Stagnation Points as Loci of Solute Concentration Extrema at the Evaporative Surface of a Random Porous Medium

Evaporation of a saline solution from a porous medium often leads to the precipitation of salt at the surface of the porous medium. It is commonly observed that the crystallized salt does not form everywhere at the porous medium surface but at some specific locations. This is interpreted at the signature of spatial variations in the salt concentration at the surface of the porous medium prior to the onset of crystallization. We explore numerically the link between the ion concentration spatial variations at the surface and porous medium heterogeneities considering strongly anisotropic short-range correlated permeability Gaussian fields corresponding to a vertical layering perpendicular to the top evaporative surface for the case of the evaporation–wicking situation. It is shown that the ion concentration extrema at the surfaces correspond to stagnation points with minima corresponding to divergent stagnation points and maxima to convergent stagnation points. Counter-intuitively, the ion concentration maxima are shown to correspond to permeability minima. However, the ion concentration absolute maximum does not necessarily always correspond to the permeability absolute minimum. More generally, the study emphasizes the key role played by the impact of heterogeneities on the velocity field induced in the medium by the evaporation process. It is also shown that the number of ion mass fraction maxima at the porous medium surface is generally much lower than the naive prediction based on the number of correlation lengths spanning the medium

Distribution, threats and protection of selected karst groundwater-dependent ecosystems in the Mediterranean region

Karst groundwater-dependent ecosystems (KGDEs) in the Mediterranean region are important in terms of ecosystem services and biodiversity but are increasingly under anthropogenic pressures and climate-change constraints. For this study, the ecohydrological characteristics, threats, and protection status of 112 selected KGDEs around the Mediterranean Sea, including caves, springs, rivers and wetlands, were evaluated, based on local expert knowledge and scientific literature. Results demonstrate that KGDEs contribute considerably to regional biodiversity. The diversity of karst landscapes, combined with the groundwater emergence at springs, leads to exceptional habitat diversity, particularly in arid climates, where KGDEs serve as a refuge for species that could not thrive in the surrounding environment. The most common threats identified among the selected sites are direct human disturbances, such as mass tourism or overfishing, water-quality deterioration and water shortage from aquifer overdraft and/or climate change. Although most of the selected sites are under protection, conservation measures are frequently insufficient. Such shortcomings are often caused by poor data availability, little knowledge on conservation needs of invertebrate species, and conflicts of interest with the local population. For this purpose, it is necessary to raise environmental awareness and promote interdisciplinary research, in order to monitor water quality and quantity in addition to the status of the biocenoses

Impact of mixing induced calcite precipitation on the flow and transport

In several hydrogeochemical situations, there is an interaction between the solid matrix and the carbonate species according to calcite dissolution and precipitation reactions (evaporate deposition in Sebkha and Chotts, calcite dissolution in seawater–freshwater mixing zone). The precipitation of such rocks can easily induce a development of the porosity and permeability as a result of the mixing processes of two different solutions. The purpose of this work is to present a relatively complete modeling tool to evaluate the porosity decrease during calcite precipitation and their impacts on the flow and transport. The modeling of such problem requires a set of highly nonlinearly coupled equations. GEODENS code, used in this work, can solve these equations by a finite element procedure. It can handle geochemical reactions such as mineral dissolution-precipitation reaction. The coupled model iteratively calculates the fluid pressure, the ion concentrations and the quantities of the different salts that may precipitate or dissolve in the domain over time. The porosity is calculated by considering the pore volume invaded during calcite precipitation in every node and each time step. The new porosity value is used to update the permeability using empirical model. The new hydrodynamic distribution is the new inputs for the next time step to solve the flow and reactive transport problem

Spatial and Temporal Variations of Water Quality of Mateur Aquifer (Northeastern Tunisia): Suitability for Irrigation and Drinking Purposes

The present study aims to assess the spatial and temporal variations of the hydrochemical characteristics of Mateur aquifer groundwaters, a crucial water resource in the northeast of Tunisia. The aquifer was subject to water quality deterioration due to salinization and nitrate contamination, and a new assessment of water quality was needed. For this purpose, 40 groundwater samples were collected during wet and dry seasons and analyzed for salinity, pH, T, O2, major cations and anions, and nutrient elements using standard methods and Water Quality Index (WQI). The results showed that most of the groundwater parameters were not within the permissible limits set by the World Health Organization in both seasons. The geochemical data were interpreted using WQI for drinking water. The spatial distribution maps of Water Quality Index showed that the highest quality was found, during both seasons, in the northwest and the southeast part of the aquifer, corresponding to the recharge zone, whereas the poor and very poor water quality was found in the outflow part of the aquifer. According to sodium adsorption ratio (SAR) and Na% values, most of the groundwater samples were not suitable for irrigation purposes and characterizing the eastern part of the aquifer and the outflow part of the aquifer, around the Ichkeul marshes

Suitability evaluation of groundwater from the Skhira coastal aquifer of east-central Tunisia for use as drinking water

The Skhira coastal aquifer is located in east central Tunisia. It is already affected by the intrusion of saline water phenomena. This study evaluated the suitability of aquifer groundwater various using data on chemical and physical data from 31 wells for the decade from 2000-10. Salinity increases in the direction of flow and exceeds 10 g/L in the northeast region. Groundwater composition is controlled primarily by the concentrations of sodium, chlorides and sulphates. The majority of the groundwater samples are not potable

Modélisation numérique de l'écoulement et du transport en milieux poreux (Application de la méthode des lignes aux écoulements densitaires)

L objectif principal de ce travail est de développer des outils numériques efficaces et fiables pour la modélisation de l écoulement et du transport de masse en milieux poreux saturé. Dans une première partie, nous avons développé une technique pour améliorer la méthode EFMH sur les quadrangles. Cette technique a permis d éliminer les oscillations rencontrées en utilisant la méthode EFMH standard pour des solutions à front raide et de gagner en précision et en temps de calcul. Dans la deuxième partie de la thèse un nouveau modèle numérique pour résoudre le systèmes d équations couplées écoulement transport a été développé. Ce modèle couple des discrétisations spatiale et temporelle efficaces en utilisant la méthode des lignes. Pour la discrétisation spatiale, nous avons combiné la méthode EFMHc pour l écoulement, la méthode MPFA pour le terme dispersif et la méthode DG pour le terme convectif de l équation de transport. Pour la discrétisation temporelle du système EDO résultant, deux solvers basés sur le schéma à ordre et pas de temps adaptatifs, BDF, sont employés. DLSODIS (resp, DASPK), utilise une méthode directe (resp, méthode itérative) pour résoudre les systèmes linéaires surgissant à chaque itération de Newton. Dans un premier temps, deux cas tests numériques standards aussi bien qu une expérience à l échelle du laboratoire sont simulés pour mettre en relief l efficacité et la robustesse du modèle basé sur DLSODIS. Dans un second temps, le solver DASPK a été utilisé pour traiter les systèmes de grande échelle. Le nouveau modèle permet de contrôler l erreur temporelle et de réduire plus que 30 fois le temps de calcul par rapport au modèle standard.The aim of this work is to develop robust numerical models for modelling flow and mass transfer in porous media. In the first part of this thesis, we developed a technique for improving the accuracy of quadrangular Mixed Hybrid Finite Elements. This technique allows to avoid non physical oscillations met when solving stiff problems with standard MHFE and to improve precision and CPU time. In the second part, we developed a robust numerical model with efficient advanced approximations for both spatial and temporal discretizations to solve the system of coupled equations flow-transport using the method of lines (MOL). For the space discretization, we combined the MHFEc method for the flow, MPFA for the dispersive term and DG for the advection term of the transport equation. For the temporal discretization of the resulting ODE system, tow solvers based on Backward Differential Formulas (BDF) methods, are used. DLSODIS (resp, DASPK), uses a direct method (resp, iterative method) for solving linear systems arising in each Newton iteration. With simulation of standard numerical test cases as well as a laboratory scale experiment we show, firstly, the efficiency, accuracy and robustness of the model based on DLSODIS solver. Secondly, the model based on DASPK solver is very suitable for solving large scale density driven flow systems. The new model allows temporal error control and reduces more than 30 times the computing time compared to the standard model.STRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceF

A water balance approach for quantifying subsurface exchange fluxes and associated errors in hill reservoirs in semiarid regions

International audienceHill reservoirs are rain water-harvesting structures that have been increasingly adopted in arid and semi-arid regions, such as North Africa, to capture and conserve runoff water and for use as alternative water resources in agricultural development. Currently, process-based information on reservoir hydrology is needed to improve reservoir management practices. The study aims to develop an approach to estimate the reservoir-subsurface exchange flux and its associated error at the annual, monthly, and intra-monthly time scales to better understand the hydrological functioning and dynamics of hill reservoirs. This approach is based on a hydrological water balance of the hill reservoir by considering all water input and output fluxes and their associated errors. The results demonstrate the ability and relevance of the approach in estimating the net reservoir-subsurface exchange flux and its error estimations at various time scales. Its application on the Kamech catchment (Northern Tunisia) for the 2009-2012 period demonstrates that the net reservoir-subsurface exchange flux is positive, i.e. the infiltration from the hill reservoir to the aquifer dominates over the discharge from the aquifer to the reservoir. Moreover, reservoir-subsurface exchange constitutes the main output component in the water balance

Analyse de la dynamique de la nappe superficielle d’un bassin versant élémentaire

MasterL’eau est indispensable à la vie des organismes vivants et de l’homme. Sous toutes ses formes, elle joue un rôle fondamental dans la plupart des processus physicochimiques qui affectent la croûte terrestre Par conséquent, il est nécessaire de raisonner et d’optimiser la gestion des ressources en eau, notamment dans les zones arides et semi-arides. Cela passe entre autres priorités par la caractérisation de la dynamique hydrologique des nappes phréatiques et lacompréhension des processus de recharge de ces nappes.Le présent travail a pour objectif de contribuer à l’étude de la dynamique des nappes superficielles en milieux sub-humides et semi-arides en se focalisant sur leur recharge. Plus spécifiquement, l’objectif est de comprendre les facteurs (climatiques, topographiques, pédologiques) conditionnant la recharge des nappes de subsurface. Ce travail s’appuie sur les données hydrologiques, piézométriques, climatiques et pédologiques issues du bassin versant de Kamech (Cap Bon, Tunisie) appartenant à l’observatoire OMERE.Pour atteindre cet objectif et bien étudier la recharge de la nappe et les facteurs susceptibles de l’affecter, nous avons suivi trois étapes. :La première étape, nous avons déterminé les caractéristiques des événements de recharge (amplitude de montée de nappe suite à la pluie, temps de montée, temps de réaction par rapport à la pluie,) à partir de chroniques piézométriques longues de 12 ans acquises à une fréquence de 5 min à 10min et de données pluviométriques journalières. Compte tenu de la masse de données piézométriques, nous avons réfléchi à l’automatisation de la détermination des caractéristiques des événements de recharge. Pour cela, nous avons contribué à la conception et au test de scripts R permettant leurs détections automatiques.Dans une deuxième étape, les facteurs de contrôle ont été calculés pour conduire une analyse de corrélation entre les caractéristiques de recharge et les facteurs de contrôle. Dans une troisième étape, une analyse de corrélation statistique a été réalisée entre d’un côté, les caractéristiques de la réaction de la surface piézométrique aux pluies (temps de réaction, amplitude de fluctuation…), et de l’autre des caractéristiques hydroclimatiques (cumul de pluie, intensité maximale de pluie, indice des pluies antérieures, profondeur initiale de la nappe)Ce développement a permis d’identifier les caractéristiques et les facteurs clé de la recharge des nappes est les corrélations existantes entres eux

Numerical exploration of the dynamics of infiltration in hill reservoir

International audienc