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Assessment of the karst network geometry based on field data is an important challenge in the accurate modeling of karst aquifers. In this study, we propose an integrated approach for the identification of effective three-dimensional (3D) discrete karst conduit networks conditioned on tracer tests and geophysical data. The procedure is threefold: i) tracer breakthrough curves (BTCs) are processed via a regularized inversion procedure to determine the minimum number of distinct tracer flow paths between injection and monitoring points, ii) available surface-based geophysical data and borehole-logging measurements are aggregated into a 3D proxy model of aquifer hydraulic properties, and iii) single or multiple tracer flow paths are identified through the application of an alternative shortest path (SP) algorithm to the 3D proxy model. The capability of the proposed approach to adequately capture the geometrical structure of actual karst conduit systems mainly depends on the sensitivity of geophysical signals to karst features, whereas the relative completeness of the identified conduit network depends on the number and spatial configuration of tracer tests. The applicability of the proposed approach is illustrated through a case study at the Hydrogeological Experimental Site (HES) in Poitiers, France.

International audience; Understanding subsurface flow, especially in fractured rocks only housing water through a few preferential pathways, is still challenging. The point is mainly associated with the poor accessibility of the subsurface and the lack of accurate representations for both heterogeneity and spatial distribution of water bearing bodies. This notwithstanding, highly-resolved geophysical investigations bring new images of the subsurface. This is exemplified over a fractured limestone aquifer at the site scale (for example, that of the radius of influence of an extraction well). On an experimental site, situated in the Cher region (France), two boreholes have been drilled for field experiments. Full Waveform Acoustic Logging (FWAL) and seismic experiments were conducted. Hybrid seismic imaging, which consists in combining refraction and reflection seismic results, has been carried out. Based on a four-step procedure, the processing of refracted and reflected waves provided two sections. After assemblage, these sections produced in a first step an extended time reflectivity section starting from the surface and, in a second step, a section over depth after calibration with Vertical Seismic Profile (VSP) and acoustic data. However, even the Very High Resolution (VHR) seismic methods do not have a sufficient vertical resolution to describe accurately the geological formation. The acoustic sections were processed to separate the different wave fields, to extract the criss-cross events and to build a criss-cross index log. A log of fracturation index, based on both criss-cross index and P-wave velocity measurements, was computed to detect the presence of fractures. After calibration, and under the assumption that the slower the P-wave velocity, the higher the permeability – porosity, a 3D seismic block of reflection can inform on preferential areas where flow should occur. At the scale of an open wellbore, acoustic loggings that measure wave velocities over a short distance within the well also inform on open features crosscut by the well. Finally, flow log measurements confirm the occurrence of flowing horizons that were previously marked by both seismic and acoustic data. Seismic and acoustic data are therefore suited to image contrasted hydraulic properties over fractured subsurface systems usually poorly documented.

International audience; Groundwater flow depends on the heterogeneity of hydraulic properties whose field characterization is challenging. Recently developed active‐Distributed Temperature Sensing (DTS) experiments offer the possibility to directly measure groundwater fluxes resulting from heterogeneous flow fields. Here, based on fundamental principles and numerical simulations, two interpretation methods of active‐DTS experiments are proposed to estimate both the porous media thermal conductivities and the groundwater fluxes in sediments. These methods rely on the interpretation of the temperature increase measured along a single heated fiber optic (FO) cable and consider heat transfer processes occurring both through the FO cable itself and through the porous media. The first method relies on the Moving Instantaneous Line Source (MILS) model that reproduces the temperature increase and provides estimates of thermal conductivity and groundwater flux as well as an evaluation of the temperature rise due to the FO cable. The second method, based on the graphical identification of three characteristic times, provides complementary estimates of flux, fully independent of the effect of the FO cable. Sandbox experiments provide an experimental validation of the interpretation methods, demonstrate the excellent accuracy of groundwater flux estimates (< 5%) and highlight the complementarity of both methods. Active‐DTS experiments allow investigating groundwater fluxes over a large range spanning 1x10‐6 to 5x10‐2 m/s, depending on the duration of the experiment. Considering the applicability of active‐DTS experiments in different contexts, we propose a general experimental framework for the application of both interpretation methods in the field, making active‐DTS field experiments especially promising for many subsurface applications.

Petrographic and mineralogical analyses combined with sequential extractions and leaching experiments as a function of pH were performed on black clayey sediments fulfilling karsts in the Hydrogeological Experimental Site (HES) of Poitiers (France) to investigate the behavior of arsenic and selenium in a fractured limestone aquifer. Sequential extractions showed that arsenic is mainly associated with pyrite (about 35%) and secondary iron oxyhydroxides (around 13%), along with a substantial exchangeable fraction (about 13%). The soluble fraction and the fraction associated to organic matter are ∼2% and ∼5%, respectively. The distribution of selenium is mainly pyritic (around 39%) or associated with organic matter (about 18%). Its association to secondary iron oxyhydroxides minerals is low (around 2%), whereas its soluble fraction is around 5%. SEM analyses revealed the presence of arsenic “hot spots” into euhedral pyrite crystals surrounded by a halo of iron oxyhydroxides resulting from their alteration, and both are enriched with arsenic. Selenium has a similar pyritic origin but after alteration, it is predominantly associated with organic matter. Despite different distributions, the leaching experiment as a function of pH showed that the mobilization of arsenic and selenium overlapped below pH 2 and above pH 8. The main differences were observed between pH 2 and 8 with a plateau at 5% of released selenium, whereas the amount of mobilized arsenic continuously decreased. The pH-dependence of both elements is attributed to the partial dissolution of pyrite in acidic conditions combined with desorption processes at higher pH values.

A 3D seismic survey was done on a near surface karstic reservoir located at the hydrogeological experimental site (HES) of the University of Poitiers (France). The processing of the 3D data led to obtaining a 3D velocity block in depth. The velocity block was converted in pseudo porosity. The resulting 3D seismic pseudo-porosity block reveals three high-porosity, presumably-water-productive layers, at depths of 30&ndash, 40, 85&ndash, 87 and 110&ndash, 115 m. This paper shows how full wave acoustic logging (FWAL) can be used to validate the results obtained from the 3D seismic survey if the karstic body has a lateral extension over several seismic. If karstic bodies have a small extension, FWAL in open hole can be fruitfully used to: detect highly permeable bodies, thanks to measurements of acoustic energy and attenuation, detect the presence of karstic bodies characterized by a very strong attenuation of the different wave trains and a loss of continuity of acoustic sections, confirm the results obtained by vertical seismic profile (VSP) data. The field example also shows that acoustic attenuation of the total wavefield as well as conversion of downward-going P-wave in Stoneley waves observed on VSP data are strongly correlated with the presence of flow.

In hydrogeology, understanding the behavior of aquifers is an important issue. May it be for forecasting the availability of water on a long term or for estimating the impact of pollutants when injected at difierent regions, modeling of an aquifer is very useful for hydrogeologists. This paper gives an overlook of the difierent types of tests on a particularsite, namely the Hydrogeological Experimental Site (HES) of Poitiers (France). The aim is to show that the HES is a huge instrumented test bed allowing difierent types of experiments. Thus, system identification users can expect to gather large series of input-output datasets so as to experiment their own tools. Note that a large database exists concerning already-run tests, but what is interesting is that new experiments can be programmed so as to account for customized datasets. As a counterpart, the use of HES data should promote proper modeling of the underlying aquifer, thus contributing to the hydrological research field.

The selenium (Se) content and the associated release mechanisms in both surface and groundwater have become a major concern worldwide over the past 30 years. Within the Hydrogeological Experimental Site of Poitiers (HESP), a large range of aqueous Se concentrations (from non-detectable to more than 30 ppb) is observed in a limited area (about 10 ha), where water flows are highly characterized. This site thus consists of an interesting spot to better understand the release mechanisms of selenium into groundwater. The present study consists of an identification and a characterization of the lithological sources of Se within the HESP. Total rock analyses applied to core samples from different depths and wells demonstrated that selenium is concentrated in argillaceous sediments enriched with organic matter, pyrite and uranium that fulfill a part of the karst cavities developed within the Bajocian host rocks. Mineralogical and petrographic investigations highlighted the heterogeneity of these filling materials and showed the presence of successive deposits under different climatic conditions. Extensive characterizations dated the selected Se-rich samples from the Upper Cretaceous and converged to demonstrate the external and continental origin of the studied filling materials and their transformation after deposition under reduced conditions. Only indirect correlations allow considering an agreement between the Se history and the very mature organic matter identified in the argillaceous samples. This association will be further favored to determine the mechanisms releasing selenium into groundwater.

International audience; This work refers to the characterization of the hydrochemistry of the southern part of the Eastern Desert in Egypt, on the basis of physico-chemical properties of groundwater occurring in the fractured Precambrian rocks inland and in sedimentary formations on the coastline of the Red Sea. Thirty-five groundwater samples have been collected from the study area for hydrochemical investigations to understand the sources of dissolved ions and assess the chemical quality of the groundwater. Several methods were used to interpret the hydrochemical data, i.e. graphical methods, principal components analysis, ions exchanges indices and saturation indices of various minerals. The results show that the major ionic relationships are Na+ > Ca2+ > Mg2+ and Cl- > > HCO3- and that groundwater chemical characteristics are controlled by natural geochemical processes but also, to a lesser extent, by anthropogenic activities. Natural minerals dissolution, ion exchanges and evaporation play a prominent role in the ion enrichment of groundwater. A comparison of groundwater quality in relation to WHO water quality standards proved that most of the water samples are not totally suitable for drinking water purpose.

International audience; Different surface seismic surveys have been recorded on an experimental hydrogeological site that has been developed for several years near Poitiers (France). The paper shows how 3D seismic imaging can be used to describe the near-surface heterogeneous aquifer. The acquisition spread is designed to perform both 3D refraction and reflection seismic surveying. Refraction survey enables us to obtain a 3D image in depth of a low velocity superficial zone contrasting with the underlying water – bearing carbonates. Refraction survey shows the main orientations (N90 and N50) of fracture corridors. These two directions have been selected as the drilling azimuths of two deviated wells C3 and C4. Reflection survey enables us to generate a 3D seismic pseudo velocity block in depth. The 3D seismic pseudo velocity block shows the large heterogeneity of theaquifer reservoir in the horizontal and vertical planes, and confirms the main structural orientations (N90 and N50) identified by the refraction survey. The low velocity areas correspond to high hydraulic conductivity. In order to quantify the porosity of the different productive layers of the aquifer, the interval seismic velocities have been converted in resistivity. For that purpose, the empirical relationship between seismic velocity and true formation resistivity proposed by Faust (1953) [Geophysics 18, 271-288] has been used. The 3D resistivity block is converted in porosity, by using the Archie law (1942) [Petrol. Technol. 146, 54-62]. The 3D seismic pseudo porosity block allows us to identify three different water productive layers: an upper layer at 35-40 m depth, an intermediate layer at 85-87 m depth and a lower layer at 110-115 m. The intermediate layer is composed of bodies having a porosity larger than 30%. These bodies represent the most karstic part of the reservoir. The very high resolution seismic surveying has led to obtain a 3D porosity seismic block which represents a deterministic high resolution reservoir model. After further calibration, that reservoir model could be used for flow simulation.

International audience; The pipe network generation proposed here assumes that flow in a 3D fracture field is highly channelled, and that there exist invariant crux nodes corresponding to crossroads connecting preferential flow paths whatever the mean head gradient's direction. The method allows preferentially for these crux nodes by conditioning the simulated network first on a ‘seeding' of nodes and second, in generating 1D pipes between these nodes. Note that usually, nodes are the consequence (the intersection) of the pipe generation. Pipes can be also conditioned on their length and other features, but this conditioning is not strict, since all pipes must pass by the nodes. In the end, the network is simple without dead ends; each cluster is flowing and connected to the boundaries of the system. This type of network is expected to facilitate further calculations, for instance solving inverse problems in deforming the network by simply moving the crux node locations.

The Time Domain Random Walk (TDRW) method has been recently developed by Delay and Bodin [Delay, F. and Bodin, J., 2001. Time domain random walk method to simulate transport by advection-dispersion and matrix diffusion in fracture networks. Geophys. Res. Lett., 28(21): 4051-4054.] and Bodin et al. [Bodin, J., Porel, G. and Delay, F., 2003c. Simulation of solute transport in discrete fracture networks using the time domain random walk method. Earth Planet. Sci. Lett., 6566: 1-8.] for simulating solute transport in discrete fracture networks. It is assumed that the fracture network can reasonably be represented by a network of interconnected one-dimensional pipes (i.e. flow channels). Processes accounted for are: (1) advection and hydrodynamic dispersion in the channels, (2) matrix diffusion, (3) diffusion into stagnant zones within the fracture planes, (4) sorption reactions onto the fracture walls and in the matrix, (5) linear decay, and (6) mass sharing at fracture intersections. The TDRW method is handy and very efficient in terms of computation costs since it allows for the one-step calculation of the particle residence time in each bond of the network. This method has been programmed in C++, and efforts have been made to develop an efficient and user-friendly software, called SOLFRAC. This program is freely downloadable at the URL (labo.univ-poitiers.fr/hydrasa/intranet/telechargement.htm). It calculates solute transport into 2D pipe networks, while considering different types of injections and different concepts of local dispersion within each flow channel. Post-simulation analyses are also available, such as the mean velocity or the macroscopic dispersion at the scale of the entire network. The program may be used to evaluate how a given transport mechanism influences the macroscopic transport behaviour of fracture networks. It may also be used, as is the case, e.g., with analytical solutions, to interpret laboratory or field tracer test experiments performed in single fractures.

A Lagrangian method in time domain is proposed to solve transport by diffusion over large grids such as those extracted from autoradiographs of rock samples. The method is rapid and well suited to simulate out-diffusion experiments in a heterogeneous rock matrix. By coupling with a Gauss–Newton optimiser, it is possible to identify automatically the diffusion coefficients of porous areas in the matrix. This offers a more comprehensive view of a potential transport of contaminants, e.g., after a leak from an underground repository site. To cite this article: F. Delay et al., C. R. Geoscience 334 (2002) 967–973.

The aim of this work was to experimentally evaluate the role of ionic strength and pore velocity on clay suspension transport and retention through a saturated porous medium. A smectite suspension was injected into columns filled with a very fine quartz sand. Experiments were carried out at constant pore velocity with increasing ionic strength adjusted with a divalent electrolyte (CaCl 2 ) and at constant ionic strength (using three chemical conditions) with decreasing pore velocity. Typical colloid breakthrough curves show two important behaviors: a constant outlet concentration value after a transient phase, and a pronounced tailing effect at the end of the injection step. No differences were observed between the mean travel time of a solute tracer and that of the clay suspension. The classical advection–dispersion equation coupled with a first-order two-site kinetics model was used to reproduce the experimental breakthrough curves. The kinetic model consisted in a site with irreversible deposition and a reversible site used to reproduce the transient phase preceding the plateau of the experimental breakthrough curves. The particle fraction kept by the porous medium increases with ionic strength; consequently the kinetic parameters of the numerical model vary with chemistry. The irreversible sorption rate ( K irr : equivalent to a clean-bed filter coefficient) increases with ionic strength and was directly determined from experimental data. With increasing ionic strength, the deposition rate ( K d ) for the reversible sorption increases whereas the release rate ( K r ) decreases. The kinetic parameters of the reversible site show an evolution with pore velocity similar to that observed in kinetics model used for modeling solute transport in double porosity media. With decreasing pore velocities, the retention of clay particles increases but the kinetic deposition coefficient of the irreversible site decreases. Particle deposition can also be described and reinterpreted in terms of collector efficiency using the concept of the sphere-in-cell model. The collector efficiency, which adds a correction to the kinetic parameter with the residence time, is a more consistent way to represent particle retention. Its value increases with increasing ionic strength and decreasing pore velocity.

The aim of this paper is to show that the hydrodispersive transport parameters of a solute in a heterogeneous medium can be predicted using parameters determined by several experiments with the different homogeneous media that constitute together the heterogeneous one. Elementary rules for predicting these parameters are defined. Three types of experiments were carried out. (1) Homogeneous medium transport experiments in 1-D columns. They were fitted with the help of a new numerical particle tracking method, including advection-dispersion and exchange between mobile and immobile water with first-order kinetics. (2) Layered heterogeneous media. The experiments were simulated by the numerical model without fitting, using the parameters of each of the layers of the homogeneous media included in the heterogeneous one. The results prove that, in such layered media, the global transport problem can be considered as the convolution of elementary transports in homogeneous blocks. We will therefore conclude that the reservoir simulation methods which represent the heterogeneous reservoir as a random set of elementary homogeneous blocks can be used to represent transport if each elementary block is given its appropriate hydrodispersive parameters. (3) Homogeneous mixtures of two pure components. The experiments produced three interesting results. The immobile water volume is the direct linear combination of its equivalent in the two components, weighted by the volumetric fraction of the component in the mixture. The dispersivity of the medium also evolves linearly, but is affected by an initial sharp rise due to a strong increase in local flow path heterogeneities whenever a “foreign body” is introduced into a pure component. The exchange rate between mobile and immobile water is also perturbed by mixing. In a binary mixture of one component with dual porosity and another one without, the exchange rate decreases with the increase of the volumetric fraction of the medium without dual porosity. It seems that, even if it does not modify the volumes of immobile and mobile water, it reduces the surface/volume ratio of the solid medium and consequently the exchange rate. Mixing rules could be adopted in order to modify the values of the hydrodispersive parameters that need to be introduced for the elementary blocks of heterogeneous reservoirs.

International audience; We present a modeling exercise of solute transport and biodegradation in a coarse porous medium widely colonized by a biofilm phase. Tracer tests in large laboratory columns using both conservative (fluorescein) and biodegradable (nitrate) solutes are simulated by means of a dual flowing continuum approach. The latter clearly distinguishes concentrations in a flowing porous phase from concentrations conveyed in the biofilm. With this conceptual setting, it becomes possible to simulate the sharp front of concentrations at early times and the flat tail of low concentrations at late times observed on the experimental breakthrough curves. Thanks to the separation of flow in two phases at different velocities, dispersion coefficients in both flowing phases keep reasonable values with some physical meaning. This is not the case with simpler models based on a single continuum (eventually concealing dead-ends), for which inferred dispersivity may reach the unphysical value of twice the size of the columns. We also show that the behavior of the dual flowing continuum is mainly controlled by the relative fractions of flow passing in each phase and the rate of mass transfer between phases. These parameters also condition the efficiency of nitrate degradation, the degradation rate in a well-seeded medium being a weakly sensitive parameter. Even though the concept of dual flowing continuum appears promising for simulating transport in complex porous media, its inversion onto experimental data really benefits from attempts with simpler models providing a rough pre-evaluation of parameters such as porosity and mean fluid velocity in the system

International audience; Different surface seismic surveys have been recorded on an experimental hydrogeological site that has been developed for several years near Poitiers (France). The paper shows how 3D seismic imaging can be used to describe the near-surface heterogeneous aquifer. The acquisition spread is designed to perform both 3D refraction and reflection seismic surveying. Refraction survey enables us to obtain a 3D image in depth of a low velocity superficial zone contrasting with the underlying water – bearing carbonates. Variogram analysis and geostatistical filtering allow to filter random and structured acquisition noise. Factorial kriging is used to filter the small scale structures (cubic structure with a range of 55 m and nugget) in order to make the large scale structures appear and to determine their orientation: a main orientation N90 and a secondary orientation N50. These two directions have been selected to implement two deviated wells C3 and C4. Reflection survey enables us to get a 3D seismic pseudo velocity block in depth. The vertical resolution is enhanced thanks todeconvolution after depth conversion. A Wiener filter, defined at a reference well C1 has beenapplied to the seismic traces to convert into velocity the amplitude sections. The results obtained are validated at four wells (MP6, MP5, M8, M9) in which acoustic data have been recorded. The 3Dseismic pseudo velocity block shows the large heterogeneity of the aquifer reservoir in the horizontaland vertical planes, confirms the main structural orientations (N90 and N50) pointed out byrefraction survey. At a given depth, the velocity distribution shows preferential connections betweenwells. As an example, well pumping tests and pressure interference confirm the hydrodynamicconnection between wells M13 and M21 defined by a low velocity zone at 88 m depth.

Matrix diffusion in saturated rocks with very low permeability is one of the major mechanisms of solute transport. Laboratory out-diffusion experiments on rock samples may provide an estimate of the bulk diffusion coefficient. However, numerous results have shown that this average parameter does not really depict the complex mechanism of diffusion as a function of the internal heterogeneity of crystalline rocks. Two-dimensional images of the porosity distribution in a granite sample were obtained by impregnation with a radioactive resin and autoradiography. Some examples based on these images and synthetic images were used to perform numerical simulations of out-diffusion using two different random walk methods. The simulated shapes of the out-diffusion curves depend on the spatial distribution of the porosity and on the pore connectivity with the border of the sample. Such relations might explain the multiple nested slopes or the convex shapes often observed on real experimental curves.

[1] A Lagrangian method has been developed to solve heterogeneous diffusion. The time domain on which the method rests allows for the coupling with a Gauss-Newton inversion in which the sensitivity to parameters is derived analytically. This tool has proved very rapid and efficient, even for large computation grids as those extracted from autoradiographs of rock samples. It offers a more comprehensive view of transport by diffusion for many applications, one of them being for instance, the safety of underground repository sites.

Hydrodynamic, hydrochemical and biological investigations were held over the Hydrogeological Experimental Site (HES) in Poitiers for the purpose of pre-identifying how a denitrification pilot plant may work in a karstified aquifer. The characterization of flow settings and geometry by water temperature loggings shows for both ambient and forced flow conditions that there exist some preferential connections between karstified layers. Even with complex flow geometry, the connectivity between wells is high; to the point mass tracer restitutions between wells may reach 90%. The existence of denizen heterotrophic denitrifying bacteria is proved. Without external stimulations, the bacterial consortium is however inactive. Thus, contrasts in nitrate concentrations would be explained by various dilution rates between waters of different origins. Stimulation of denitrifying bacteria was performed in an ex-situ lab-experiment. Transport experiments in columns allow proposing different homogenized models for both transport and denitrification by a biofilm phase. The development of biofilm in a porous medium generates local heterogeneity with multiple porosities, thus yielding contrasted fluid velocities. A double continuum (rapid versus slow) seems to be a relevant assumption for interpreting column tracer tests and could be tractable for applications at the site scale. Grounded on data from ex-situ experiments, the calibrated first-order kinetics mimicking the denitrification process is independent from experimental hydrodynamic settings. To validate all these results, it urges now going farther and to proceed with a transport experiment between a doublet of wells of the HES., Pour pré-identifier le fonctionnement d'un pilote de dénitrification au sein d'un aquifère karstifié, des investigations hydrodynamiques, hydrochimiques et biologiques ont été menées sur le Site Expérimental Hydrogéologique (SEH) de Poitiers. La caractérisation de la géométrie des écoulements par mesures de température conclut, en ambiant et en pompage, à l'existence de connexions préférentielles entre les plans karstiques. Malgré ces écoulements complexes, la connectivité entre les puits est élevée avec une restitution de soluté pouvant atteindre 90 %. La présence de bactéries dénitrifiantes hétérotrophes autochtones est vérifiée. Sans stimulation exogène, ces bactéries sont inactives au droit du SEH. Les contrastes de concentrations en nitrates s'expliqueraient alors par des processus de dilution entre masses d'eaux, l'écoulement divergent régional et les activités anthropiques dans la zone de recharge de la nappe. La stimulation des bactéries dénitrifiantes sur un pilote ex situ implanté sur le SEH permet de proposer des modèles homogénéisés de transport intégrant les effets de dénitrification en présence d'un biofilm. L'implantation du biofilm dans le milieu génère un système hétérogène à porosités multiples caractérisé par une forte variabilité des vitesses d'écoulement. Le modèle double continuum écoulement rapide vs. écoulement lent testé pour expliquer cette hétérogénéité semble le plus à même d'être transposé à l'hydrodynamique du SEH. Sur le pilote ex situ, la cinétique d'ordre 1 mimant la dénitrification est indépendante des conditions hydrodynamiques. Il importe maintenant de conduire l'expérience à l'échelle d'un doublet de puits pour valider ces résultats.