds. Conditions prone to bacterial sulfate reduction developed episodically at the sediment-water interface as the result of spills of anoxic pore waters onto the seafloor. These spills were probably released by synsedimentary fault movements. Such continuous limestone beds being formed under rather common conditions during the earliest stage of diagenesis of shale deposits must be more frequent in the geological record than hitherto identified.

International audience; Radiocarbon decay is rarely used to assess the residence time of modern groundwater due to the low resolution of its long half-life in comparison to the expected range of ages. Nonetheless, the modern 14 C peak induced by the nuclear bomb tests traces efficiently the impacts of recent human activities on groundwater recharge, as well as for tritium. A simple lumped parameter model (LPM) was implemented in order to assess the interest of 14 C and 3 H nuclear peaks in a highly anthropized aquifer system of southeastern Spain under intense agricultural development. It required i) to assess a correction factor for modern 14 C activities and ii) to reconstruct the 3 H recharge input function, affected by irrigation. In such a complex hydrogeological context, an exponential model did not provide satisfying results for all samples. A better solution was reached by taking into account the qualitative recent variation of the recharge rates into a combined exponential flow and piston flow model. Apart from presenting an uncommon approach for 14 C dating of modern groundwater, this study highlights the need of considering not only the variation of the tracer but also the variability of recharge rates in LPMs.

Assessing the impact of current and future pressures on groundwater resources requires an assessment of recharge process. For these purposes, the isotopic tools, and more precisely the stable isotopes of the water molecule were used, as part of this work to understand the flow mechanisms within the unsaturated zone. A new analytical protocol allowing a better accuracy of pore water isotope measurements has been developed. A numerical calculation code taking into account the evolution of stable isotopes of water, from precipitation to groundwater, to estimate recharge has been established. Stable isotopes of the watermolecule have been shown to be powerful tools for constraining, by inverse modeling, the values of the hydrodynamic parameters needed to describe and predict water transport in the unsaturated zone at the profile scale and at the watershed scale.; Évaluer l’impact des pressions actuelles et futures qui s’exercent ou s’exerceront sur les ressources en eau souterraines passe obligatoirement par une évaluation de la recharge. A ces fins, les outils isotopiques, et plus précisément les isotopes stables de la molécule d'eau ont été utilisés, dans le cadre de ces travaux de thèse pour comprendre les mécanismes d’écoulement au sein de la zone non saturée. Un nouveau protocole analytique permettant une meilleure précision de la mesure de la signature isotopique de l’eau porale a été développé. Un code de calcul numérique prenant en compte l’évolution des isotopes stables de l’eau, depuis les précipitations jusqu’aux eaux souterraines, pour estimer la recharge a été mis en place. Les isotopes stables de la molécule d’eau se sont avérés être des outils puissants pour contraindre, par modélisation inverse, les valeurs des paramètres hydrodynamiques nécessaires pour décrire et prédire le transport d’eau dans la zone non saturée, à l’échelle du profil de sol et du bassin versant.

In the face of modern anthropogenic pressures, the preservation of freshwater resources, particularly groundwater, is paramount. This study investigates interactions between small lakes and groundwater in Quebec, where more than 10% of the territory is covered by freshwater including one million inventoried lakes, to determine whether these lakes may be used as indicators of groundwater change. Twenty-one kettle lakes, set in fluvioglacial deposits, have been specifically targeted for this study due to the high likelihood of connection to shallow unconfined aquifers. Groundwater flows were quantified via lake water balances coupled with two natural tracers of groundwater: stable isotopes of water and Radon-222. On a regional scale the majority of these lakes are characterized by an important annual groundwater inflow and a short-to-medium groundwater flushing time. Daily multi-layer modeling of one of the study lakes, Lake Lacasse, highlights the importance of conducting measurements of these complementary tracers on the water column at the local scale. Different climate and environmental change scenarios were tested to determine the long-term evolution of lake geochemistry. The results indicate that the sensitivity of the isotopic composition of lake water to changes in recharge by year 2050 is mainly controlled by the amount of the total lake balance contributed by groundwater, with an optimum sensitivity for 50-80% of the total inflows contributed by groundwater. This research demonstrates that the geochemical signatures of lakes can be used to indicate future groundwater change in response to climate and environmental evolution, which may assist in improvements to the sustainable management of freshwater resources., Face à l’augmentation actuelle des pressions anthropiques sur les réserves d’eaux douces, leur préservation, et notamment celle des eaux souterraines, est primordiale. Les interactions entre les lacs et les eaux souterraines sont étudiées ici afin d’identifier si certains lacs peuvent être utilisés comme indicateurs de changements qualitatifs et quantitatifs des eaux souterraines, spécialement au Québec où plus de 10% du territoire est recouvert d’eaux douces avec un million de lacs recensés. Une vingtaine de petits lacs de Kettle, situés dans des dépôts fluvioglaciaires, ont été spécifiquement choisis pour leur très probable interconnexion avec les aquifères libres peu profonds. Les flux d'eaux souterraines ont été quantifiés via les bilans hydrologiques des lacs couplés aux deux traceurs naturels des eaux souterraines que sont les isotopes stables de la molécule d’eau et le radon 222.A l’échelle régionale, ces lacs présentent un apport annuel et un temps de renouvellement relativement rapide par les eaux souterraines. A l’échelle locale, la modélisation journalière multicouches d’un lac témoin (le lac Lacasse) montre l’importance de la mesure des traceurs, et leur complémentarité, sur la colonne d’eau en fonction des saisons. Différents scénarii d’évolution climatique et environnementale ont ensuite été testés sur les bilans hydrologiques à long terme. Les résultats montrent que la sensibilité de la composition isotopique des lacs à l’horizon 2050 face à l’évolution de la recharge est principalement contrôlée par le pourcentage que représente l’apport des eaux souterraines dans le total de leurs apports en eau, avec une sensibilité optimale pour une fraction de 50 à 80% des apports totaux. Par l’amélioration des connaissances sur les interactions entre lacs et eaux souterraines et sur les outils qui les mettent en évidence, ce travail de recherche a montré que la signature géochimique des lacs peut être utilisée comme indicateur du devenir des ressources en eaux souterraines face aux changements climatiques et environnementaux, pour une meilleure gestion durable de ces hydro systèmes.

The response of continental groundwater systems to recent climate fluctuations can be reconstructed via the continuous measurement of groundwater level, spring flow and climatic chronicles. When any data exist, recent reconstructions of groundwater dynamics may be reached through various recorders of environmental and hydrological conditions such as travertine. Although the relationship between geochemical records in travertine and environmental parameters seems to be accepted, the details of processes and their respective weight in the paleo-information are not clearly established.Rate of CO2 degassing in CO2-rich spring likely influences calcite precipitation rate and the related δ18O and δ13C composition. Isotopic equilibrium is rarely maintained during travertine deposition and the degassing rate is the main controlling factor of the disequilibrium. Due to the lack of knowledge, fractionation processes, either kinetic or equilibrium, occurring between CO2-rich water, gas and travertine required specific pH and temperature-controlled laboratory tests. These tests were conducted on synthetic water at different pH to focus only on the degassing processes. Other tests were conducted on trace elements partitioning during calcite precipitation, to identify the origin of isotopic signature variability, and to constraint the way of recording past conditions.All these tests confirmed that during a degassing leading to travertine precipitation, the speciation of dissolved inorganic carbon species is a major parameter to be tackled since a linear relationship between εDIC-CO2(g) and pH is observed. Indeed, we highlighted that for a high degassing rate, the isotopic equilibrium is not reached because the reaction greater involves light isotopes than heavy ones. There is thus different reaction rate between the species of dissolved inorganic carbon, the reactions occurring faster in the water than the one between water and gas, the latter being controlled by diffusion.; Si la relation entre l’enregistrement géochimique au sein de ces travertins et les paramètres environnementaux semble admise, le détail des processus, notamment cinétique et dégazage, ainsi que leur poids respectifs dans les reconstitutions paléoenvironnementales ne sont clairement établis. Afin de mieux contraindre ces processus, un travail minutieux de reconnaissance des mécanismes de précipitation et d’enregistrement de la signature géochimique a été effectué sur les travertins –récents et fossiles- d’une source carbogazeuse située dans le Massif Central (France). Il couple expériences de laboratoire innovantes et travail sérié sur le terrain.Le taux de dégazage du CO2 dans les sources carbogazeuses est l’un des facteurs principaux conduisant aux conditions de précipitation via la modulation du taux de précipitation de la calcite et par conséquent, sa composition en 18O et 13C. Les tests en laboratoire ont été conduits en deux étapes : (1) caractérisation des mécanismes de dégazage seuls, et (2) des expériences combinant dégazage et précipitation pour mieux appréhender les processus cinétiques lors de la précipitation des travertins. En parallèle, un travail de terrain spécifique par pose de plaques de plexiglas le long de différents écoulement de la source, nous a permis de connaitre la résolution de précipitation des travertins et d’obtenir une meilleure estimation des facteurs de fractionnement.Les tests en laboratoire confirment que, lors d’un dégazage important, i.e. conduisant à la précipitation de travertins, la spéciation des espèces du carbone inorganique dissout (CID) et les flux entre ces espèces sont des paramètres primordiaux à considérer. Le facteur de fractionnementεDIC-CO2(g) qui dépend du pH suit alors une distillation de Rayleigh dans un système cinétique. Les évolutions des espèces carbonées en solution ont ainsi pu être modélisées à l’aide d’une équation de diffusion faisant intervenir les paramètres propres à l’expérience.Les résultats de terrain permettent de démontrer que les variations de conditions de précipitation sont principalement dues aux variations de conditions hydrodynamiques et, secondairement, aux variations de température, ce qui a pu être modélisé à l’aide de l’équation définie par Plummer en 1968 et qui a montré la variabilité des conditions de précipitation in-situ. Dans le cas des expériences de précipitation en laboratoire et par comparaison avec les données de terrain, le facteur de fractionnement εCID-calcite va dépendre des concentrations de chaque espèce de CID mais aussi de la partition du carbone entre phase gaz et phase solide. Pour l’oxygène 18, de 0°C à 10°C, les résultats acquis sur les travertins actuels nous ont permis d’utiliser la droite définie récemment par Coplen (2007) pour lier de façon fiable la température au moment de la précipitation et le facteur de fractionnement. Toutefois, pour des températures plus élevées, le facteur de fractionnement est plus important que ce qu’il devrait et confirmant ainsi la non-représentativité des droites de Coplen pour la détermination de la température de précipitation. Lors des expériences de précipitation la tendance est inverse à celle observée pour les travertins confirmant l’influence de la vitesse de dégazage et de l’indice de saturation sur le fractionnement isotopique. Nous avons également confirmé avec les expérimentations, l’impact de la vitesse de précipitation sur les teneurs isotopiques enregistrées dans les travertins.Ce travail a mis en lumière des mécanismes cinétiques complexes et qui ne sont pas complètement contraints par la seule utilisation des outils isotopiques. L’utilisation du partitionnement des éléments traces est un outil primordial pour appuyer l’étude de ces mécanismes.

Karst aquifers are among the most difficult to characterize. They are known for their complex hydrological responses to rainfall events due to the high heterogeneity of their porosity, going from matrix conduit porosity. The porosity of conduit induces a high transfer velocity in the aquifer which is the cause of the karst aquifer vulnerability. The objective of this study is to define the modes of water transfer and solute in a karst aquifer of Dogger in Burgundy. Two complementary methods are used: (i) the analyse of piezometric and debitmetric time series, and (ii) the geochemical and isotopic characterization of groundwater based on the using of large panel of tracers, notably some dating tracers. These two methods are applied at different temporal scales: a high frequency monitoring (1 hour time step) during discharge event to study fast water transfer and a long time monitoring to characterize slow water transfer and study the seasonal variability of water transfer processes. This study highlights a large distribution of residence time of groundwater, from several hours during a recharge event, underlined by short response time rainfall-piezometric level (less to 70 hours) to several years for the baseflow showed by the using of dating tracers (39Kr, CFC-12 et SF6). In response to rainfall events, several water bodies are described. The first one, with slow transfer velocity, corresponds to the baseflow and is showed by dating tracers. The second one, with a direct recharge and a residence time in the karst aquifer that varies with the porosity from several hours to several days, is described by a continous monitoring of electrical conductivity, majors elements and tritium. Finally, the water stored in the unsaturated zone, remobilized during recharge events, is revealed by the variation of geochemical composition in tritium and Ca2+ in groundwater during recharge events. Despite of the high heterogeneity of water transfer processes, the fast water transfer is spatially organized. The response time of piezometric level to rainfall events and the residence time increase toward the hydrogeological downhill. The temporal study of the impulse response using the method of the sliding-window cross-correlation shows that the response time varies seasonally, being shorter during the summer. The variability of the response time is significant, up to a hundred hours. This temporal variability could be explained in part by a variability of the rainfall intensity which induces a variability of water flow processes in the epikarst. During intensive rainfall, the saturation of the epikarst is higher which induces lateral transfers and allows transferring water toward large conduits.; Les aquifères karstiques sont parmi les milieux souterrains les plus difficiles à caractériser. Ils sont connus pour avoir des réponses complexes aux événements pluvieux en raison de l’hétérogénéité de la porosité allant de la porosité matricielle (micro-porosité) à la porosité de conduits (macro-porosité). La porosité de conduits génère des vitesses de transfert élevées au sein de l’aquifère, à l'origine de la vulnérabilité des aquifères karstiques. L'objectif de cette thèse est de définir les modalités du transfert de l'eau et des solutés au sein d'un aquifère karstique du Dogger situé en Bourgogne. Deux approches complémentaires ont été utilisées: (i) l'analyse de chroniques piézométriques et débitmétriques à partir d'outils de traitement du signal et (ii) une caractérisation géochimique et isotopique des eaux souterraines basée sur l'utilisation d'une large palette de traceurs, notamment des traceurs de datation. Ces deux approches ont été appliquées à différentes échelles temporelles: un suivi haute fréquence (jusqu'à 1h) lors de pics de crue pour étudier les transferts rapides et un suivi à long terme (pluri-annuel) pour caractériser les transferts lents et étudier la variabilité saisonnière des modalités d'écoulement. Ce travail a permis de mettre en évidence une très large distribution des temps de résidence de l’eau souterraine, de quelques heures, lors d’événements de crue souligné par des temps de réponse pluie-niveau piézométrique faible (inférieurs à 70 h) à quelques années pour l’écoulement de base mis en évidence par l’utilisation de traceurs de datation (39Kr, CFC-12 et SF6). En réponse aux précipitations, différentes masses d'eau ont pu être décrites: (i) une eau à vitesse d’écoulement lente, qui correspond à l’écoulement de base mis en évidence par l'utilisation des traceurs de datation; (ii) une eau de recharge rapide dont le temps de résidence au sein de l'aquifère varie, selon les porosités empruntées, de quelques heures à quelques jours soulignés grâce au suivi en continu de la conductivité électrique, des éléments majeurs et du tritium; et (iii) des eaux stockées au sein de la zone non saturée, remobilisées lors de pics de crue mises en évidence par la variation des compositions géochimiques en tritium et Ca2+ des eaux souterraines lors de pics de crue. Malgré une forte hétérogénéité des processus d'écoulement, une logique spatiale des écoulements rapides apparaît. Le temps de réponse du niveau piézométrique à un événement pluvieux et le temps de résidence augmentent vers l’aval du dôme hydrogéologique. L'étude temporelle de la réponse impulsionnelle par la méthode du corrélogramme croisé coulissant montre que le temps de réponse varie de façon saisonnière, et est plus court en été. La variation du temps de réponse pour un forage entre été et hiver est significative et peut atteindre une centaine d’heures. Cette variabilité temporelle s'explique en partie par une variabilité de l'intensité des pluies (plus importantes en été) qui induit une variation des mécanismes d'écoulement au sein de l'épikarst. Lors d'événements de pluie intense, la saturation de l'épikarst est plus importante provocant des transferts latéraux en son sein et permettant de transférer les eaux vers les conduits les plus ouverts.