13 research outputs found
Réponse d'un systÚme aquifÚre multicouche aux variations paléoclimatiques et aux sollicitations anthropiques - Approche par modélisation couplée hydrodynamique, thermique et géochimique
The study of the hydrodynamic parameters and the piezometric level of the aquifers is not enough to understand the hydrogeological behaviour of large sedimentary basins. It is necessary to take into account the whole of the layers and their hydrogeological behaviour on the long term. These aquifer systems are indeed too often analysed on the short term on the supposition that the hydrodynamic state was stabilized before the human exploitation. Thus it is under a new vision that the understanding of the aquifer of the Infra-Molassic Sands is approached in the aim of integrating it in a hydrodynamic model with the overall of the south-Aquitaine multi-layered aquifer system.The interpretation of the isotopic and geochemical data showed that only vertical flows in addition or not to a horizontal flow can explain the results and it made it possible to present the idea of a major phase of recharge at the end of the Pleistocene. The reconstitution of the local paleoclimate was undertaken in order to propose a scenario of the potential periods of recharge of the south-Aquitain aquifer system over the last 40,000 years. These results revealed the instability of the conditions of recharge of this multi-layered aquifer system and thus its transitory state.A major study of the geological, hydrogeological and geothermical characteristics was undertaken to suggest a hydrodynamical and thermal 3D model. This model makes it possible to study the recent evolution but also the long-term trends. This model is made up of eleven layers and the extension is approximately 32,000 kmÂČ. The calibration of the temperature in the model was carried out. The calibration in transient state of exploitation with the pumping and the activity of two storages of gas was done. The simulation of the eustatic variations on the last 100,000 years showed the weak influence of those variations on the Infra-Molassic Sands groundwater. The simulation of the variations of recharge according to the paleoenvironmental fluctuations over 40,000 years, final purpose of this work, was carried out and enabled to establish three principal axis of flow in coherence with the geochemical constraints. The importance of the vertical and horizontal flow could be done using water budget. To finish with, the piezometric fall currently observed would be the principal consequence of the pumping of the last thirty years and of the natural drainage of the system started 150 years ago, following an important recharge during the Little Ice Age.This study shows the existence of exchanges between the various layers (aquifers and aquitards), the important recharge by vertical drainage in different areas and the transient-state groundwater flow over several hundreds to several thousands of years. In short, the behaviour of this type of multi-layered aquifer of regional extension requires to be detached from the common vision of a functioning which would have begun only with the anthropic exploitation on a hydrodynamical stabilized system.L'Ă©tude des paramĂštres hydrodynamiques et de la charge piĂ©zomĂ©trique des aquifĂšres ne suffit pas pour apprĂ©hender le fonctionnement hydrogĂ©ologique des grands bassins sĂ©dimentaires. Il est nĂ©cessaire de tenir compte de l'ensemble des couches et de leurs comportements hydrogĂ©ologiques sur le long terme. Ces systĂšmes sont en effet trop souvent analysĂ©s sur le court terme avec l'hypothĂšse que le rĂ©gime hydrodynamique Ă©tait stabilisĂ© avant l'exploitation humaine. C'est donc sous une vision nouvelle que la comprĂ©hension de l'aquifĂšre des Sables Infra-Molassiques, et plus globalement du systĂšme multicouche sud-aquitain, est abordĂ©e avec comme objectif son intĂ©gration dans un modĂšle hydrodynamique.L'interprĂ©tation des donnĂ©es isotopiques et gĂ©ochimiques a montrĂ© que seuls des transferts verticaux en complĂ©ment ou non Ă un Ă©coulement horizontal Ă©taient Ă mĂȘme d'expliquer les rĂ©sultats des analyses et a permis d'exposer l'idĂ©e d'une phase de recharge importante Ă la fin du PlĂ©istocĂšne. La reconstitution du palĂ©oclimat local a Ă©tĂ© entreprise afin de proposer un scĂ©nario des pĂ©riodes potentielles de recharge du systĂšme aquifĂšre sud-aquitain sur les 40000 derniĂšres annĂ©es. L'ensemble de ces rĂ©sultats a rĂ©vĂ©lĂ© l'instabilitĂ© des conditions d'alimentation de ce multicouche et donc son Ă©tat transitoire.Une analyse majeure des caractĂ©ristiques gĂ©ologiques, hydrogĂ©ologiques et gĂ©othermiques a Ă©tĂ© engagĂ©e et les donnĂ©es mises en cohĂ©rence afin de proposer un modĂšle couplĂ© hydrodynamique et thermique 3D permettant d'Ă©tudier l'Ă©volution rĂ©cente mais Ă©galement de longue durĂ©e. Comportant onze couches, l'extension du modĂšle est d'environ 32000 kmÂČ. L'ajustement dans le modĂšle des donnĂ©es de tempĂ©rature a Ă©tĂ© rĂ©alisĂ©. Le calage en transitoire court d'exploitation avec les prĂ©lĂšvements et l'activitĂ© de deux stockages de gaz a Ă©tĂ© effectuĂ©. La simulation des variations eustatiques sur les derniers 100000 ans a montrĂ© la faible influence de celles-ci sur la nappe des Sables Infra-Molassiques. La simulation des variations de recharge liĂ©es aux fluctuations palĂ©oclimatiques sur 40000 ans, objectif final de ce travail, a Ă©tĂ© exĂ©cutĂ©e et a permis d'Ă©tablir trois principaux axes d'Ă©coulement en cohĂ©rence avec les contraintes gĂ©ochimiques. L'importance des transferts verticaux et horizontaux a pu ĂȘtre approchĂ©e Ă l'aide de bilans de flux. Enfin, la baisse piĂ©zomĂ©trique actuellement observĂ©e serait la consĂ©quence des prĂ©lĂšvements de ces trente derniĂšres annĂ©es et, dans une moindre mesure, de la vidange naturelle du systĂšme amorcĂ© il y a 150 ans, suite Ă une recharge importante au cours du Petit Ăge Glaciaire.Ce travail a mis en exergue l'existence d'Ă©changes entre les diffĂ©rentes couches (aquifĂšres et formations peu permĂ©ables), la recharge par drainance verticale importante dans certains secteurs ainsi que le rĂ©gime transitoire sur plusieurs centaines voire milliers d'annĂ©es avec un caractĂšre continu. Au final, la comprĂ©hension de ce type de multicouche d'extension rĂ©gionale nĂ©cessite de se dĂ©tacher de la vision commune d'un fonctionnement qui n'aurait dĂ©butĂ© qu'avec l'exploitation anthropique sur un systĂšme hydrodynamiquement stabilisĂ©
Réponse d'un systÚme aquifÚre multicouche aux variations paléoclimatiques et aux sollicitations anthropiques (approche par modélisation couplée hydrodynamique, thermique et géochimique)
Le fonctionnement hydrogĂ©ologique des grands bassins sĂ©dimentaires est souvent analysĂ© sur le court terme avec l'hypothĂšse que le rĂ©gime hydrodynamique Ă©tait stabilisĂ© avant l'exploitation humaine. Le comportement du systĂšme multicouche sud-aquitain, et en particulier de l'aquifĂšre des Sables Infra-Molassiques, est Ă©tudiĂ© ici sur le long terme afin d ĂȘtre intĂ©grĂ© dans un modĂšle hydrodynamique. 'interprĂ©tation des donnĂ©es de la chimie de l eau a montrĂ© que des transferts verticaux associĂ©s ou non Ă un flux horizontal pouvaient expliquer ces rĂ©sultats et a permis d'exposer l'idĂ©e d'une phase de recharge importante Ă la fin du PlĂ©istocĂšne. Un scĂ©nario des phases potentielles de recharge sur 40000 ans Ă©tĂ© proposĂ© Ă partir d une reconstitution palĂ©oclimatique. n modĂšle couplĂ© hydrodynamique et thermique comportant onze couches sur une extension de 32000 km a Ă©tĂ© rĂ©alisĂ©. La calibration des paramĂštres hydrodynamiques Ă l aide de variations induites par deux stockages de gaz et des conditions thermiques a Ă©tĂ© effectuĂ©. La simulation des variations eustatiques sur les derniers 100000 ans a montrĂ© la faible influence de ces fluctuations. La simulation des variations de recharge sur 40000 ans a Ă©tĂ© effectuĂ©e et a permis d Ă©tablir trois principaux axes d'Ă©coulement en cohĂ©rence avec les contraintes gĂ©ochimiques. L'importance des transferts verticaux et horizontaux a pu ĂȘtre approchĂ©e Ă l'aide de bilans de flux. Enfin, la baisse piĂ©zomĂ©trique actuellement observĂ©e serait la consĂ©quence principale des prĂ©lĂšvements de ces trente derniĂšres annĂ©es et de la vidange naturelle du systĂšme amorcĂ© il y a 150 ans, suite Ă une recharge importante au cours du Petit Age Glaciaire.The hydrogeologic behaviour of large sedimentary basins is often analysed on the short term on the assumption that the hydrodynamic state was stabilized before the human exploitation. The behaviour of the south-Aquitaine multi-layered system, and in particular of the Infra-Molassic Sands aquifer, is studied here on a long term period in order to be integrated in a hydrodynamic model.The interpretation of the data of the water chemistry showed that vertical flows combined or not with a horizontal flow could explain these results and it made it possible to present the idea of a major phase of recharge at the end of the Pleistocene. A scenario of the potential phases of recharge over 40,000 years was realized starting from a paleoenvironmental reconstitution.A hydrodynamic and thermal coupled model including eleven layers on an extension of 32,000 km was carried out. The calibration of the hydrodynamic parameters using variations induced by two storages of gas and of the thermal conditions was done. The simulation of the eustatic variations on the last 100,000 years showed the weak influence of these fluctuations. The simulation of the variations of recharge over 40,000 years was carried out and enabled to establish three principal axis of flow in coherence with the geochemical constraints. The importance of the vertical and horizontal transfers could be done using water budget. Finally, the piezometric fall currently observed would be the principal consequence of the pumping of the last thirty years and of the natural drainage of the system started 150 years ago, following an important refill during the Little Ice Age.BORDEAUX3-BU Lettres-Pessac (335222103) / SudocSudocFranceF
A platform to harmonize the regional hydrodynamic models in the southwest of France
International audienceTo support the authorities in water resource management, different hydrodynamic models have been developed in south-western France (the new Large Region: Aquitaine - Limousin - Poitou-Charentes) for nearly twenty years. These models also contribute to understand the functioning of multilayered aquifers on this territory. Four developed models coexist and cover a total area of 82,570 kmÂČ (Jurassic and Cretaceous models in Poitou-Charentes, the North Aquitaine Model and the South Aquitaine Model). They cover 87% of the sedimentary formations of the new Large Region. In addition of these regional models, others models with refined geometry or mesh have been developed on a part of the same territory in order to respond to local and specific problems. All these models, developed independently from each other, will be integrated in a common platform in order to harmonize their functioning with the experience gained during the development and the use of each of them.Thus, the use of a geological modeler upstream to create geometrical models allows some flexibility in order to regularly integrate the latest geological data (used for the North Aquitaine Model). Moreover, the use of climate spatialized data (SAFRAN data from MĂ©tĂ©o-France) coupled with the use of a specific recharge module allows to homogenize and calculate in the same way the groundwater recharge in the different models (used for the Jurassic model). Including the restitution of surface runoff and rivers flow in modeling allows as well to properly constrain the distribution of effective precipitation between infiltration and runoff.Finally, a database of water withdrawals is built to handle and treat in the same way data from different origins with variable time step and specific issues to be considered in each model (water withdrawals in groundwater and/or surface for various uses as water supply, industry, irrigation, dam releases, sewage discharges, etc.).All of these works aim to consolidate regional hydrodynamic models and to allow the update and faster adaptation of these models
Groundwater management of large aquifers in southwestern France by regional hydrodynamic models
International audienceIn southwestern France, groundwater resources are withdrawn for various uses (drinkable water, irrigation, industry, geothermal energy, forestry, conservation of wetlands, shellfish breeding among others) which can sometimes cause conflicts between users. To help the management of these resources, regional hydrodynamic models have been developed. These models have been a support to the public policies for over 20 years.In the northern region, groundwater is in close relation with rivers, especially during periods of low-water levels. Indeed, groundwater used for irrigation impact not only the stream flows, but also the water supply for the second large wetland of France: the Marais Poitevin. The developed models have permitted to test the impact of several scenarios of water withdrawals. These tests help to determinate the amount of water that can be pumped to respect defined objectives as water levels in piezometer and river flows.These models were also used to test the impact of the eventual implementation of water tanks (400,000 to 800,000 m3). Further south, the major problem is the important reduction of the level of the Eocene aquifer in the department of Gironde. In this area, the regional hydrodynamic model has been developed since 1990. Its development allows the management of deep groundwater resources and contributes to validate strategies of exploitation based on different simulations. This model also allows to answer problems of overexploitation and to analyse areas where water savings could be done to avoid this overexploitation, and to estimate the impact of new resources.The extreme southern region has a significant particularity: on two different sites, groundwater is used for the storage of gas. The cyclic injection and withdrawal of gas impacts significantly the aquifer level. The proposed model is adapted for the groundwater resources knowledge and management of this case. Finally, these models were used to evaluate impacts of climate change on groundwater resources in order to allow authorities to evaluate strategies to adapt to this change (Project Explore 2070 for example)
Developing Substitution Resources as Compensation for Reduced Groundwater Entitlements: The Case of the Poitou Marshes (France)
International audienceThis chapter describes the groundwater management policy implemented in the Poitou marshes, a 100 000 ha wetland located on the Atlantic coast in Western France. Similarly, to other French basins, irrigated agriculture has rapidly developed since the 1980's, mainly based on groundwater exploitation. Clear signs of groundwater overexploitation appeared in 1992-95, with the intrusion of brackish water in the aquifer. Because of the overexploitation, ecosystems were severely affected and the French Government was sued by the European Commission for non-compliance with the Bird Directive (1999). The chapter describes the progressive implementation of a groundwater management policy aiming at ensuring the long-term sustainability of an emblematic groundwater dependent wetland. To do so, the State imposed a very significant reduction in historical water entitlements. This case study illustrates the difficulties encountered in implementing this reduction, in a context of extreme competition between economic uses (agriculture, urban and touristic) and environmental objectives. The case study also reports on the complexity of developing an integrated management plan in basins where groundwater, rivers, wetlands and canals are highly interdependent. It highlights the importance of a (shared) knowledge on water resource and uses, of involving stakeholders in the different steps, and of trying to share scarcity in an equitable way
The cumulative impacts of small reservoirs on hydrology: A review
International audienceThe number of small reservoirs has increased due to their reduced cost, the availability of many favourable locations, and their easy access due to proximity. The cumulative impacts of such small reservoirs are not easy to estimate, even when solely considering hydrology, which is partially due to the difficulty in collecting data on the functioning of such reservoirs. However, there is evidence indicating that the cumulative impacts of such reservoirs are significant.The aim of this article is to present a review of the studies that address the cumulative impacts of small reservoirs on hydrology, focusing on the methodology and on the way in which these impacts are assessed.Most of the studies addressing the hydrological cumulative impacts focused on the annual stream discharge, with decreases ranging from 0.2% to 36% with a mean value of 13.4% ± 8% over approximately 30 references. However, it is shown that similar densities of small reservoirs can lead to different impacts on stream discharge in different regions. This result is probably due to the hydro-climatic conditions and makes defining simple indicators to provide a first guess of the cumulative impacts difficult. The impacts also vary in time, with a more intense reduction in the river discharge during the dry years than during the wet years. This finding is certainly an important point to take into consideration in the context of climate change.Two methods are mostly used to estimate cumulative impacts: i) exclusively data-based methods and ii) models. The assumptions, interests and shortcomings of these methods are presented. Scientific tracks are proposed to address the four main shortcomings, namely the estimation of the associated uncertainties, the lack of knowledge on reservoir characteristics and water abstraction and the accuracy of the impact indicators
Cumulative impact of small reservoirs;: a review of estimations and methods
International audienceDue to a reduce cost, availability of many favorable locations, easy access due to proximity, the number of small reservoirs has increased, especially in arid and semi-arid regions. The cumulative impact of reservoirs in a catchment is considered as the modifications induced by the reservoir network taken as a whole. The impact may exert on the flow regimes and sediment, nutrient and contaminant transfer, and thereby modify the ecological behaviour of the aquatic environment, the continuity of rivers and the habitats of organisms living in them. The cumulative impact is not necessarily the sum of individual and local modifications, because reservoirs may be inter-dependent. This is the case for instance in cascading reservoirs along a stream course. The cumulative impact is not straightforward to estimate, even solely considering hydrological impact, in part due to the difficulty to collect data on the functioning of those reservoirs. However, there are evidences that the cumulative impacts are not negligible.This work is dedicated to a review of the studies dealing with the cumulative impact of small reservoirs on hydrology, focusing on the methodology as well as on the way the impacts are reported. It is shown that similar densities of small reservoirs can lead to different impacts on the quantitative water resource in different regions. This is probably due to the hydro-climatic conditions, and makes it difficult to define simple indicators to provide a first guess of the cumulative impact. The impacts vary also on time, with a more intense reduction of the river discharge during the dry years than during the wet years. This is certainly an important point to take into account in a context of climate change
Le climat de la France au XXIe siÚcle. Vol 5 : Changement climatique et niveau de la mer : de la planÚte aux cÎtes françaises
Ce rapport traite du lien entre le changement climatique et le niveau de la mer. Il a Ă©tĂ©rĂ©digĂ© en complĂ©ment au volume 4 du rapport de la mission Jouzel « ScĂ©nariosrĂ©gionalisĂ©s. Le climat de la France au XXIe siĂšcle » publiĂ© en aoĂ»t 2014. Il met Ă jour levolume 3 de ce rapport dĂ©jĂ consacrĂ© Ă la question du niveau de la mer et publiĂ© enfĂ©vrier 2012. Il vise Ă faire un point des connaissances sur lâĂ©volution passĂ©e et future duniveau de la mer de lâĂ©chelle planĂ©taire Ă celle des cĂŽtes françaises, et sur les principauximpacts physiques de la montĂ©e du niveau marin (submersion marine, Ă©rosion cĂŽtiĂšre,intrusions salines dans les aquifĂšres cĂŽtiers et des impacts sur les infrastructures cĂŽtiĂšreset portuaires). En revanche, il ne traite pas la question des impacts socio-Ă©conomiques nicelle de la gestion des risques associĂ©s. Il sâappuie sur lâexpertise du GIEC et sur lesrĂ©sultats de travaux menĂ©s au sein de la communautĂ© scientifique français
Cumulative impact of reservoirs on the aquatic environment. Joint scientific appraisal
Water storage has increased considerably worldwide since the 1950s. Water is collected and stored in reservoirs to supply towns and cities with drinking water, and for agricultural and industrial purposes, fish farming, leisure activities and low-flow period support. In the late 20th Century, there was a considerable increase in small reservoirs in France, reaching a total of 125,000 by the early 2000s. The creation of new water storage structures continues in parallel with the exploration of water use reduction solutions. These developments raise a whole host of environmental issues, such as the impact of reservoirs on the aquatic environment, particularly in areas where there are already a number of reservoirs and water resources are in high demand. By law, building a new reservoir requires a planning application or government authorization, which require an environmental impact study. Such studies must now assess the cumulative effects of the project together with other known similar projects. The âcumulativeâ aspect of the impact of water storage structures on a single catchment area is often poorly understood, probably due to a lack of relevant knowledge and methods. Consultants and government services therefore face a lack of operational tools for processing new reservoir applications, which gives rise to other problems around water management planning and the supervision of the development of new reservoirs. In this context, the French Ministry of the Environment, Energy and Marine Affairs (MEEM), supported by ONEMA, requested a joint scientific assessment (ESCo) from Irstea, in partnership with INRA, on the cumulative impact of reservoirs on the aquatic environment. It was produced by around fifteen experts from a range of disciplines and research organizations, and is based on analysis of a thousand or so international scientific articles and reports.Le stockage de lâeau a considĂ©rablement augmentĂ© dans le monde depuis les annĂ©es 1950. Les retenues collectent et stockent lâeau, pour lâalimentation des villes en eau potable, Ă des fins agricoles, industrielles, piscicoles, de loisir ou de soutien dâĂ©tiage. En France, les retenues de petites tailles se sont multipliĂ©es Ă la fin du 20iĂšme siĂšcle. Au dĂ©but des annĂ©es 2000, on en comptait environ 125 000. La crĂ©ation de nouveaux ouvrages de stockage se poursuit, parallĂšlement Ă une recherche de rĂ©duction des usages de lâeau. Ces crĂ©ations soulĂšvent de nombreuses questions environnementales, notamment en termes dâimpact sur le milieu aquatique, en particulier dans les zones dĂ©jĂ trĂšs Ă©quipĂ©es et oĂč les ressources en eau sont dâores et dĂ©jĂ trĂšs mobilisĂ©es. La construction dâune nouvelle retenue nĂ©cessite rĂ©glementairement une dĂ©claration ou la sollicitation dâune autorisation auprĂšs des services de lâEtat, impliquant de rĂ©aliser une Ă©tude dâimpact environnemental du projet. Une telle Ă©tude doit dorĂ©navant Ă©valuer les effets cumulĂ©s avec les autres projets Ă©quivalents connus. Cette dimension « cumulĂ©e » de lâimpact dâouvrages de stockage dâeau sur un mĂȘme bassin versant est souvent mal apprĂ©hendĂ©e, les connaissances et les mĂ©thodologies Ă©tant peu dĂ©veloppĂ©es sur cet aspect. Bureaux dâĂ©tude et services de lâEtat font ainsi face Ă un manque dâoutils opĂ©rationnels leur permettant dâinstruire les projets de nouvelles retenues. Ces difficultĂ©s en entrainent dâautres au niveau de la planification de la gestion de lâeau et de lâencadrement Ă la crĂ©ation de telles retenues. Dans ce contexte, le MinistĂšre de lâEnvironnement, de lâEnergie et de la Mer (MEEM), avec lâappui de lâOnema, a sollicitĂ© une expertise scientifique collective (ESCo) auprĂšs dâIrstea, en partenariat avec lâInra, sur lâimpact cumulĂ© des retenues dâeau sur le milieu aquatique. Cette ESCo a mobilisĂ© une quinzaine dâexperts de diffĂ©rents organismes de recherche et de disciplines variĂ©es ; elle est fondĂ©e sur lâanalyse dâun millier dâarticles scientifiques et de rapports internationaux
Guide technique Interactions nappe/riviĂšre: Des outils pour comprendre et mesurer les Ă©changes
National audienceLa dĂ©marche et les outils prĂ©sentĂ©s dans ce guide permettent de diagnostiquer qualitativement et quantitativement les Ă©changes nappe/riviĂšre en tenant compte du degrĂ© plus ou moins Ă©levĂ© dâanthropisation. De tels diagnostics contribueront Ă la prĂ©servation de la ressource en eau, des nappes phrĂ©atiques et de la biodiversitĂ© aquatique sur les bassins versants Ă enjeux importants. Ils sont issus de travaux de recherche et applicables en contexte alluvionnaire et sĂ©dimentaire. Le guide propose une dĂ©marche (parties A et B) qui aidera le lecteur Ă choisir les outils adaptĂ©s Ă son terrain et Ă ses enjeux et Ă en interprĂ©ter les rĂ©sultats. Les outils (partie C) sont prĂ©sentĂ©s sous forme de fiches techniques donnant un accĂšs rapide Ă lâinformation nĂ©cessaire pour les comprendre et les mettre en Ćuvre : principe, donnĂ©es nĂ©cessaires, protocole de mesure, interprĂ©tation, exemple de rĂ©sultats, valeurs guides, points forts et points faibles... Les outils proposĂ©s sont les suivants : analyse gĂ©omatique des niveaux dâeau, modĂšles hydrodynamiques distribuĂ©s Ă base physique, modĂšles hydrothermiques Ă base physique et dispositifs de mesure de tempĂ©rature et dâĂ©coulements dâeau, image thermique infrarouge, gĂ©ochimie, vĂ©gĂ©tation aquatique, invertĂ©brĂ©s souterrains. Ce guide sâadresse avant tout aux gestionnaires de lâeau qui, pour gĂ©rer de façon durable leurs ressources en eau, doivent sâintĂ©resser aux interactions nappe/riviĂšre. Les informations et la dĂ©marche proposĂ©es ont donc pour objectif principal dâaider lâutilisateur Ă mettre en Ćuvre et rĂ©aliser par lui-mĂȘme une Ă©tude pour caractĂ©riser les Ă©changes nappe/riviĂšre ou Ă rĂ©diger le cahier des charges pour la sous-traiter