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- PublicationAccès libreModelling groundwater salinisation in irrigated coastal areas: from solute recycling concepts to quantitative risk assessmentThe 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.
- PublicationAccès libreFracture characterisation using geoelectric null-arraysThe 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.
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- PublicationAccès libreMarkierungsversuche und Modellierung zur Bewertung der Gefährdung eines Trinkwasserbrunnens(2008)
;Goldscheider, Nicola ; ;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.
- PublicationAccès libreSimultaneous identification of a single pollution point-source location and contamination time under known flow field conditionsA 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.
- PublicationAccès libreDirect simulation of solute recycling in irrigated areasSolute 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.
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