Structure des écoulementset propriétés de transportdes aquifères cristallinsfracturés et altérés :Application au site deChoutuppal (Inde du Sud)

Mémoire de Géosciences-Rennes n°150. ISBN : 2-914375-92-1The fractured and weathered crystalline aquifers are often the only viable waterresource in arid and semi-arid regions. Nevertheless, these highly heterogeneous mediaare still poorly understood, especially the major structures that control groundwaterows and contaminant transport. To improve knowledge of those media, wehave conducted a detailed analysis of the hydrological properties of the Choutuppalexperimental site (Andhra Pradesh, Southern India) which has a dense network ofobservation boreholes. The study focus on 1) the identication of relevant structuresand the spatial variability of hydraulic properties and 2) the dominant solutetransport processes in these fractured media. First, the evolution of properties withdepth highlights the most permeable structures. The properties were then studied inhighly contrasted hydrological conditions. In high water level conditions, the saprolite{ granite interface controls groundwater ows at watershed scale. By contrast,when groundwater level is lower than this interface, hydrological compartmentalizationappears due to the decrease of the number of permeable fractures with depthwhich in turns decreases considerably connectivity with depth. A conceptual modelof groundwater ow is proposed at the watershed scale to illustrate these contrastingbehaviors. To identify the respective role of advective and diusive processes aectingboth solute transport, two kind of tracer experiments were conducted underdierent ows congurations. Combination of tracer experiments between boreholesand single borehole tests (push-pull) permit to highlight the predominant role of heterogeneousadvection. Matrix diusion can be neglected at least for the time scalesconsidered. The push-pull experiments have also allowed identifying the impact ofinvestigation scale on anomalous solute transport. All results give a better understandingof the properties and the vulnerability of those media subject to stronganthropogenic pressure.Les milieux cristallins fractures et alteres representent souvent la seule ressourceen eau viable pour les regions arides et semi-arides. Toutefois, ces milieux fortementheterogenes restent encore mal connus, notamment les principales structuresqui contr^olent les ecoulements et le transport de contaminants. An d'ameliorerla connaissance de ces milieux, nous avons eectue une analyse detaillee des proprietes hydrologiques du site experimental de Choutuppal (Andhra Pradesh, Indedu Sud) qui benecie d'un reseau dense de forages d'observation. L'etude porte ala fois sur 1) l'identication des structures pertinentes et la variabilite spatiale desproprietes hydrauliques et 2) sur les processus dominant le transport de solutes dansces milieux fractures. Dans un premier temps, l'evolution des proprietes en fonctionde la profondeur a permis d'identier les structures les plus permeables. Les proprietes du milieu ont ensuite ete etudiees dans des conditions hydrologiques trescontrastees. En conditions de hautes eaux, l'interface saprolite { granite contr^oleles ecoulements souterrains a l'echelle du bassin versant. En revanche, lorsque lesniveaux piezometriques sont plus bas que cette interface, une compartimentationhydrologique apparait en raison de la diminution de la connectivite et du nombre defractures permeables en profondeur. Un modele conceptuel d'ecoulement souterrainest propose a l'echelle du bassin versant pour illustrer ces comportements hydrologiquescontrastes. Pour identier le r^ole respectif des processus advectifs et diusifsaectant tous deux le transport de solutes, deux types d'essais de tracage ont eterealises sous dierentes congurations d'ecoulement. La combinaison d'experiencesde tracage entre puits et en puits seul ("push-pull") a permis de mettre en avantle r^ole predominant de l'advection heterogene. La diusion dans la matrice peut^etre negligee, au moins pour les echelles de temps considerees. Les experiences de"push-pull" ont egalement permis d'identier l'impact de l'echelle d'investigation surle transport anormal de solutes. L'ensemble de ces resultats fournit une meilleureconnaissance des proprietes et de la vulnerabilite de ces milieux soumis a une fortepression anthropique

Flow structure and transport properties of fractured and weathered crystalline aquifers : application to Choutuppal site (South India)

Les milieux de socle cristallins fracturés et altérés représentent souvent la seule ressource en eau viable pour les régions arides et semi-arides. Toutefois, ces milieux fortement hétérogènes restent encore mal connus, notamment les principales structures qui contrôlent les écoulements et le transport de contaminants. Afin d'améliorer la connaissance de ces milieux, nous avons effectué une analyse détaillée des propriétés hydrologiques du site expérimental de Choutuppal (Andhra Pradesh, Inde du Sud) qui bénéficie d'un réseau dense de forages d'observation. L'étude porte à la fois sur 1) l'identification des structures pertinentes et la variabilité spatiale des propriétés hydrauliques et 2) sur les processus dominant le transport de soluté dans ces milieux fracturés. Dans un premier temps, l'évolution des propriétés en fonction de la profondeur a permis d'identifier les structures les plus perméables. Les propriétés du milieu ont ensuite été étudiées dans des conditions hydrologiques très contrastées. En conditions de hautes eaux, l'interface saprolite-granite contrôle les écoulements souterrains à l'échelle du bassin versant. En revanche, lorsque les niveaux piézométriques sont plus bas que cette interface, une compartimentation hydrologique apparaît en raison de la diminution de la connectivité et du nombre de fractures perméables en profondeur. Un modèle conceptuel d'écoulement souterrain est proposé à l'échelle du bassin-versant pour illustrer ces comportements hydrologiques contrastés. Pour identifier le rôle respectif des processus advectifs et diffusifs affectant tous deux le transport de soluté, deux types d'expériences de traçages artificiels ont été réalisés sous différentes configurations d'écoulements. La combinaison d'expériences de traçages entre puits et sur puits seul (push-pull) ont permis de mettre en avant le rôle prédominant de l'advection hétérogène. La diffusion dans la matrice peut être négligée, au moins pour les échelles de temps considérées. Les expériences de push-pull ont également permis d'identifier l'impact de l'échelle d'investigation sur le transport anormal de soluté. L'ensemble de ces résultats fournit une meilleure connaissance des propriétés et de la vulnérabilité de ces milieux soumis à une forte pression anthropique.The fractured and weathered crystalline aquifers are often the only viable water resource in arid and semi-arid regions. Nevertheless, these highly heterogeneous media are still poorly understood, especially the major structures that control groundwater flows and contaminant transport. To improve knowledge of those media, we have conducted a detailed analysis of the hydrological properties of the Choutuppal experimental site (Andhra Pradesh, Southern India) which has a dense network of observation boreholes. The study focus on 1) the identification of relevant structures and the spatial variability of hydraulic properties and 2) the dominant solute transport processes in these fractured media. First, the evolution of properties with depth highlights the most permeable structures. The properties were then studied in highly contrasted hydrological conditions. In high water level conditions, the saprolite – granite interface controls groundwater flows at watershed scale. By contrast, when groundwater level is lower than this interface, hydrological compartmentalization appears due to the decrease of the number of permeable fractures with depth which in turns decreases considerably connectivity with depth. A conceptual model of groundwater flow is proposed at the watershed scale to illustrate these contrasting behaviors. To identify the respective role of advective and diffusive processes affecting both solute transport, two kind of tracer experiments were conducted under different flows configurations. Combination of tracer experiments between boreholes and single borehole tests (push-pull) permit to highlight the predominant role of heterogeneous advection. Matrix diffusion can be neglected at least for the time scales considered. The push-pull experiments have also allowed identifying the impact of investigation scale on anomalous solute transport. All results give a better understanding of the properties and the vulnerability of those media subject to strong anthropogenic pressure

Impact of groundwater extraction on the subsurface thermal regime

National audienceBeing the world's largest freshwater resource, groundwater is under simultaneous threat from increasing human water consumption. Beside substantial drops in groundwater levels that modify the recharge/discharge relationships between large-scale hydrogeological units, this hydraulic forcing is also responsible for changes in thermal regimes within the critical zone. While the impact of global groundwater pumping on the hydrogeological cycle has long been demonstrated, we still have insufficient knowledge on the influence of human activities on groundwater temperatures and, as a consequence, on stream thermal regimes. In this contribution we discuss temperature anomalies that develop in the shallow subsurface as a result of localized groundwater extraction. We study different hydrogeological settings, i.e., porous and fractured aquifers, that we explore via numerical modelling and comparison with field observations. In the field, we use repeated temperature-depth borehole profiles separated by decades, the advantage of which is that differencing the temperature logs for individual boreholes yields real temperature change and eliminates steady-state sources of curvature. Thus, it enables us to detect changes in subsurface thermal regimes, resulting from transient conditions, i.e., climate change and changes in groundwater hydrodynamics

Impact of groundwater extraction on the subsurface thermal regime

National audienceBeing the world's largest freshwater resource, groundwater is under simultaneous threat from increasing human water consumption. Beside substantial drops in groundwater levels that modify the recharge/discharge relationships between large-scale hydrogeological units, this hydraulic forcing is also responsible for changes in thermal regimes within the critical zone. While the impact of global groundwater pumping on the hydrogeological cycle has long been demonstrated, we still have insufficient knowledge on the influence of human activities on groundwater temperatures and, as a consequence, on stream thermal regimes. In this contribution we discuss temperature anomalies that develop in the shallow subsurface as a result of localized groundwater extraction. We study different hydrogeological settings, i.e., porous and fractured aquifers, that we explore via numerical modelling and comparison with field observations. In the field, we use repeated temperature-depth borehole profiles separated by decades, the advantage of which is that differencing the temperature logs for individual boreholes yields real temperature change and eliminates steady-state sources of curvature. Thus, it enables us to detect changes in subsurface thermal regimes, resulting from transient conditions, i.e., climate change and changes in groundwater hydrodynamics

Flow-bearing structures of fractured rocks: Insights from hydraulic property scalings revealed by a pumping test

International audienceA long duration pumping test conducted over 151 days in a fractured sandstone and shale formation displays a nonstandard drawdown response and anomalous pressure diffusion, which cannot be properly interpreted using existing frameworks (e.g., homogeneous, double porosity, boundary conditions, and fractal models). An alternative framework with simple geometry and more complex hydraulic properties is thus proposed to interpret such kind of drawdown responses. The analytical development allows first to demonstrate all scaling relations in this interpretation framework. Then, and most importantly, the multi-scale hydraulic test provides consistent scalings of transmissivity, T, to storativity, S, over distances ranging from 83 to 383 m in a faulted area. Drawdown analysis in several monitoring wells shows persistent decrease of transmissivity in highly channelized fracture flow structures. In one structure, the cubic dependency of transmissivity to storativity identifies a well-defined fault and also demonstrates the validity of Poiseuille flow at a scale rarely investigated. In the other structure, the linear dependency of transmissivity to storativity indicates that the flow-bearing structure is the surrounding fracture network. Well-designed pumping tests combined with scaling analysis driven by geological evidence thus provide essential information on flow-bearing structures for site characterization and modeling tasks. At least for moderate to low permeable fractured rocks, the scaling of transmissivity to storativity appears to be more informative than any separate interpretation of hydraulic property scaling exponents

Periodic Hydraulic Testing for Discerning Fracture Network Connections

International audienceDiscrete fracture network (DFN) models often predict highly variable hydraulic connections between injection and pumping wells used for enhanced oil recovery, geothermal energy extraction, and groundwater remediation. Such connections can be difficult to verify in fractured rock systems because standard pumping or pulse interference tests interrogate too large a volume to pinpoint specific connections. Three field examples are presented in which periodic hydraulic tests were used to obtain information about hydraulic connectivity in fractured bedrock. The first site, a sandstone in New York State, involves only a single fracture at a scale of about 10 m. The second site, a granite in Brittany, France, involves a fracture network at about the same scale. The third site, a granite/schist in the U.S. State of New Hampshire, involves a complex network at scale of 30-60 m. In each case periodic testing provided an enhanced view of hydraulic connectivity over previous constant rate tests. Periodic testing is particularly adept at measuring hydraulic diffusivity, which is a more effective parameter than permeability for identify the complexity of flow pathways between measurement locations. Periodic tests were also conducted at multiple frequencies which provides a range in the radius of hydraulic penetration away from the oscillating well. By varying the radius of penetration, we attempt to interrogate the structure of the fracture network. Periodic tests, therefore, may be uniquely suited for verifying and/or calibrating DFN models

Clogging detection and productive layers identification along boreholes using Active Distributed Temperature Sensing

International audienceFiber-Optic Active Distributed Temperature Sensing (FO-ADTS) experiments were performed on an Aquifer Thermal Energy Storage system (ATES) site located on the university campus of Bordeaux, France. The experiments consisted in heating the steel core of the FO cable while monitoring the rate of temperature increase during the heating periods. The changes in temperature, that were monitored through time at every depth under various hydraulic conditions and in different boreholes, were used to evaluate both aquifer properties and wells conditions. A first ADTS experiment was conducted under cross borehole configuration using a pumping well and a monitoring well separated by a distance of 8.5 meters. Then, to check the reciprocity of the results, a second experiment was conducted by switching the monitoring and the pumping well. The results obtained through the use of analytical solutions for reproducing and interpreting the data lead to the following conclusions: (i) ADTS can be used to estimate both thermal conductivity and Darcy velocity distribution along boreholes, crucial properties for ATES performance. (ii) The proposed method is a promising tool to detect clogging locations in the boreholes when it occurs. This can be of great practical interest to maintain systems performance, since, once FO cables deployed, experiments could be easily repeated without opening boreholes and stop the system operation

Characteristic Length Scales in Fracture Networks: Hydraulic Connectivity through Periodic Hydraulic Tests

International audienceDetermining hydraulic and transport connectivity in fractured bedrock has long been an important objective in contaminant hydrogeology, petroleum engineering, and geothermal operations. A persistent obstacle to making this determination is that the characteristic length scale is nearly impossible to determine in sparsely fractured networks. Both flow and transport occur through an unknown structure of interconnected fracture and/or fracture zones making the actual length that water or solutes travel undetermined. This poses difficulties for flow and transport models. For, example, hydraulic equations require a separation distance between pumping and observation well to determine hydraulic parameters. When wells pairs are close, the structure of the network can influence the interpretation of well separation and the flow dimension of the tested system.This issue is explored using hydraulic tests conducted in a shallow fractured crystalline rock. Periodic (oscillatory) slug tests were performed at the Ploemeur fractured rock test site located in Brittany, France. Hydraulic connectivity was examined between three zones in one well and four zones in another, located 6 m apart in map view. The wells are sufficiently close, however, that the tangential distance between the tested zones ranges between 6 and 30 m. Using standard periodic formulations of radial flow, estimates of storativity scale inversely with the square of the separation distance and hydraulic diffusivity directly with the square of the separation distance. Uncertainty in the connection paths between the two wells leads to an order of magnitude uncertainty in estimates of storativity and hydraulic diffusivity, although estimates of transmissivity are unaffected. The assumed flow dimension results in alternative estimates of hydraulic parameters. In general, one is faced with the prospect of assuming the hydraulic parameter and inverting the separation distance, or vice versa. Similar uncertainties exist, for instance, when trying to invert transport parameters from tracer mean residence time. This field test illustrates that when dealing with fracture networks, there is a need for analytic methods of complexity that lie between simple radial solutions and discrete fracture network models

Continuous in-situ monitoring of dissolved gases for the characterization of the Critical Zone with a MIMS

International audienceIn the perspective of a temporal and spatial exploration of the Critical Zone, we developed an in situ monitoringinstrument for continuous dissolved gas analysis (N2, O2, CO2, CH4, He, Ne, Ar, Kr, Xe). With a large resolution(5 orders of magnitude) and a capability of high frequency multi-tracer analysis (1 gas every 1.5 seconds), theMIMS (Membrane Inlet Mass Spectrometer) is an innovative tool allowing the investigation of a large panel ofphysical and biogeochemical processes.First of all, this study presents the results of groundwater tracer tests using dissolved gases in order to evaluatetransport properties of a fractured media in Brittany, France (Ploemeur, ORE H+). The tracer test experimentshowed that the MIMS is perfectly suitable for field work. The instrument provides precise measurements accurateenough to produce breakthrough curves during groundwater tracer tests. The results derived from 4He data givestransport parameters in good agreement with the results obtained with a fluorescent tracer.Combined with a pump and a multi-parameter probe, the MIMS is also capable to perform accurate dissolved gaseswell-logs allowing a real-time estimation of recharge conditions (temperature, excess air), aquifer stratification,redox conditions and groundwater residence time by 4He dating.Therefore, the MIMS is a valuable tool for in situ characterization of biogeochemical reactivity in aquatic systems,the determination of aquifer transport properties, the monitoring of groundwater recharge conditions and thecharacterization of aquifer-river exchanges

Characterization of the role of heterogeneous advection and diffusion on transport in weathered and fractured granite.

International audienc