Analyse et modélisation des transferts de masse et de chaleur au sein des décharges d'ordures ménagères

En considérant les décharges d'ordures ménagères comme des milieux poreux réactifs, nous nous sommes intéressés aux transferts couplés de masse et de chaleur qui s'y développent, pour tenter d'expliquer les interactions entre les processus physiques, thermiques et biochimiques qui gouvernent la biodégradation des déchets. Un casier de décharge a été instrumenté pour mesurer les évolutions spatiales et temporelles de la température, de la pression et de la composition chimique du biogaz dans les déchets. Les propriétés des déchets (composition, humidité, perméabilité, conductivité thermique...) ont également été caractérisées. Parallèlement, deux pilotes de 300 litres contenant un déchet modèle, ont été mis en oeuvre en laboratoire pour reproduire en milieu contrôlé les comportements observés sur site. Le second réacteur a été conçu pour étudier plus particulièrement la phase aérobie. Cette approche expérimentale à deux échelles a fourni des résultats originaux concernant les principaux mécanismes et les temps caractéristiques des transferts thermiques et massiques au cours de la biodégradation. Elle a en particulier montré l'influence importante de la phase aérobie sur l'établissement ultérieur de champs thermiques uniformes et d'un régime de méthanogénèse stable. Une modélisation des transferts couplés d'oxygène et de chaleur au sein des décharges d'ordures ménagères a donc été développée. Le modèle numérique a été appliqué à la prévision du comportement thermique des déchets en fonction de certains paramètres liés au mode d'exploitation des sites. Des premières recommandations pour accélérer la production de méthane dans les décharges ont ainsi pu être proposées. ABSTRACT : Waste biodegradation results from microbial, physical and thermal mechanisms. In order to improve knowledge on these interdependent phenomena, we studied coupled heat and mass transfers in sanitary landfills, considered as reactive porous media. Probes were placed in a landfill to monitor the spatial and temporal evolutions of temperature, pressure and biogaz chemical composition in refuses. Waste properties (composition, moisture, permeability, thermal conductivity...) were also characterised. At the same time, two experimental reactors of 300 litters, filled with a model waste, were achieved to simulate in laboratory the main behaviours observed in landfills. The second one was especially performed to study the aerobic phase of waste decomposition. These two-scales experimental studies gave some interesting results about mechanisms and characteristic times of heat and mass transfers during biodegradation. They particularly highlighted the important role of aerobic digestion on the future establishment of thermal fields and methanogenesis. A modelling of coupled oxygen and heat transfers in landfills was then proposed. The numerical model was applied to predict thermal behaviour of refuses according to several parameters linked to landfilling techniques. Some ways were thus suggested to accelerate methane production in sanitary landfills

Évaluation des ressources en eau de la Martinique : calcul spatialisé de la pluie efficace et validation à l’échelle du bassin versant

L’évaluation des différents termes du bilan hydrologique à l’échelle d’un bassin versant constitue l’un des points clés de la gestion des ressources en eau, et ce, tout particulièrement dans les régions montagneuses présentant de fortes variations spatiales de la pluviométrie et de l’évapotranspiration. Une méthodologie, basée sur le modèle classique de Thornthwaite, est proposée. Elle prend en compte les différents types de sols, l’occupation des sols ainsi que les effets topographiques et calcule les différents termes du bilan hydrologique (pluie, évapotranspiration, pluie efficace, etc.). L’approche a été mise en oeuvre à l’échelle du kilomètre carré, pour l’ensemble de l’île de la Martinique (1 080 km2), puis validée à l’échelle du bassin versant, en comparant les pluies efficaces calculées avec les débits mesurés aux stations de jaugeage. Malgré l’absence de calage des différents paramètres du modèle, les résultats sont très satisfaisants. Une surestimation de la pluie efficace est néanmoins observée pour la plupart des bassins versants utilisés pour la validation du modèle. Cet écart est attribué à une sous-estimation de l’évapotranspiration potentielle, la plupart des bassins versants comportant une composante forestière significative, non prise en compte dans le modèle.The assessment of the various components of the hydrologic budget at catchment scale represents a key challenge for water resources management. This is especially true for regions characterized by important spatial variability in rainfall or evapotranspiration due, for example, to topographical effects. A methodology, based on the classical Thornthwaite model, is proposed to account for soil types, land cover changes and topographical effects on the main components of the water cycle (rainfall, evapotranspiration and efficient rainfall). The approach is developed for the whole Martinique Island (French West Indies, 1080 km2) using a 1-km2 resolution and validated at catchment scale comparing computed efficient rainfall with measured discharge at several gauging stations. Despite the absence of any calibration of the model parameters, the results are satisfying. A slight overestimation of the efficient rainfall is generally observed for the validation watersheds. This discrepancy is interpreted as an underestimation of potential evapotranspiration as the classical Penman-Montheit formula for grass is used despite the presence of forested areas in most of the watersheds

Using IDPR to characterize recharge area of karst aquifers from catchment to regional scales

International audienceThe shape and the nature of hydrographic networks on karst environments are closely linked to the infiltration capacity of geological formations. Thus, the presence or absence of a river in a favorable topographic context can be interpreted in terms of karstic aquifers recharge potential. We present the GIS built index of infiltration and runoff properties of landscape (called IDPR) as an interesting tool for the characterization of the karst extension and for the further recharge estimation of such aquifers. A new version of the IDPR tool has been released over France in 2017 which spatial resolution (25 m) becomes very useful for the detection of contributive zones for karst aquifers recharge. Examples of the IDPR application at the river basin scale (Fontaine de Vaucluse) and at the regional scale (Rhone river basin) will be presented. Base flow estimation methods are used for the calibration of the IDPR index in terms of infiltration capacity, leading the IDPR index to be useful for recharge estimation

Recharge des aquifères à l'échelle de la France : estimation, évolution et incertitudes associées

International audienceEvaluating future groundwater recharge with associated uncertainty at the scale of France Among aquifer recharge processes, effective precipitation infiltration is the most directly impacted by climate. We propose a methodology for estimating this recharge at the scale of large territories, based on global hydrological models requiring few data and parameters. The originality of our work lies in the use of the IDPR (Network Development and Persistence Index) which is a qualitative cartographic index of infiltration potentiality. The study of more than 350 hydrological basins made it possible to highlight a linear relation between the BaseFlow Index and the IDPR for the sedimentary basins. Based on this relationship and assuming that the effective rainfall infiltration ratio could be assimilated to the BFI, an map of this ratio was generated all over France, allowing the effective precipitation map to be converted into a recharge map. The methodology was then applied to estimate future recharge using the outputs of climate models for two contrasted radiative concentration pathway scenarios. By 2030, whatever the scenario, the modeling results predict a moderate decrease in the direct recharge of aquifers everywhere in France except in Paris basin and around the Mediterranean sea. By 2080, the optimistic scenario allows to recover recharge values close to the current ones, outside the north of the Paris basin. At the opposite, within the pessimistic scenario, a majority of aquifers would undergo a recharge decrease by 10 to 30%, with stress zones in the Paris basin, around the Mediterranean sea and in Corsica. Nonetheless, these first results should be considered carefully as the associated uncertainties are significant and can reach 50% for the most pessimistic forecasts.Parmi les processus qui participent à la recharge des aquifères, la recharge par infiltration des précipitations efficaces est la plus directement impactée par le climat. Nous proposons une méthodologie qui permet d'estimer cette recharge à l'échelle de grands territoires, sur la base de modèles hydrologiques globaux nécessitant peu de données et de paramètres. L'étude de plus de 350 bassins hydrologiques a permis de mettre en évidence pour les bassins sédimentaires, une relation entre le BaseFlow Index et l'IDPR (Indice de Développement et de Persistance des Réseaux), qui est un indice cartographique qualitatif de potentialité d'infiltration. Sur la base de cette relation, une carte du ratio d'infiltration de la pluie efficace a pu être générée pour toute la France métropolitaine, permettant de transformer la carte de pluie efficace en carte de recharge. La méthodologie a été appliquée pour estimer la recharge future en utilisant les sorties de modèles climatiques pour deux scenarios de forçage radiatif. A l'horizon 2030, quel que soit le scénario, les résultats de la modélisation prévoient une baisse modérée de la recharge directe des aquifères sur la majorité du territoire, en dehors du bassin parisien et du pourtour méditerranéen. A l'horizon 2080, le scénario optimiste permet de revenir à des valeurs de recharge proches de l'actuel, en dehors du nord du bassin parisien. En revanche, dans le scénario pessimiste, une majorité des aquifères voit sa recharge diminuer de 10 à 30%, avec des zones plus tendues dans le bassin parisien, le pourtour méditerranéen et la Corse. Les incertitudes associées à ces résultats sont néanmoins importantes et peuvent atteindre 50% sur les prévisions les plus pessimistes

Comparison of climate change impacts on the recharge of two karst systems computing different modelling approaches

International audienceKarst systems constitute aquifers in which infiltration and groundwater flows are generally complex processes and are characterized by limited knowledge in terms of geometry and structure. Nonetheless, they often represent interesting groundwater resources, some of them being subjected to intensive exploitation and others non exploited due to their poor understanding. In the future, it is likely that climate change impact on water resources will increase the interest for such a kind of aquifers due to their strong infiltration and storage capacity, in a broad context of higher water scarcity.The Lez and the Lison karst systems in Southern and Eastern France, respectively, provide 2 examples of such systems of several km² under two contrasted climate conditions, the first one being heavily exploited. This study presents a comparative climate change assessment onboth karst systems. Nine climate scenarios corresponding to the Fourth assessment report of the IPCC (SRES A1B scenario), downscaled using weather-type methods by the CERFACS, have been applied to various recharge modelling approaches, as standard analytical solutions of recharge estimation and soil-water balance models. Results are compared and discussed in order to assess the influence on climate change impacts of i) the climate conditions(geographic location), ii) the groundwater exploitation and iii) the modelling approach

Soluções semianalíticas em estado estacionário para avaliar a distribuição bidimensional da carga hidráulica induzida por uma barragem subterrânea em um aquífero inclinado com recarga e bombeamento artificial de aquífero

Underground dams are a technology for artificially increasing existing groundwater resources. They modify the natural groundwater flow in aquifers and, typically, cause hydraulic heads to rise upstream and fall downstream of the dam. However, such modifications must be defined to forecast their environmental, economic and/or social impacts. A steady-state semianalytical solution is proposed for evaluating the two-dimensional distribution of hydraulic head caused by an underground dam fully penetrating a homogeneous and inclined aquifer. The dam is impermeable, of rectangular shape, and its length concerns a limited part of the aquifer width. The developed solution is based on the method of fundamental solutions. Analysis of the semianalytical solution included sensitivity tests and a satisfactory comparison with numerical modelling. Dimensionless graphs relating the dam geometry to maximum hydraulic-head variations upstream and downstream of the dam are given. The proposed solution was applied at two field sites, giving satisfactory results. A semianalytical solution is also developed for an artificial recharge area and/or a pumping well near the underground dam. Interestingly, in the case of highly permeable aquifers, the increase in hydraulic head created by the dam may be much higher than that created by managed aquifer recharge (MAR), despite high injected flux. These semianalytical solutions will be useful applications for assessing the long-term spatial distribution of hydraulic head induced by underground dams, or for testing the combination of dams with pumping wells or MAR technology. They are intended to guide the design of such structures, especially to quickly test various configurations.Las presas soterradas son una tecnología para aumentar artificialmente los recursos de aguas subterráneas existentes. Modifican el flujo natural de las aguas subterráneas en los acuíferos y, por lo general, hacen que las cargas hidráulicas aumenten aguas arriba y disminuyan aguas abajo de la presa. Sin embargo, estas modificaciones deben definirse para prever sus repercusiones ambientales, económicas y/o sociales. Se propone una solución semianalítica en estado estacionario para evaluar la distribución bidimensional de la carga hidráulica causada por una presa soterrada que penetra completamente en un acuífero homogéneo e inclinado. La presa es impermeable, de forma rectangular y su longitud afecta a una parte limitada del ancho del acuífero. La solución desarrollada se basa en el método de las soluciones fundamentales. El análisis de la solución semianalítica incluye pruebas de sensibilidad y una comparación satisfactoria con la modelización numérica. Se presentan gráficos adimensionales que relacionan la geometría de la presa con las variaciones máximas de la carga hidráulica aguas arriba y aguas abajo de la presa. La solución propuesta se aplicó en dos emplazamientos dando resultados satisfactorios. También se desarrolla una solución semianalítica para una zona de recarga artificial y/o un pozo de bombeo cerca de la presa soterrada. Curiosamente, en el caso de acuíferos muy permeables, el aumento de la carga hidráulica creado por la presa puede ser mucho mayor que el creado por la recarga gestionada de acuíferos (MAR), a pesar del elevado flujo inyectado. Estas soluciones semianalíticas serán aplicaciones útiles para evaluar la distribución espacial a largo plazo de la carga hidráulica inducida por presas en el subsuelo, o para probar la combinación de presas con pozos de bombeo o tecnología MAR. Están pensadas para orientar el diseño de tales estructuras, especialmente para probar rápidamente diversas configuraciones.Les barrages souterrains servent à augmenter artificiellement les ressources en eaux souterraines existantes. Ces constructions modifient l’écoulement naturel des eaux souterraines et, généralement, font monter la charge hydraulique en amont du barrage et la font chuter à l’aval. Ces modifications doivent ainsi être évaluées pour prévoir leurs impacts environnementaux, économiques et/ou sociaux. Une solution semianalytique en régime permanent est proposée pour évaluer la distribution spatiale de la charge hydraulique induite par la mise en place d’un barrage souterrain occultant sur toute sa hauteur un aquifère homogène et incliné. Le barrage est imperméable, de forme rectangulaire, et sa longueur concerne une partie limitée de la largeur de l’aquifère. La solution développée est basée sur la méthode des solutions fondamentales. L’analyse de la solution a compris des tests de sensibilité et une comparaison satisfaisante avec une modélisation numérique. Des graphiques aux paramètres sans dimension reliant la géométrie du barrage aux variations maximales de la charge hydraulique en amont et en aval du barrage sont donnés. La solution proposée a été appliquée à deux sites expérimentaux, donnant des résultats satisfaisants. Une autre solution semianalytique incluant une recharge artificielle et/ou un puits de pompage à proximité du barrage souterrain est également proposée. De façon intéressante, dans le cas d’aquifères très perméables, l’augmentation de la charge hydraulique créée par le barrage peut être beaucoup plus élevée que celle créée par un système de recharge maîtrisée, malgré un flux injecté élevé. Ces solutions semianalytiques sont des outils utiles pour évaluer la distribution spatiale à long terme de la charge hydraulique induite par les barrages souterrains, ou pour tester la combinaison de barrages avec des puits de pompage ou des dispositifs de recharge maîtrisée. Elles sont destinées à guider la conception de telles structures, notamment pour tester rapidement différentes configurations.As barragens subterrâneas são uma tecnologia para aumentar artificialmente os recursos hídricos subterrâneos existentes. Eles modificam o fluxo natural das águas subterrâneas nos aquíferos e, normalmente, fazem com que as cargas hidráulicas subam a montante e caiam a jusante da barragem. Contudo, tais modificações devem ser definidas para prever os seus impactos ambientais, económicos e/ou sociais. Uma solução semianalítica em estado estacionário é proposta para avaliar a distribuição bidimensional da carga hidráulica causada por uma barragem subterrânea que penetra totalmente em um aquífero homogêneo e inclinado. A barragem é impermeável, de forma retangular, e o seu comprimento diz respeito a uma parte limitada da largura do aquífero. A solução desenvolvida baseia-se no método das soluções fundamentais. A análise da solução semianalítica incluiu testes de sensibilidade e comparação satisfatória com modelagem numérica. São fornecidos gráficos adimensionais relacionando a geometria da barragem com as variações máximas da carga hidráulica a montante e a jusante da barragem. A solução proposta foi aplicada em dois locais de campo apresentando resultados satisfatórios. É também desenvolvida uma solução semianalítica para uma área de recarga artificial e/ou poço de bombagem próximo da barragem subterrânea. Curiosamente, no caso de aquíferos altamente permeáveis, o aumento na carga hidráulica criada pela barragem pode ser muito maior do que aquele criado pela recarga gerenciada de aquíferos (RGA), apesar do elevado fluxo injetado. Estas soluções semianalíticas serão aplicações úteis para avaliar a distribuição espacial a longo prazo da carga hidráulica induzida por barragens subterrâneas, ou para testar a combinação de barragens com poços de bombeamento ou tecnologia de RGA. Eles têm como objetivo orientar o projeto de tais estruturas, principalmente para testar rapidamente diversas configurações

A generic analytical solution for modelling pumping tests in wells

International audienceThe behaviour of transient flow due to pumping in fractured rocks has been studied for at 11 least the past 80 years. Analytical solutions were proposed for solving the issue of a well 12 intersecting and pumping from one vertical, horizontal or inclined fracture in homogeneous 13 aquifers, but their domain of application—even if covering various fracture geometries—was 14 restricted to isotropic or anisotropic aquifers, whose potential boundaries had to be parallel or 15 orthogonal to the fracture direction. The issue thus remains unsolved for many field cases. For 16 example, a well intersecting and pumping a fracture in a multilayer or a dual-porosity aquifer, 17 where intersected fractures are not necessarily parallel or orthogonal to aquifer boundaries, 18 where several fractures with various orientations intersect the well, or the effect of pumping 19 not only in fractures, but also in the aquifer through the screened interval of the well. 20 Using a mathematical demonstration, we show that integrating the well-known Theis 21 analytical solution (Theis, 1935) along the fracture axis is identical to the equally well-known 22 analytical solution of Gringarten et al. (1974) for a uniform-flux fracture fully penetrating a 23 homogeneous aquifer. This result implies that any existing line-or point-source solution can 24 be used for implementing one or more discrete fractures that are intersected by the well. 25 Several theoretical examples are presented and discussed: a single vertical fracture in a dual-26 porosity aquifer or in a multi-layer system (with a partially intersecting fracture); one and two 27 inclined fractures in a leaky-aquifer system with pumping either only from the fracture(s), or 28 also from the aquifer between fracture(s) in the screened interval of the well. For the cases 29 Journal of Hydrology 559 (2018) 89–99 2 with several pumping sources, analytical solutions of flowrate contribution from each 30 individual source (fractures and well) are presented, and the drawdown behaviour according 31 to the length of the pumped screened interval of the well is discussed. Other advantages of 32 this proposed generic analytical solution are also given. 33 The application of this solution to field data should provide additional field information on 34 fracture geometry, as well as identifying the connectivity between the pumped fractures and 35 other aquifers. 36 To protect this original concept of a generic solution for modelling pumping tests in fracture

Analytical Solution for Modeling Discharge into a Tunnel Drilled in a Heterogeneous Unconfined Aquifer

International audiencePredicting transient inflow rates into a tunnel is an important issue faced by hydrogeologists. Most existing analytical solutions overestimate the initial discharge due to the assumption that drilling was instantaneous over the entire tunnel length. In addition, they assume a homogeneous system. An alternative model was recently developed for tunnels intersecting heterogeneous formations, but its application was reduced to the case of confined flow to deep tunnels in weakly diffusive aquifers. In this paper, we adapt existing analytical solutions for drainage systems to the specific case of a tunnel progressively drilled in a highly diffusive heterogeneous unconfined aquifer. The case of a tunnel overlying an impervious layer is analytically solved by applying the superposition principle, while the case of a tunnel constructed some distance above an impervious layer is solved by discretizing the tunnel length into subsectors. Both models can simulate transient discharge into a tunnel drilled at various speeds through a heterogeneous unconfined aquifer, and allow the prediction of discharge rates in shallow tunnels located in highly diffusive aquifers. We successfully applied this approach to a tunnel in heterogeneous volcanic rock

Improvement of Borehole Thermal Energy Storage Design Based on Experimental and Modelling Results

International audienceUnderground Thermal Energy Storage appears to be an attractive solution for solar thermal energy storage. The SOLARGEOTHERM research project aimed to evaluate the energetic potential of borehole thermal energy storage by means of a full-scale experimental device and heat transfer models. Analysis of the experimental data showed that a single borehole is not efficient for storage. Models showed that the heat transfer fluid in the geothermal probe lost 15 per cent of its energy at a depth of 100 m and 25 per cent at 150 m. A relation was established that enables comparison of the storage characteristic time of any vertical BTES to an optimum one. Finally, guidelines are formulated to optimise the design of vertical borehole fields with an objective of inter-seasonal heat storage