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International audience; The monitoring of temporal variabilities of groundwater flows is a critical point in many hydrogeological contexts, especially for the characterization of coastal aquifers, sub-surface heterogeneities or else groundwater/stream interactions. Considering the lack of available methods, we investigate the possibility of monitoring and quantifying groundwater fluxes variations over time through active-Distributed Temperature Sensing (DTS) measurements. Active-DTS, consisting in heating a fiber optic cable, performs very well for investigating the spatial distribution of groundwater fluxes but the method has never been tested to continuously monitor groundwater fluxes changes. In this context, both numerical simulations and sandbox experiments were performed in order to assess the sensitivity of temperature elevation to variable flow conditions. Results first demonstrate that when a flow change is followed by a long-enough steady-state flow stage the temperature elevation stabilizes independently of previous fluxes conditions. Thus, the stabilization temperature can easily be interpreted to estimate groundwater fluxes using the analytical model commonly used under steady flow conditions to interpret active-DTS measurements. Under certain flow conditions, depending on the nature of flow variations, the approach also allows the continuous monitoring of fluxes variations over time. If instantaneous flow changes occur, the superposition principle can even be used to reproduce the temperature signal over time. In summary, we demonstrated through these preliminary results the possibility of for monitoring and/or quantifying the temporal dynamic of groundwater fluxes at different temporal scales including diurnal and periodic fluxes variations, which open very interesting perspectives for the quantification of subsurface processes.

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.

Our ability to characterize aquifers, predict contaminant transport and understand biogeochemical reactions occurring in the subsurface directly depends on our ability of characterizing the distribution of groundwater flow. In this context, recently-developed active-Distributed Temperature Sensing (DTS) experiments are particularly promising, offering the possibility to characterize groundwater flows resulting from heterogeneous flow fields. Here, based on theoretical developments and numerical simulations, we propose a general framework for estimating active-DTS measurements, which can be easily applied and takes into account the spatial distribution of the thermal conductivities of sediments. Two independent methods for interpreting 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. In order to validate these interpretation methods with independent experimental data, active-DTS measurements were collected under different flow-conditions during laboratory tests in a sandbox. First, the combination of a numerical model with laboratory experiments allowed improving the understanding of the thermal processes controlling the temperature increase. Then, the two complementary and independent interpretation methods providing an estimate of both the thermal conductivity and the groundwater flux were fully validated and the excellent accuracy of groundwater flux estimates ( Our results suggest that 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. The active-DTS method could thus be potentially applied to a very wide range of flow systems since groundwater fluxes can be investigated over more than three orders of magnitude. In the field, the reliable and direct estimation of the distribution of fluxes could replace the measurement of hydraulic conductivity, whose distribution and variability still remains difficult and time consuming to evaluate.

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.

Active-Distributed Temperature Sensing is a new method that has been recently developed for quantifying groundwater fluxes in the sub-surface along fibre-optic cables with a great spatial resolution. It consists in measuring and modelling the increase of temperature due to a heat source, dissipated through heat conduction and heat advection, depending on groundwater fluxes. Here, we propose to estimate the applicability and limitations of the method using sandbox experiments where flow rate and temperature are well controlled. For doing so, active-DTS experiments have been achieved under different flow rates and experimental conditions. In addition, we compare three different and complementary methods to estimate in practice the spatial resolution of DTS measurements. Active-DTS experiments have been conducted by deploying a fiber optic cable in a large PVC tank (1.6m long; 1.2 m width and 0.3 m height) and filled with 0.4-1.3 mm diameter sand. The height of water in water reservoirs on either side of the sandbox can be adjusted to control the head gradient and the flow rate through the sand. Heating was done by injecting during at least 8 hours for each experiment, a well-controlled electrical current along the steel armouring of the fiber optic cable. The three methods for estimating spatial resolution were applied and compared using FO-DTS measurements obtained on the same fiber-optic cable but with two different DTS units having different spatial resolution. Results show that a large range of groundwater fluxes may be estimated with a very good accuracy. Finally, we compare the advantages and complementarities of the different methods proposed for estimating the spatial resolution of measurements. In particular, the spatial resolution estimated using a temperature step change is both dependent on the effective spatial resolution of the DTS unit but also on heat conduction induced because of the high thermal conductivity of the cable. By showing the applicability of the method for a large range of flow rates and with an excellent spatial resolution, these experiments demonstrate the potentialities of the method for quantifying fluid fluxes in porous media for a large range of 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.

International audience; Le service d'observation H+ (hplus.ore.fr), créé en 2002, est un réseau de sites hydrogéologiques, pour la mesure et la modélisation du transfert et de la réactivité des eaux dans les aquifères hétérogènes. Sa mission première est de maintenir et de coordonner un réseau de sites expérimentaux capables de fournir des données pertinentes pour la compréhension du cycle de l'eau et des éléments transportés dans les aquifères à diverses échelles spatiales et temporelles.

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.

Hydraulic interference pumping tests are widely used for identifying mid-to large-scale aquifer hydraulic parameters. Fractured reservoirs often behave as fractal media for which classical interpretations may fail because of the decrease in permeability and fracture porosity (or, equivalently, transmissivity and storage capacity) with the pumping time and the extent over which the pumping stress has propagated. A solution to hydraulic interference tests in fractal media is developed in dimensional variables, which allows for a very rapid and unique fitting to data. Hydraulic parameters, their scale exponents and the fractal dimension of the medium can be determined for each pressure draw-down curve without any prior information or other calculation. The method is applied to a large series of tests completed on the calcareous fractured aquifer of the experimental site in Poitiers (France). Even though data of pressure draw-downs are various and show a fractal behavior, permeability seems to rapidly homogenize for distances of 100-200 m. On the other hand, the fracture porosity shows a much progressive decrease with distance and homogenization is reached for scales beyond 300-500 m. It is also shown that the classical Cooper-Jacob interpretation of pressure draw-downs may yield artifacts in the identification of permeability and fracture porosity. Orders of magnitude are comparable to that from fractal interpretation but their evolution with respect to time or distance (scaling laws) may strongly differ.

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.

Characterization of fractured aquifers can present several problems. The main complexities that affect the characterization are the heterogeneous properties of the aquifer, the geometry and the physical and hydraulic parameters. Nevertheless their characterization is necessary to optimize their exploitation and management. These characterization issues have been afforded at the Poitiers Hydrogeological Experimental Site (HES, France); this work analyzes the main characteristics of the experimental site. An innovative methodology is the utilization of 3D seismic techniques (refraction and reflection with P waves) in order to characterize the aquifer heterogeneities on the experimental site. These have also allowed the characterization of large areas and more productive the direction of zones and/or more vulnerable zones.

In hydrogeology, estimating aquifer permeability is an important issue. This can be useful in understanding the flow of pollutants from one area of an aquifer to another. For this aquifer analysis sake, the Hydrogeological Experimental Site of Poitiers (France) covering a limestone aquifer is an appropriate instrumented test bed enabling measurement of hydraulic responses of a series of observation wells due to a step-type pumping out excitation at a given well. Given the input-output data, black-box continous-time modeling is quite a straight forward process as shown in this paper. The aim is then to be able to use the identified parameters to classify the different wells according to how sensitive they are to the one having been excited. A correlation between black-box parameters and hydrogeological ones is then established.

Electrical Resistivity Tomography (ERT) surveys were undertaken to investigate the Dogger Limestone fractured-karst aquifer at the Hydrogeological Experimental Site (HES) of Poitiers, France. Three-dimensional resistivity imaging was obtained from full inversion of combined 2D ERT data collected along five parallel 470 m long profiles with a 50 m line spacing. A 3D block measuring 515 x 203 m in size with a maximum depth of 100 m was surveyed. Dogger Limestone occurs at a depth ranging between 30 and 120 m and is overlain by argillaceous limestone. This paper compares the imaging obtained from different array sequences. Calibration of the 3D resistivity block with well logs indicates that: the Wenner-Schlumberger (WS) array shows the tendency to enhance layering, to locate bodies at a shallower depth and to laterally extend them; the Pole-Dipole (PD) array shows larger lateral heterogeneities, more compact and vertically extended bodies and poor data fitting; the hybrid array sequence, obtained by the combination of WS and PD array sequences, despite a poor data fitting, similar to PD, shows a better correlation with respect to well log results. In this setting, the hybrid array sequence shows better imaging, due to the combination of the large vertical resolution of WS, large lateral resolution and penetration depth of PD. It allows passing through the thick, low resistivity shallow layer. Indeed, the results are affected by the occurrence of the shallow, 30 m thick, low resistivity argillaceous limestone that reduced the investigation depth as revealed by synthetic datasets modelling and sensitivity analysis. Modelling also revealed that the occurrence of the argillaceous limestone led to a severe underestimate of the Dogger Limestone resistivity values with respect to well resistivity logs; it also allowed verifying the detectability limits when investigating shallow karst limestone intervals located at depths of up to 50 m.

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.

MAGES is a software developed at INRS-Eau (Canada) for forecasting pollution hazards in groundwater. The transport model uses stationary truncated temporal moment equations instead of the classical time dependent advection-dispersion equation. The aim of this work is to propose a numerical validation of the method by comparison with both analytical solutions in homogeneous medium and a sophisticated lagrangian model of transport in heterogeneous medium. It is shown that the temporal moment equations perform well, while saving on computation. This gives MAGES some abilities in water management problems which compensate for the poor knowledge of the transport parameters through numerous stochastic simulations.

MAGES is software for forecasting pollution hazards of groundwater which is in the process of development at INRS-Eau (Canada). The main distinctive feature of the model is the use of stationary truncated temporal moment equations instead of the classical time dependent advection-dispersion equation to solve the transport of contaminants. The aim of this work is to describe the theory of truncated temporal moment equations and to show how the curves of the concentration versus time can be calculated from temporal moments. The discrete method used to solve the equations and its stability is also discussed.

In the last decade, the characterization of transport in porous media has benefited largely from numerical advances in applied mathematics and from the increasing power of computers. However, the resolution of a transport problem often remains cumbersome, mostly because of the time-dependence of the equations and the numerical stability constraints imposed by their discretization. To avoid these difficulties, another approach is proposed based on the calculation of the temporal moments of a curve of concentration versus time. The transformation into the Laplace domain of the transport equations makes it possible to develop partial derivative equations for the calculation of complete moments or truncated moments between two finite times, and for any point of a bounded domain. The temporal moment equations are stationary equations, independent of time, and with weaker constraints on their stability and diffusion errors compared to the classical advection–dispersion equation, even with simple discrete numerical schemes. Following the complete theoretical development of these equations, they are compared firstly with analytical solutions for simple cases of transport and secondly with a well-performing transport model for advective–dispersive transport in a heterogeneous medium with rate-limited mass transfer between the free water and an immobile phase. Temporal moment equations have a common parametrization with transport equations in terms of their parameters and their spatial distribution on a grid of discretization. Therefore, they can be used to replace the transport equations and thus accelerate the achievement of studies in which a large number of simulations must be carried out, such as the inverse problem conditioned with transport data or for forecasting pollution hazards.

A new method to simulate flow and solute transport in 1-D heterogeneous media is presented. This method integrates the time domain random walk method (TDRW) and the finite fragment method (FFM). The TDRW, similar in concept to the classical random walk method, calculates the arrival time of a particle cloud at a given location and provides the solute breakthrough curve. The FFM, which can be seen as an enhancement of the finite difference scheme, allows a heterogenous media to be divided into homogeneous zones to which the TDRW can be applied. The main advantage of the resulting coupling is that the restrictions can be avoided on the space increments and the time steps which exist with the classical methods of finite differences and random walk. In a homogeneous zone of soil, the breakthrough curve can be calculated directly at a given distance with a reasonable number of particles. A 1-D heterogeneous domain to be simulated is then split into homogeneous zones in which the hydraulic heads and the velocities are calculated by the FFM and at the end of which the temporal solute distributions are calculated by the TDRW. A few hundred particles are generally sufficient to clearly define the breakthrough curve. Comparisons with analytical and numerical solutions and experimental data shows the reliability and advantages of this new method.

A new method to simulate solute transport in 1-D heterogeneous media is presented. This time domain random walk method (TDRW), similar in concept to the classical random walk method, calculates the arrival time of a particle cloud at a given location (directly providing the solute breakthrough curve). The main advantage of the method is that the restrictions on the space increments and the time steps which exist with the finite differences and random walk methods are avoided. In a homogeneous zone, the breakthrough curve (BTC) can be calculated directly at a given distance using a few hundred particles or directly at the boundary of the zone. Comparisons with analytical solutions and with the classical random walk method show the reliability of this method. The velocity and dispersivity calculated from the simulated results agree within two percent with the values used as input in the model. For contrasted heterogeneous media, the random walk can generate high numerical dispersion, while the time domain approach does not.

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.

Transfer function based models are widely used in hydrological modeling, especially for rainfall-runoff simulation. This paper deals with a new model associating three elementary transfer functions in series along four parallel branches. Each transfer function is a parametric probability density function with some physical meaning regarding hydrological transfers. Model inversion is performed using the so-called particle swarm optimization (PSO) technique. The systemic model and the PSO method were tested on the Sevre Niortaise water catchment in France.

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

A new method for solving the transport equation based on the management of a large numbe of particles in a discretized 2-D domain is presented. The method uses numerical variables to represent the number of particles in a given mesh and is more complex than the 1-D problem. The first part of the paper focuses on the specific management of particles in a 2-D problem. The method also would be valid for three dimensions as long as the medium can be modeled similar to a layered system. As the particles are no longer tracked individually, the algorithm is fast and does not depend on the number of particles present. The numerical tests show that the method is nearly numerical dispersion free and permits accurate calculations even for simulations of low-concentration transport. Because each mesh is considered as a closed system between two successive time steps, it is easy to add adsorption phenomenon without any problem of numerical stability. The model is tested under conditions that are extremely demanding for its operating mode and gives a good fit to analytical solutions. The conditions in which it can be used to best advantage are discussed.

International audience; Five modelling approaches were tested against a well-studied limestone aquifer constituting the Hydrogeological Experimental Site (HES) of Poitiers, France. The modelling exercise consisted of predicting the drawdown responses of a series of observation wells for two dipole (pumping-injection) flow experiments involving two distinct well pairs at the HES. The differences between model predictions appear to be mainly related to the hydraulic datasets used for model parameterisation and, to a much lesser extent, to the conceptual modelling approach (equivalent porous medium vs. discrete fracture networks), the model dimensionality (two-dimensional vs. three-dimensional), and/or the parameterisation approach (forward vs. inverse). Despite the abundance and diversity of calibration/parameterisation data, all of the models failed to predict the drawdowns with a reasonable degree of accuracy. Only the order of magnitude of the drawdowns was correctly predicted by three of the five models, whereas all models failed to predict the drawdown behaviour at both intermediate and late times. The primary source of error is attributed to a lack of information in the single-well pumping test data, which did not capture the multi-permeability structure of the aquifer as revealed a posteriori by the responses to dipole flow experiments. This highlights the need to develop novel (or improved) approaches to characterise the hydraulic properties of limestone aquifers.

International audience; Fractured karst aquifers are well known to conceal channeled flow where a few productive-water bearing conduits comprise a relatively small volume within the flow system. This study investigates the ability of combined water temperature measurements and flow logs in wells to reveal the structure of these channeled flows. Temperature and flow logs discussed in this study were performed under various hydrodynamic conditions from ambient flow to radial forced flow at the large scale. Both techniques complement each other since flow logs measure local flow velocities in wells whereas water column temperature profiles delineate inflow depths. For example, temperatures indicate the origin of water (e.g., shallow versus deep) which is not indicated by flow logs. On the other hand, complex temperature profiles due to the interception by a well of several productive layers are better explained when local flow directions are provided by flow logs. The results indicate hydrodynamic conditions affecting the connections between different water-bearing bodies. Pumping the monitored well at a low rate will capture a short-term and local effect related to the convergence of flow into the well bore. When pumping at high discharge rates in a distant well, temperatures and flow logs will capture the long-term re-adjustment of hydraulic heads over a wide portion of the aquifer. In the end, the association of temperature and flow logs reveals a handy and cost-effective tool, well suited to delineate the complex structure of flow in fractured karst aquifers. Knowing this structure would probably improve the ability to design proper scenarios of water resources exploitation and management in complex aquifers

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.

International audience; In most cases, analysis and modeling of flow and transport dynamics in ground water systems require long-term, high-quality, and multisource data sets. This paper discusses the structure of a multisite database (the H+ database) developed within the scope of the ERO program (French Environmental Research Observatory, http://www.ore.fr). The database provides an interface between field experimentalists and modelers, which can be used on a daily basis. The database structure enables the storage of a large number of data and data types collected from a given site or multiple-site network. The database is well suited to the integration, backup, and retrieval of data for flow and transport modeling in heterogeneous aquifers. It relies on the definition of standards and uses a templated structure, such that any type of geolocalized data obtained from wells, hydrological stations, and meteorological stations can be handled. New types of platforms other than wells, hydrological stations, and meteorological stations, and new types of experiments and/or parameters could easily be added without modifying the database structure. Thus, we propose that the database structure could be used as a template for designing databases for complex sites. An example application is the H+ database, which gathers data collected from a network of hydrogeological sites associated with the French Environmental Research Observatory.

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.

International audience; The time domain random walk (TDRW) method has been developed for simulating solute transport in discrete fracture networks. The following transport processes have been considered: advective transport in fractures, hydrodynamic dispersion along the fracture axis, sorption reactions on the fracture walls and decay reactions. The TDRW method takes advantage of both random walk and particle-tracking methods. It allows for the one-step calculation of the particle residence time in each bond of the network while avoiding mass balance problems at fracture intersections with contrasted dispersion coefficients. The accuracy of the TDRW method has been addressed by means of synthetic test problems into single fractures and into a 2D discrete fracture network. In each case, simulated and theoretical results compare very well.

[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.

The equations of hydrodynamic dispersion based upon the classical theory of mass transfer in porous media are not always applicable to migration in fractured media where the anisotropic characteristics involve a divergence between experimental and computed results. In this paper, a dispersion coefficient is calculated for a medium consisting of fractures with apertures having an exponential distribution; equations of concentration have been worked out for convection on the one hand, and for dispersion in the fractures on the other. The results are compared with results ∗ relating to a medium consisting of tubes whose cross-sectional areas have also an exponential distribution.

Three-dimensional total porosity blocks of a Dogger Limestone fractured-karst aquifer were obtained at the Hydrogeological Experimental Site of Poitiers, France. Dogger Limestone occurs at a depth ranging between 30 and 120 m and is overlain by argillaceous limestone. Three karst intervals at depth of 35-40 m, 85-87 m and 110-115 m have been revealed from vertical flowmeter data and highresolution imagery of borehole walls. This paper compares the total porosity blocks obtained from 3D seismic and resistivity imaging. Reflection seismic survey allowed generating a 3D seismic pseudo velocity block. The seismic interval velocities have been converted into resistivity. The empirical relationship between seismic velocity and true formation resistivity proposed by Faust (1953) has been used. Resistivity values were then converted into porosity values, by using the Archie’s law (1942). The 3D seismic pseudo porosity block allowed identifying three different large porosity layers that are consistent with flowmeter and borehole imagery data. Three-dimensional resistivity imaging was obtained from full inversion of combined 2D ERT data collected using both Wenner-Schlumberger (WS) and Pole-Dipole (PD) array sequences. 3D resistivity block obtained from the hybrid (combination of WS and PD) sequence seemed to apparently identify the first two main karst intervals. It was converted into porosity by using the Archie law (1942). The total porosity blocks obtained from seismic and resistivity imaging show very different resolutions (above all at the karst layers depth), but agree in identifying the karst layers depth. The resistivity block shows an overestimation of the total porosity values: this is due to the occurrence of the shallow, thick, low resistivity layer (drift deposits and argillaceous limestone) that led to a severe underestimate of the limestone bulk resistivity values. Both the seismic and the resistivity blocks agree in suggesting that not karst carbonates would be affected by low permeability but significant storability.

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.