A study of karst hydrosystem recharge at the parcel scale, using modeling and correlation analysis - Low noise underground laboratory of Rustrel site

La caractérisation des flux d’eaux qui rechargent réellement les hydrosystèmes souterrains reste un frein à la compréhension du fonctionnement hydrogéologique des milieux souterrains. Lors d’événements pluvieux, quelle part de l’eau est évapo-transpirée ? Quelle part est temporairement stockée dans le sol ? Ces incertitudes sont particulièrement fortes dans le cas de la recharge des milieux hétérogènes tel que le karst. En général, les calculs de recharge des hydrosystèmes karstiques se basent sur une représentation simplifiée de l’évapotranspiration qui considère seulement le climat et pas le fonctionnement de la végétation. Dans cette étude, un modèle de végétation permettant de simuler les transferts d’eaux entre le sol et l’atmosphère en contexte forestier (le modèle CASTANEA), a été appliqué à une parcelle de Chêne vert. L’infiltration efficace (un indicateur de la recharge) estimé avec CASTANEA a été comparée à celle estimée par des approches classiques ainsi qu’à des séries long terme de flux d’eaux souterraines (9 années). Les résultats de cette analyse révèlent que l’infiltration efficace modélisée à partir d’un modèle de végétation comme CASTANEA est plus satisfaisante que les approches classiques ne tenant pas compte du fonctionnement de la végétation. Ce travail ouvre des perspectives intéressantes pour mieux tenir compte du fonctionnement de la végétation et de l’usage du sol sur la recharge des hydrosystèmes karstiques.Assessing the recharge of underground hydrosystems remains an obstacle to understand their hydrologeological functioning. During a rain event, which part of the rain is evapotranspired ? And how much is temporarily stored within the soil ? These questions are particularly relevant in heterogeneous media such as karst hydrosystems. Currently, the models used to compute recharge of karst hydrosystems, rely on simplistic formulations of evapotranspiration that do not account for vegetation functioning. In this study, we used the vegetation process based model CASTANEA, which is designed to compute water transfer between soil, plant and atmosphere. We computed effective infiltration (an index of recharge) with CASTANEA and with other classical approach (based on precipitation minus ETP), and for a welldocumented holm oak site in Provence. Our results provide evidences that effective infiltration computed with CASTANEA yield more satisfactory correlation with measured outflow than simulations based on the classical approach. Our results provide a promising way to improve the simulation of karst hydrosystem recharge

Experimintal study of mass transfert in heterogeneous porous media

Étant donné le caractère hétérogène du milieu naturel, l'étude de l'hydrodynamique et du transfert de masse dans une nappe nécessite la détermination de paramètres équivalents : la perméabilité équivalente et la macrodispersivité. Il s'agit ici de détermiWith the heterogeneity of the natural environment, the study of the hydrodynamics and mass transfer in groundwater requires the determination of equivalent parameters: the equivalent permeability and the macrodispersivity. This work deals with the determ

Etude expérimentale du transfert de masse en milieu poreux hétérogène

Etant donné le caractère hétérogène du milieu naturel, l'étude de l'hydrodynamique et du transfert de masse dans une nappe nécessite la détermination de paramètres équivalents : la perméabilité équivalente et la macrodispersivité. Il s'agit ici de déterminer ces paramètres à partir de résultats d'essais de traçage.Un Modèle d'Analyse et de Recherche sur la Contamination des EAUx Souterraines (MARCEAUS) a été développé pour réaliser des expériences contrôlées dans un milieu hétérogène de grandes dimensions (6 x 1 x 1 m3) et ayant les propriétés statistiques voulues. La concentration, via la conductitivité électrique, et la piézométrie sont mesurées dans le domaine. Après la mise au point du dispositif, 7 expériences ont été réalisées en écoulement uniforme ou radial. L'interprétation des courbes de concentration souligne l'influence des hétérogénéités locales sur le transport de traceur : l'établissement d'écoulements préférentiels et tridimensionnels.Les résultats obtenus montrent qu'il est possible de déterminer des paramètres équivalents de l'hydrodynamique et du transport à partir de mesures de concentrations. Un modèle numérique adapté est proposé pour chaque mode d'écoulement (uniforme et radial). Les valeurs expérimentales sont différentes d'une expérience à l'autre et supérieures aux valeurs théoriques. Mais les développements théoriques utilisés ne tiennent pas compte des limites finies du domaine. Les perméabilités sont toutes du même ordre de grandeur et bornées par les valeurs théoriques de Cardwell et Parson. Les valeurs de macrodispersivité sont inférieures dans le cas de l'injection-pompage. La comparaison des valeurs entre-elles montre l'importance de la zone de capture des puits.La simulation tridimensionnelle des expériences s'avère complexe et pose de nouvelles questions : celles de l'anisotropie du milieu, de l'influence de la dispersion locale ou de la densité du traceur.With the heterogeneity of the natural environment, the study of the hydrodynamics and mass transfer in groundwater requires the determination of equivalent parameters: the equivalent permeability and the macrodispersivity. This work deals with the determination of these parameters with tracer tests.A Model, "Modèle d'Analyse et de Recherche sur la Contamination des EAUx Souterraines" (MARCEAUS) was developed to carry out controlled experiments in a large-sized heterogeneous medium (6 x 1 x 1 m3) which has some statistical properties. Concentration, via the electric conductitivity, and piezometry are measured in the field. After the development of the device, 7 experiments were carried out in uniform or radial flow. The interpretation of the breakthrough curves shows the influence of local heterogeneities on the transport of the tracer: establishment of preferential paths and three-dimensional flows.The results obtained show that it is possible to determine equivalent hydrodynamical and transport parameters owing to measurements of concentrations. An adapted numerical model is proposed for each type of flow (uniform and radial). The experimental values are different from one experiment to another and higher than the theoretical values. But the theoretical developments used do not hold account of the boundary conditions. The permeabilities are of the same order of magnitude and are bounded by the theoretical values of Cardwell and Parson. The values of macrodispersivity are lower in the case of injection-pumping. The comparison of the values shows the importance of the capture zone of the wells.The three-dimensional simulation of the experiments are difficult and raises new questions: those of the anisotropy of the medium, the influence of local dispersion or the density of the tracer.STRASBOURG-Sc. et Techniques (674822102) / SudocCACHAN-ENS (940162301) / SudocSudocFranceF

Experimental determination of equivalent parameters with tracer tests

International audienceWith the heterogeneity of the natural environment, the study of the hydrodynamics and mass transfer in groundwater requires the determination ofequivalent parameters: the equivalent permeability and the macrodispersivity. This work deals with the determination of these parameters with tracer tests. A Model, "Modèle d’Analyse et de Recherche sur la Contamination des EAUx Souterraines" (MARCEAUS) was developed to carry out controlled experiments in a largesized heterogeneous medium (6x1x1 m3) which has some statistical properties. Concentration, via the electric conductitivity, and piezometry are measured in the field. After the development of the device, 7 experiments were carried out in uniform or radial flow. The interpretation of the breakthrough curves shows the influence of local heterogeneities on the transport of the tracer: establishment of preferential paths and three-dimensional flows. The results obtained show that it is possible to determine equivalent hydrodynamical and transport parameters owing to measurements of concentrations. An adapted numerical model is proposed for each type offlow (uniform and radial). The experimental values are different from one experiment to another and higher than the theoretical values. But the theoretical developments used do not hold account of the boundary conditions. The permeabilities are of the same order of magnitude and are bounded by the theoretical values of Cardwell and Parson. The values of macrodispersivity are lower in the case of injection-pumping. The comparison of the values shows the importance of the capture zone of the wells

Sequence stratigraphy as an inter-disciplinary tool for karst network forecast. Multiphase and multiprocess karst genesis in the Urgonian Formation

International audienceCommonly, in carbonate reservoirs, karst systems result from long-term, multiphase and multiprocess speleogenesis that can occur from very early after the sedimentation of the limestone to presentday geomorphological events. This leads to the construction of inherently complex karst networks with a piece of architecture difficult to understand and to accurately predict. The dynamic appraisal of these reservoirs gives greater insight into the overall organization of the actual karst systems, while totally ignoring the detailed features that impact the local fluid flow behavior. In order to reduce the uncertainties in predicting the behavior of complex systems, a reconstruction of the development of the karst conduits is needed. It requires that, for each speleogenesis stage, the processes and their controlling parameters are correctly assessed.The issue of defining the prevalent parameters for the karst development is usually one on which the different specialists involved in the study of karst systems disagree. The difference in views is all the more important in the fact that specialists do not necessary address the question according to the karst origin (epigenic, hypogenic, island).For geologists, lithology and geological structure impose a general control on most cave genesis. The karst is considered to be facies-controlled, with some rock fabrics and textures described as more favorable for the development of conduits than others. Faults and fracture zones influence the secondary porosity development, through karst enhanced fractures and highly developed cavern systems. However, lithologic and structural conditions for speleogenesis evolve throughout time and change from one phase to another: progressively, the importance of fabric-selective porosity and stratigraphic elements diminishes in favor of fracture porosity.For hydrologists, hydraulic boundary conditions play as a controlling factor in karst genesis, as conduits develop mainly vertically in vadose zone and horizontally along the groundwater table, following the hydraulic gradient. Moreover, the hydraulic gradient can limit the extension or completely inhibit the development of horizontal conduits even if lateral flow path was structurally preferred. In the Urgonian Formation, a cretaceous series outcropping in south-eastern France, the development of karst conduits has been considered through geological time, and examined from many different perspectives. For several stages of speleogenesis, the relationships between various geological and hydraulic drivers with the formation of karst features have been established. An integration of most drivers has been possible through sequence stratigraphy, which influenced both the development of pore and fracture systems, the extension of seepage areas, the evolution of water table, the organization of drainage networks, and the location of inception surfaces on sequence boundaries. Sequence stratigraphy appears as an impressive tool for proposing an integrated and predictive understanding of caves development. More generally, this study highlights how much karst genesis is a multi-factorial process, which requires an inter-disciplinary approach

Simulating Solute Transport in Porous or Fractured Formations Using Random Walk Particle Tracking: A Review

International audienceSince the first attempts some 20 yr ago in the field of hydrology, random walk (RW) particle tracking as applied to solute transport has experienced profound changes. Concepts and mathematical techniques have improved to the point that numerically difficult problems (e.g., advection-dominated transport in highly heterogeneous media, or reactive transport) are now much easier to address. Random walk has never been widely used for multiphase flow, probably because numerical dispersion is not a major problem for modeling exercises at large scales. However, vadose zone hydrologic studies often point out very strong variations in fluid velocity over relatively short distances. Random walk methods may be well suited for such studies, a possibility which motivated us to write this review. We first give a comprehensive discussion of the theoretical context of the method. The Fokker-Planck-Kolmogorov equation (FPKE) is established for solute transport, as well as the ordinary Langevin equation and its simplifications for transport of small particles (e.g., colloids). Next, numerical methods are developed for the motion of particles in space. An important section is subsequently dedicated to recent RW concepts in the time domain, and to their application to anomalous (non-Fickian) transport and inverse problems. Adaptations of RW to transport with solute-solid reactions are also provided, as well as several numerical recipes for resolving a few computational difficulties with the RW method. We purposely did not include any comparisons with Eulerian and Lagrangian approaches. These approaches are discussed at length in several references cited in this review. We note, however, that today's computing capabilities provide new incentives to using RW methods for problems where Eulerian methods are potentially unstable or hampered by numerical diffusion

Characterization of flows in the vadose zone by direct measurements in karst aquifer

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Dynamic outcrop analogues in groundwater reservoirs. A disruptive approach for reservoir characterization and modelling

International audienceThe usual workflow for delivering production forecasts for oil-bearing subsurface reservoirs consists in the building of very detailed geological and petrophysical models on which fluids flows are simulated. Populating models with heterogeneous distributions of reservoir properties is a very challenging issue because of the lack of data in typical datasets, that comprise boreholes with sparse lateral sampling, and seismic data with low vertical resolution.Against this background of limited data, reservoir analogues have long been used to improve understanding of subsurface fields. From outcrops, geological data have been collected. Their interpretation has been used to build conceptual static models, and to run sensitivity studies of dynamic response to geological and petrophysical heterogeneity. Although quite advanced, this linear workflow does not achieve the objective of reservoir characterization as there is not any hydraulic information for calibration of the outcrop interpretation.In the same time, hydrogeologists have developed observatories, dedicated to the observation and monitoring of groundwater reservoirs, for the exploitation of water resources and the prevention of environmental risks. These experimental sites provide data relevant to the understanding of the water cycle and the behavior of the aquifers. However, despite the acquisition of geological, geophysical and petrophysical data in boreholes, the description of geology is usually lacking in detail, thereby compromising the quality of the coupling between measurements, theories, and models.A new concept of dynamic outcrop analogue arose from a double need: on one side, the integration of hydrodynamics and geophysics in outcrops studies, on the other side the use of detailed geology in observatories studies. The dynamic outcrop analogue allows the characterization of the reservoir at different scales and from an integrated point of view. The analogue is assessed through multiphysics experiments on various supports : outcrops, but also plugs, cores, boreholes and between wells, caves, tunnels… Coupling the hydrodynamic response of groundwater reservoir to a detailed static description of its heterogeneity very early before the building of models provides a key knowledge on the understanding of reservoir architecture. It contributes to improve the way models are populated and to enhance the forecast of reservoir.This new methodology has been developed and tested through a pilot project, ALBION. The demonstration project is dedicated to the Urgonian carbonates (a barremo-aptian series outcropping in South-East France) and proposes a multi-scales set of analogues in the Fontaine de Vaucluse catchment area. On other sites, further projects are dedicated to various topics: multi-scales permeability in tidal sandstones, anisotropy of permeability in anastomosed fluviatile channels, heterogeneity in unconsolidated alluvial coarse gravels, fractures and karstic networks, and permeability in a fault zone

Influence des conditions aux limites sur le transport en présence d’un dipôle en milieu poreux hétérogène

International audienceFlow and transport of a tracer with a dipole in a heterogeneous porous media bounded by two parallel no-flow limits is investigated. Using a first order approach, and under an ergodicity implicit assumption, a semianalytical solution is achieved to determine mean concentration in the pumping well. We then study the influence of the location of the limits on this solution and especially on breakthrough time.On s’intéresse à l’écoulement et au transport d’un traceur en présence d’un dipôle en milieu poreux hétérogène limité par deux frontières imperméables parallèles équidistantes des puits. Par une approche du premier ordre et sous l’hypothèse implicite d’ergodicité du milieu, une solution semi-analytique est obtenue pour déterminer la concentration moyenne dans le puits de pompage. On étudie alors l’influence de la position des limites sur cette solution et notamment sur le temps de percée