Milnes, Ellen

2023, Scherrer, Cyrille, Milnes, Ellen

L'objectif principal de cette thèse est de contribuer aux efforts humanitaires visant à fournir un approvisionnement en eau souterraine réactif, adapté et fiable aux populations déplacées vivant dans des camps de réfugiés à travers l'Afrique subsaharienne. De manière générale, une transition à grande échelle est en cours, de nombreuses zones rurales devenant périurbaines et urbaines, ce qui entraîne le passage d'un approvisionnement en eau basé sur des pompes à main, dispersées dans les villages, à des forages motorisés, centralisés et reliés à des réseaux de distribution. Avec ce changement de stratégie d'approvisionnement en eau, la nécessité d'atteindre des débits d'un ordre de grandeur supérieur à ceux généralement recherchés pour les pompes manuelles va de pair avec la nécessité de changer la stratégie d'exploration, de ‘forer là où sont les gens’ à ‘forer là où est l'eau’. Dans le contexte humanitaire, il est souvent impossible de réaliser des études hydrogéologiques complètes, car le temps est limité et les données sont rares. C'est dans ce contexte que cette recherche appliquée a été menée, visant à développer un outil pratique qui puisse informer rapidement les parties prenantes dans la planification des emplacements de forage des meilleurs endroits où forer tout en visant une exploitation durable des eaux souterraines. La première partie de cette thèse est consacrée au développement d'une méthodologie de cartographie qui permet de déterminer rapidement le potentiel en eau souterraine. Elle repose sur la superposition des deux variables principales pertinentes pour la quantification des eaux souterraines. Ces deux variables sont: la disponibilité en eau, qui est basée sur des unités de paysages hydrogéomorphologiques, et la capacité du réservoir, qui représente une approximation des propriétés hydrauliques. En retenant toujours la plus faible de ces propriétés, le potentiel en eau souterraine peut être défini en unités de potentiel. La méthodologie a été appliquée au cas du camp de réfugiés de Bidibidi (nord de l'Ouganda) et utilisée pour localiser huit nouveaux forages. La validation de la méthodologie a révélé que son application avait permis d’augmenter de manière significative le rendement moyen des forages de Bidibidi, justifiant alors son application à d’autres camps. Le processus de superposition de la méthodologie a permis de constater que la variable de la disponibilité en eau est presque toujours inférieure, mais dépendante, à celle de la capacité du réservoir, ce qui a conduit à proposer une révision de la méthodologie basée uniquement sur la cartographie de la disponibilité en eau. Cette version révisée a été validée en utilisant à nouveau l'étude de cas de Bidibidi, et a donné des résultats similaires. Ensuite elle a donc été exportée à quatorze autres camps de réfugiés situés dans des paysages régolithiques similaires en Afrique subsaharienne. Une comparaison des débits de forage avec les unités du potentiel en eau souterraine déterminés par la méthodologie a été effectuée pour tous les sites, révélant un haut degré de prédictibilité. Bien que l'approche révisée se soit avérée très utile et pratique pour visualiser la probabilité spatiale du potentiel en eau souterraine, elle ne comportait aucune information sur l'exploitation durable, notamment le nombre de forages à haut rendement qu'une unité pourrait contenir. Le manque d'informations sur l'exploitation durable des forages dans la méthodologie révisée a conduit à la deuxième phase de cette thèse. Cette phase est consacrée à l'exploration de la relation entre les unités de potentiel des eaux souterraines et les composantes du bilan hydrique, en particulier la recharge des eaux souterraines. Une analyse hydrogéomorphologique a été réalisée sur vingt bassins versants de référence, sélectionnés dans des contextes géologiques (régolithiques) et climatiques similaires de l'Afrique subsaharienne, pour lesquels toutes les composantes du bilan hydrique ont été collectées (c'est-à-dire incluant les données des stations de jaugeage) et pour lesquels les unités de potentiel en eau souterraine ont été traduites en unités hydrogéomorphologiques. Les contextes hydrogéologiques de ces environnements s'inscrivent tous dans le cadre de nappes phréatiques contrôlées par la topographie, où des unités hydrogéomorphologiques peuvent être associées à la dynamique des eaux de surface et des eaux souterraines. L'analyse du bilan hydrique à travers l'hydrogéomorphologie a abouti à une formulation empirique de la recharge basée sur les unités hydrogéomorphologiques cartographiées, les précipitations et l'évapotranspiration. Une fois cette approche disponible pour estimer la recharge dans tout bassin versant ayant des caractéristiques géologiques similaires, il a été possible d'introduire le concept de durabilité dans la méthodologie révisée. Afin de réaliser cela sur un cas réel, la recharge a été estimée en appliquant la relation empirique au cas de Bidibidi, pour lequel les unités hydrogéomorphologiques ont été obtenues directement par analogie avec les unités de potentiel en eau souterraine. Les précipitations et l'évapotranspiration ont été quant à elles obtenues à partir de produits de télédétection. La carte du potentiel des eaux souterraines de Bidibidi a ensuite été divisée en sous-bassins versants, dans lesquels l'extraction cumulée des forages motorisés a été comparée au potentiel durable en eau souterraine, défini comme un tiers de la recharge du sous-bassin versant. Cela a permis de cartographier le degré de durabilité de l'extraction des eaux souterraines dans les sous-bassins versants. Certains sous-bassins versants ont été identifiés comme étant dans un état de surexploitation, tandis que d'autres ont été identifiés comme étant dans un état où le développement des eaux souterraines pourrait être plus important. Pour refléter le potentiel durable des eaux souterraines de manière visuelle, des sections de drainage correspondant aux différentes extractions durables cumulées des sous-bassins versants ont été ajoutées à la carte obtenue avec la méthodologie révisée. Dans la dernière partie de cette thèse, la corrélation entre les unités hydrogéomorphologiques et les composantes du bilan hydrique, exprimée dans la relation empirique, a été abordée en formulant le modèle conceptuel par une approche analytique simplifiée et en appliquant cette solution aux vingt bassins versants de référence. La comparaison des résultats analytiques avec la solution empirique a suggéré une relation métaphysique entre les unités hydrogéomorphologiques, les termes du bilan hydrique et les propriétés hydrauliques, capturant ainsi certaines interactions complexes essentielles entre les paysages régolithiques et le climat. D'une part, cela soutient la solution empirique plus facile à mettre en œuvre et, d'autre part, cela a ouvert un large éventail de nouvelles perspectives, montrant la manière d'étendre ce type d'analyses hydrogéomorphologiques à d'autres paysages géologiques se trouvant dans des conditions climatiques diverses et changeantes.
Abstract
The main objective of this thesis is to contribute to humanitarian efforts to provide responsive and reliable groundwater supply to displaced populations living in refugee camps or settlements across Sub-Saharan Africa. A large-scale transition is underway from rural towards peri-urban and urban settings, hence, from scattered hand pumps towards centralized motorized boreholes connected to distribution networks. However, with this shift in water supply strategy, the need to achieve yields an order of magnitude higher than those typically sought for hand pumps goes hand in hand with the need for changing the exploration strategy from ‘drilling where the people are’ to ‘drilling where the water is’. However, comprehensive hydrogeological assessments can often not be carried out as time is limited and data are scarce. It is against this backdrop that this applied research has been carried out, aiming towards developing a practical tool which can rapidly inform stakeholders in planning the siting of boreholes and on sustainable groundwater exploitation. The first section of this thesis is dedicated to the development of a rapid groundwater potential mapping methodology (RGWPM). It is based on the overlay of the two main groundwater relevant variables, i.e. the water availability (WA) reflecting hydrogeomorphological landscape units and the reservoir capacity (RC) being a proxy of the hydraulic properties, always retaining the lowest to define the groundwater potential (GWP). The RGWPM methodology was applied to the real case-study of Bidibidi refugee settlement (Northern Uganda) and used to implement eight new boreholes. The cross-validation revealed that its application significantly increased the average yield, justifying further applications in other settlements and camps. The overlay process of the RGWPM methodology revealed that the WA variable is almost always inferior and dependent on the RC, leading to a proposed revision to the RGWPM, relying only on the WA mapping. The revised version was again evaluated using the Bidibidi case-study with similar results and was subsequently applied to fourteen different refugee camps situated in similar regolithic landscapes in Sub-Saharan Africa. A cross-validation of the borehole yield with the mapped RGWPM units was carried out for all sites, again revealing a high degree of predictability. Although the revised approach was very useful and practical in visualising the spatial probability for GWP, it did so far not include any information on sustainable exploitation, as for instance how many high-yielding boreholes one unit can accommodate. The lack of information on sustainable exploitation of boreholes in the revised RGWPM methodology led to the second part of the thesis, which is dedicated to the exploration of the relationship between RGWPM units and water balance components, in particular groundwater recharge. A hydrogeomorphological analysis was carried out on twenty reference catchments, selected in similar geological (regolithic) and climatic contexts in Sub-Saharan Africa, for which all water balance components were known (i.e. including data from gauging stations) and for which the revised RGWPM units were translated into hydrogeomorphological landscape (HGM) units. The hydrogeological frameworks of these environments all fall into the topography-driven water table settings, where HGM units can be associated with surface and groundwater dynamics. The hydrogeomorphological-water balance analysis resulted in an empirical formulation of groundwater recharge based on the mapped HGM units, precipitation, and evapotranspiration. Once there was an approach to estimate groundwater recharge in any catchment with similar geological characteristics it was possible to introduce the notion of sustainable exploitation into the revised methodology. To do this on a real case-study, groundwater recharge was estimated using the empirical relationship for the Bidibidi settlement, for which the HGM units were directly obtained by analogy with the RGWPM units, while precipitation and evapotranspiration were obtained from remote sensing products. The Bidibidi RGWPM map was subsequently divided into sub-catchments, within which the cumulative extraction from motorised boreholes was compared to the sustainable groundwater potential, defined as a third of the sub-catchment groundwater recharge. This allowed mapping of the degree of sustainability of groundwater extraction within the sub-catchments. Some sub-catchments were identified to be in a state of over-exploitation, while others were identified where further groundwater development could be envisaged. In order to translate the sustainable groundwater potential into a map, the drainage sections corresponding to different sustainable cumulative sub-catchment extractions were added to the revised methodology. In the last section of the thesis, the correlation between the HGM units and the water balance components, expressed in the empirical relationship, was addressed by articulating the conceptual framework by a simplified analytical approach, and applying the obtained solution to the twenty reference catchments. Comparing the analytical results with the empirical solution indeed suggested a meta-physically based relationship between the HGM units, the water balance components, and the hydraulic properties, thereby capturing some essential complex interactions between regolithic landscapes and climate. On the one hand, this supports the easier-to-implement empirical solution, and, on the other hand, it opened a wide range of new perspectives showing the path to expanding this type of hydrogeomorphological analysis to other geological landscapes, also under diverse and changing climatic conditions.

2011, Milnes, Ellen

Groundwater salinisation is a major groundwater contamination issue world-wide and can be caused by different processes, such as seawater intrusion, agrochemical pollution, geogenic contamination and irrigation-induced salinisation. In many areas, several salinisation processes are superimposed. Since remedial measures vary for different salinisation processes, correct identification is fundamental for adequate design of management strategies: different strategies may be required in one and the same aquifer, depending on which salinisation process is active where in the domain.

A simulation-based salinisation risk assessment methodology is proposed, based on the principle of linear superposition of total dissolved solutes in groundwater. In a first step, the measured bulk salinity distribution is used to calibrate a numerical groundwater flow and transport model, accounting for all identified salinisation processes. Then, the bulk salinity distribution is decomposed into different salinity components by adapting the boundary conditions, running a simulation for each salinisation process separately. These simulation results yield the necessary components to calculate the risk index distributions, which are a measure of the respective future potential salinity increase. Overlaying the risk index distributions with a defined threshold concentration reveals risk areas requiring remediation or conservation measures with respect to each process. The risk area maps resulting from this methodology are a promising tool for the design of groundwater management schemes. They condense relevant information from complex dynamic processes obtained from numerical simulations and visualise the results in simple and static maps, accessible to decision makers who are not familiar with groundwater dynamics.

The different steps of the salinisation risk assessment procedure are first described and illustrated on a synthetic example and then applied to a real aquifer system in Southern Cyprus (Akrotiri), where three major salinisation processes are superimposed.

2006, Milnes, Ellen, Perrochet, Pierre

Solute recycling from irrigation can be described as the process that occurs when the salt load that is extracted from irrigation wells and distributed on the fields is returned to the groundwater below irrigated surfaces by deep percolation. Unless the salt load leaves the system by means of drains or surface runoff, transfer to the groundwater will take place, sooner or later. This can lead to solute accumulation and thus to groundwater degradation, particularly in areas where extraction rates exceed infiltration rates (semi-arid and arid regions). Thus, considerable errors can occur in a predictive solute mass budget if the recycling process is not accounted for in the calculation. A method is proposed which allows direct simulation of solute recycling. The transient solute response at an extraction well is shown to be a superposition of solute mass flux contributions from n recycling cycles and is described as a function of the travel time distribution between a recycling point and a well. This leads to an expression for a transient ‘recycling source’ term in the advection–dispersion equation, which generates the effect of solute recycling. At long times, the ‘recycling source’ is a function of the local capture probability of the irrigation well and the solute mass flux captured by the well from the boundaries. The predicted concentration distribution at steady state reflects the maximum spatial concentration distribution in response to solute recycling and can thus be considered as the solute recycling potential or vulnerability of the entire domain for a given hydraulic setting and exploitation scheme. Simulation of the solute recycling potential is computationally undemanding and can therefore, for instance, be used for optimisation purposes. Also, the proposed method allows transient simulation of solute recycling with any standard flow and transport code.

, Falco, Pierik, Negro, François, Szalai, Sándor, Milnes, Ellen

The term “geoelectric null-array” is used for direct current electrode configurations yielding a potential difference of zero above a homogeneous half-space. This paper presents a comparative study of the behaviour of three null-arrays, midpoint null-array (MAN), Wenner-γ null-array and Schlumberger null-array in response to a fracture, both in profiling and in azimuthal mode. The main objective is to determine which array(s) best localise fractures or best identify their orientation.
Forward modelling of the three null-arrays revealed that the Wenner-γ and Schlumberger null-arrays localize vertical fractures the most accurately, whilst the midpoint null-array combined with the Schlumberger null-array allows accurate orientation of a fracture. Numerical analysis then served as a basis to interpret the field results. Field test measurements were carried out above a quarry in Les Breuleux (Switzerland) with the three null-arrays and classical arrays. The results were cross-validated with quarry-wall geological mapping. In real field circumstances, the Wenner-γ null-array proved to be the most efficient and accurate in localising fractures. The orientations of the fractures according to the numerical results were most efficiently determined with the midpoint null-array, whilst the Schlumberger null-array adds accuracy to the results. This study shows that geoelectrical null-arrays are more suitable than classical arrays for the characterization of fracture geometry.

2015, Blandenier, Lucien, Perrochet, Pierre, Milnes, Ellen, Brunner, Philip

Avec une population de près de 400'000 personnes, le camp de réfugiés de Dadaab est le plus grand au monde. Il est situé dans la région semi-aride de l’est du Kenya, proche de la frontière somalienne. L’unique ressource permanente en eau potable pour les réfugiés et les communautés locales provient de l’aquifère de Merti, qui consiste en une large lentille d’eau douce souterraine de 250 km sur 50 km, et qui est entourée par de l’eau salée. L’augmentation des volumes pompés, des signes d’augmentation de la salinité de l’eau et l’incertitude sur la diminution du niveau de l’eau souterraine ont fait prendre conscience du risque d’épuisement de la ressource. Ces observations ont abouti à la nécessité de mieux caractériser la recharge de l’aquifère ainsi que la dynamique entre la lentille d’eau douce et l’eau salée environnante.
Dans un premier temps, une nouvelle méthodologie a été développée pour quantifier la recharge concentrée à travers un modèle numérique, basé sur la physique des écoulements, couplant l’eau souterraine et l’eau de surface. Ce modèle a eu pour but de reproduire les surfaces inondées durant les crues et a été calibré à l’aide d’images satellites. Deuxièmement, la dynamique de la lentille d’eau douce a été investiguée par une série de modèles numériques synthétiques. Ces modèles ont permis d’analyser les effets des taux de recharge et leurs mécanismes (pluie, recharge concentrée) sur la géométrie de la lentille et de comparer ces géométries simulées avec la géométrie réelle de la lentille de l’aquifère de Merti. Ces deux approches ont été contre-validées à l’aide d’un réseau de monitoring de l’eau souterraine composé de vingt stations de mesures à haute résolution temporelle (15 min), installé sur toute la lentille en septembre 2013. Finalement, les résultats de ces trois axes de recherche ont été combinés dans un modèle numérique régional.
L’approche développée dans cette étude a permis de quantifier une recharge concentrée entre 195 et 329 x 10⁶ m³/a. La recharge diffuse sur la lentille d’eau douce est quant à elle estimée entre 12 et 62 x 10⁶ m³/a. Comparée à ces taux de recharge, l’extraction courante de l’eau souterraine (environ 4.8 x 10⁶ m³/a) est considérée comme durable à l’échelle de l’aquifère. Cependant, la recharge totale est environ 50 à 100 fois plus grande que l’inféroflux traversant le niveau exploité, calculé à l’aide du gradient hydraulique de l’aquifère et des valeurs de transmissivité issues des essais de pompage. Cet écart a mené à postuler la présence d’un aquifère multi-couche considérablement plus épais qu’uniquement l’horizon couramment exploité, mais avec une importante incertitude sur son épaisseur et sur le gradient vertical de salinité entre la lentille d’eau douce et l’eau salée sous-jacente.
Les modèles numériques ont révélé la très grande inertie de l’aquifère et ont également confirmé que la recharge a principalement lieu de manière concentrée lors des évènements de crue, juste en amont de la lentille d’eau douce. La grande inertie de l’aquifère est cohérente avec les faibles variations des niveaux d’eau et de la conductivité électrique (salinité) observées avec le réseau de monitoring.
En conclusion, ce travail ouvre de nouvelles perspectives pour la quantification de la recharge en milieu aride à semi-aride lors d’évènements de crue. Il a également permis de mettre en avant la nécessité de poursuivre le monitoring de l’aquifère et de mener de nouvelles investigations sur l’épaisseur de l’aquifère afin de confirmer les résultats de la recharge., The Dadaab refugee camp, the largest refugee camp in the world with a population of approximately 400’000 persons, is located in the arid to semi-arid eastern Kenya, close to the Somali border. The only permanent water resource for the refugees and the host communities comes from the Merti aquifer which consists in a large continental freshwater lens of 250 km by 50 km surrounded by salty water. The increasing groundwater abstractions as well as signs of increasing salinity and uncertainty on the water level depletion led to the necessity to better characterise the aquifer recharge and the dynamics between the freshwater lens and the surrounding salty water.
Firstly, a new methodology was developed for quantifying the concentrated groundwater recharge through a physically-based coupled surface/groundwater numerical model reproducing inundated surfaces during flood events which is calibrated with inundated surfaces derived from satellite images. Secondly, the dynamics of freshwater lenses are investigated with a series of synthetic numerical models. These models aim to analyse the effect of the recharge rates and mechanisms (rainfall, concentrated recharge) on the freshwater lens geometries and to compare these geometries with the observed geometry of the freshwater lens of the Merti aquifer. These two approaches are cross-validated owing to a groundwater monitoring network of twenty high time-resolution devices installed over the whole freshwater lens in September 2013. Finally, results from these three axes are combined in a regional numerical model.
The approaches developed in this study allowed to quantify a concentrated groundwater recharge to be between 195 and 329 x 10⁶ m³/y. Diffuse recharge contributing to the freshwater lens is estimated to be between 12 and 62 x 10⁶ m³/y. Compared to these recharge rates, the current groundwater extraction (about 4.8 x 10⁶ m³/y), is considered as sustainable on the regional scale. However, this recharge is about 50 to 100 higher than the axial flow estimated with the gradient and the transmissivities obtained with pumping tests. This discrepancy led to postulate the presence of a multi-layer aquifer much thicker than the currently exploited horizon but with uncertainties on its thickness and the vertical salinity gradient.
The synthetic numerical model revealed a very high inertia of the Merti aquifer and confirmed that the recharge of the aquifer is mainly controlled by concentrated recharge on flood plains in the upstream area of the freshwater lens. The high inertia of the aquifer is consistent with the very small groundwater level and electrical conductivity variations observed with the monitoring network.
As conclusion, this work opens new perspectives for the quantification of groundwater recharge in arid to semi-arid areas occurring during large scale flood events. However, it also showed the necessity to continue the monitoring of the aquifer and to carry out further investigations on the aquifer thickness if further exploitations are foreseen.

2008, Goldscheider, Nicola, Milnes, Ellen, Fries, W., Joppen, M.

In einem Porengrundwasserleiter aus Niederterrassenschottern bei Pratteln, Schweiz, wurde ein Markierungsversuch mit Uranin durchgeführt, um die Gefährdung eines Trinkwasserbrunnens durch ein 760 m talaufwärts gelegenen Ablagerungsstandort zu bewerten. Dabei wurde eine maximale Abstandsgeschwindigkeit von 127 m/d und ein Wiedererhalt von 0,93 % festgestellt. Der Tracer wurde auch in zwei intermediären Beobachtungsbrunnen nachgewiesen. Diese Studie diskutiert die Ursachen dieser hohen Fließgeschwindigkeit. Durch drei verschiedene analytische Modelle konnten die Durchgangskurven simuliert und Transportparameter bestimmt werden. Erst die Anwendung eines zweidimensionalen numerischen Modells (FEFLOW) mit vereinfachter Geometrie liefert aber eine hydrogeologisch konsistente, mögliche Erklärung aller Versuchsergebnisse. Demnach handelt es sich vermutlich um einen relativ homogenen Grundwasserleiter. Der steile Gradient (6 ‰) und die hohe Durchlässigkeit (3 • 10^–2 m/s) verursachen die hohen Fließgeschwindigkeiten. Der Hauptteil der Tracerwolke strömt seitlich an den Brunnen vorbei. Durch später durchgeführte Kleinpumpversuche wurde diese Modellvorstellung weitgehend bestätigt. Diese Befunde sollten bei Schutz- und Sanierungskonzepten berücksichtigt werden., A tracer experiment with uranine in a gravel aquifer aimed to assess the risk of a drinking water well near Pratteln, Switzerland, resulting from a contaminated site 760 m further upgradient. The experiment revealed a maximum linear flow velocity of 127 m/d and a mass recovery of 0.93 %. The tracer was also detected at two intermediate monitoring wells. This paper discusses the causes of this high flow velocity. Three different analytical models allowed simulation of the breakthrough curves and determination of transport parameters. A two-dimensional numerical model (FEFLOW) with simplified geometry provided a hydrogeologically consistent and probable explanation of all results. The aquifer is most likely relatively homogeneous, the steep hydraulic gradient (6 ‰) and high conductivity (3 • 10^–2 m/s) cause high flow velocities, and most of the tracer passed northeast of the wells. Recently conducted small scale pumping tests largely confirmed this conceptual model. These findings should be considered for future protection measures.

2005, Milnes, Ellen, Perrochet, Pierre

The main objective of this thesis is the quantitative investigation of groundwater salinisation induced by solute recycling from irrigation, and its implications for the overall salinisation in coastal settings. Since the modelling approaches proposed in literature to simulate seawater-intruded areas rarely account for the coupled and superimposed effects of solute recycling and seawater intrusion, simulation procedures have been developed, to evaluate the impact of salinisation induced by seawater intrusion and solute recycling. The problem of solute recycling is identified and illustrated on an example from Cyprus, the Kiti aquifer, where field investigations suggested that the observed salinity distribution is not only related to seawater intrusion, but also to solute recycling. Two numerical simulation scenarios were carried out, with and without solute recycling. The simulation scenario with solute recycling led to a wide saline zone inland, which compared well with field observations and indicates that considerable errors may occur in a predictive solute mass budget if the recycling process is not accounted for in the calculation. A mathematical description of the solute recycling process is first carried out for a 1-D advective system and then extended to arbitrary advective-dispersive systems by means of the transfer function theory. This yields a formulation for the transient solute mass flux at an irrigation well, which is obtained from the solute mass flux captured by the well from the boundaries and the recycling transfer function (RTF). The RTF is derived from the sum of the n-fold convolutions of the travel time probability density function between the irrigated surface and the extraction well. This allows definition of a distributed 'recycling source' in the general form of the advection-dispersion equation. The solute recycling process is thereby reduced to a simple flow and transport problem, allowing evaluation of the effect of solute recycling on spatial groundwater salinisation with any standard groundwater simulation code for average steady state hydraulic conditions. At late times, the 'recycling source' is a function of the capture zone probability and the lateral solute mass flux only and yields the salinisation potential, which describes the maximum salinity distribution that will be attained for the given hydraulic setting in response to solute recycling. The effect of transient hydraulic conditions on groundwater salinisation induced by solute recycling is solved numerically in a time-stepping procedure. Then, a framework for a process-based salinisation risk assessment methodology is proposed in which seawater intrusion and solute recycling salinisation are evaluated separately. By decomposing the overall salinity into a seawater intrusion and solute recycling component, a salinisation risk index is defined as the potential of further salinisation with respect to either salinisation process. The risk index is obtained by comparing the respective 'present state' salinisation with the salinisation potential. The obtained risk index maps reveal areas prone to further salinity increase due to solute recycling and seawater, respectively. In the last section, a 3-D finite element model, reflecting the main features of another aquifer in Cyprus, the Akrotiri aquifer, was used as a 'hypothetical' reality to illustrate the proposed salinisation risk assessment procedure. The results obtained from the simulations indicate zones running danger of further salinisation with respect to solute recycling and seawater intrusion, which correlate with the spatial distribution of the dominant salinity sources derived from field investigations. But they also revealed that data essential for calibration and cross-validation related to solute recycling is rarely monitored in coastal aquifers. This leads to a discussion on the qualitative estimation of key-factors, identified during the mathematical analysis of the solute recycling process, based on classical hydrogeological data. Such estimations can be a preliminary and inexpensive field approach to identify areas potentially endangered by solute recycling, indicating where the installation of monitoring networks would be advisable in order to obtain the data necessary for a quantitative salinisation risk assessment.

2013, Falco, Pierik, Perrochet, Pierre, Milnes, Ellen

Le terme "géoélectrique null-array" est utilisé pour les configurations géoélectriques qui mesurent une différence de potentiel nulle sur un espace homogène. Ce papier présente une étude du comportement de trois de ces null-arrays - le midpoint null-array, le Wenner-γ null-array et le Schlumberger null-array - sur différentes structures verticales, en mesure en profil et en mesure azimutale. Une analyse numérique est présentée avec une approche qui utilise un logiciel de calcul hydrodynamique, en utilisant l'analogie entre les théories hydrodynamiques et électriques. Cette analyse numérique concorde avec les mesures de terrain effectuées avec les méthodes géoélectriques classiques et les null-arrays. Une campagne de terrain a été effectuée sur une carrière aux Breuleux (Jura, Suisse) afin de détecter les fractures et failles verticales ainsi que leurs orientations, qui pouvaient aussi être déterminées et mesurées sur le mur de la carrière. Une autre campagne a été menée à Vers chez le Brandt (Neuchâtel, Suisse) pour localiser la grotte et déterminer son orientation. Une dernière campagne à été menée sur la plaine de Buix (Jura, Suisse) dans le but de détecter et caractériser la position et l'orientation de chenaux. Cette étude a montré que certains null-arrays donnaient des résultats plus précis et sont mieux adaptés pour détecter et caractériser précisément ces structures verticales que d'autres méthodes classiques. Le Wenner-γ null-array a montré qu'il est le plus efficient pour la détection de structures verticales et plus sensible que les autres null-arrays. Le Schlumberger null-array donne aussi de bons résultats, mais sa sensibilité est légèrement plus faible que pour le premier. De plus, cette configuration est nettement moins rapide à mettre en oeuvre que les autres. Le midpoint null-array est le plus efficace pour la détermination de l'orientation des structures, alors que le Schlumberger null-array permet d'ajouter de la précision à la mesure. Finalement, cette étude a mené à l'élaboration de conseils d'application de ces méthodes, comme le choix des configurations ou les dimensions à utiliser en pratique., The term "geoelectric null-array" is used for those geoelectrical configurations measuring a zero potential difference above a homogenous half-space. This thesis presents a study of the behaviour of three of these null-arrays - the midpoint null-array, the Wenner-γ null-array and the Schlumberger null-array - above different vertical structures, using profiling and azimuthal measurements. A numerical analysis is presented with an approach which uses a hydrodynamic software based on the analogy between hydrodynamic and electric theories. This numerical analysis provides similar results to the field data acquired with the classical and null-array geoelectrical methods. Field measurements have been carried out on a quarry located in Les Breuleux (Jura, Switzerland) with the aim to detect vertical fractures and faults as well as to determine their orientations. Other measurements were performed in Vers chez le Brandt (Neuchâtel, Switzerland) with the aim to localise a cave and determine the orientation. A last data set was acquired at Buix (Jura, Swizerland) with the aim to detect and characterise the position and orientation of channels. These studies showed that some of these null-arrays provided more accurate results and are more capable of detecting and characterising precisely these vertical structures than other classical methods. The Wenner-γ null-array was the most efficient for the detection of vertical structures and the most sensitive of the three null-arrays. The Schlumberger null-array provided good results, but the sensitivity was slightly lower. Moreover, this configuration was clearly more time-consuming than the others. The midpoint null-array is the best to detect orientation of structures, while the Schlumberger null-array added accuracy to the measurements. Finally, this thesis led to the elaboration of advices for a field application of the methods, as the choice of the array or of the array size.

2007, Milnes, Ellen, Perrochet, Pierre

A theoretical framework is presented that allows direct identification of a single point-source pollution location and time in heterogeneous multidimensional systems under known flow field conditions. Based on the concept of the transfer function theory, it is shown that an observed pollution plume contains all the necessary information to predict the concentration at the unknown pollution source when a reversed flow field transport simulation is performed. This target concentration C₀ is obtained from a quadratic integral of the observed pollution plume itself. Backwards simulation of the pollution plume leads to shrinkage of the C₀-contour due to dispersion. When the C₀-contour reduces to a singular point, i.e. becomes a concentration maximum, the position of the pollution source is identified and the backward simulation time indicates the time elapsed since the contaminant release. The theoretical basis of the method is first developed for the ideal case that the pollution plume is entirely known and is illustrated using a synthetic heterogeneous 2D example where all the hydro-dispersive parameters are known. The same example is then used to illustrate the procedure for a more realistic case, i.e. where only few observation points exist.

2004, Milnes, Ellen, Renard, Philippe

In coastal aquifers which are exploited for agricultural purposes, salinisation by salt recycling from irrigation is superimposed on the effects of seawater intrusion. Water quality degradation of irrigation pumping wells caused by seawater intrusion further enhances salinisation by irrigation, as the extracted solute mass is recycled and is not withdrawn from the system.

The main objective of this study is the investigation and quantification of the impact of solute recycling from irrigation relative to seawater intrusion. A solute mass budget was established by expressing the solute mass return flow as fraction of the extracted solute mass from wells by means of a solute mass return flow ratio (r_r). The obtained expression for the relative contribution of solute recycling from irrigation is an exponential function of the return flow ratio r_r and normalised time only (time versus system turnover time).

This expression was applied to an example, the Kiti aquifer (Southern Cyprus), where field observations suggest that solute return flow is a super-imposed salinisation mechanism. The contribution from solute recycling normalised with the solute mass flux entering from the sea after 20 years was found to be 1.5–8.5% in the extracted solute mass flux, depending on the estimation of the system turnover time.

Subsequently, a coupled finite element model, reflecting the main features of the Kiti aquifer was used as a possible ‘synthetic reality’, to test the relative impact of solute recycling on the spatial salinity distribution in a complex hydrogeological and geometrical setting. This was done by running two simulation scenarios : (1) recycling all the extracted solute back into the system and (2) leaving solute recycling aside and comparing the results of these two scenarios relative to each other and to patterns observed in the field. The results showed, that by introducing solute recycling into the numerical model as coupled boundary condition does not only respect the overall solute mass balance but can have an important impact on the salinity distribution, leading to a significant spreading of the mixing zone, similar to what was observed in the field.