Fuzzy logic modeling of the resistivity parameter and topography features for aquifer assessment in hydrogeological investigation of a crystalline basement complex

A linguistic fuzzy logic system (LFLS)-based expert system model has been developed for the assessment of aquifers for the location of productive water boreholes in a crystalline basement complex. The model design employed a multiple input/single output (MISO) approach with geoelectrical parameters and topographic features as input variables and control crisp value as the output. The application of the method to the data acquired in Khondalitic terrain, a basement complex in Vizianagaram District, south India, shows that potential groundwater resource zones that have control output values in the range 0.3295–0.3484 have a yield greater than 6,000 liters per hour (LPH). The range 0.3174–0.3226 gives a yield less than 4,000 LPH. The validation of the control crisp value using data acquired from Oban Massif, a basement complex in southeastern Nigeria, indicates a yield less than 3,000 LPH for control output values in the range 0.2938–0.3065. This validation corroborates the ability of control output values to predict a yield, thereby vindicating the applicability of linguistic fuzzy logic system in siting productive water boreholes in a basement complex.     

Numerical simulation of groundwater flow in regional rock aquifers, southwestern Quebec, Canada

The St. Lawrence Lowlands platform, Quebec, Canada, is a densely-populated area, heavily dependent on groundwater resources. In 1999, the Geological Survey of Canada initiated a large-scale hydrogeological assessment study over a 1,500 km² region northwest of Montreal. The objectives were to define the regional groundwater flow, and to give quantitative estimates of the groundwater dynamic parameters and of the available groundwater resources. The applied approach consisted of defining the hydrogeologic framework, hydraulic properties of the aquifer units, and groundwater dynamic components. Lower Paleozoic sedimentary rocks represent regional aquifer units. Coarse Quaternary fluvio-glacial sediments locally overlay the rock sequence and constitute an interface aquifer unit. Fine marine sediments confine most of the regional aquifers. Collected GIS based information was synthesized in a finite element numerical model. The regional saturated steady-state flow was calibrated under current stress conditions assuming an equivalent porous medium approach. Water budget calculations show that the total groundwater flow in regional aquifers amounts to 97.7 Mm³/y. Infiltration from precipitation provides 86.6% of the groundwater supply, while 9.6% comes from subsurface inflow and the remaining 3.8% is induced recharge from surface waters. Discharge from regional aquifers occurs through flow to streams (76.9%), groundwater withdrawal (18.4%), and underground outflow (4.7%).

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Resumen La plataforma de Tierras Bajas San Lorenzo, Quebec, Canadá, es un área densamente poblada que depende fuertemente de recursos de agua subterránea. En 1999 el Servicio Geológico de Canadá inició un estudio de evaluación hidrogeológica a gran escala sobre un área de 1,500 km² en la región noroeste de Montreal. Los objetivos fueron definir el flujo regional de agua subterránea y aportar estimados cuantitativos de los parámetros dinámicos de agua subterráne y de los recursos disponibles de agua subterránea. El enfoque aplicado consistió en definir el marco hidrogeológico, propiedades hidráulicas de las unidades acuíferas, y los componentes dinámicos de agua subterránea. Rocas sedimentarias del Paleozoico Inferior representan unidades regionales de acuíferos. Sedimentos fluvio-glaciares Cuaternarios gruesos sobreyacen localmente la secuencia rocosa y constituyen una unidad acuífera interfacial. Sedimentos marinos finos confinan la mayoría de acuíferos regionales. Información colectada basada en SIG se sintetizó en un modelo numérico de elemento finito. El flujo regional saturado en régimen permanente se calibró bajo condiciones de stress asumiendo un enfoque de medio poroso equivalente. Los cálculos de balance hídrico muestran que el flujo total de agua subterránea en acuíferos regional alcanza 97.7 Mm³/año. Infiltración a partir de lluvia aporta el 86.6% del abastecimiento al agua subterránea, mientras que el 9.6% proviene de entradas subsuperficiales y el restante 3.8% consiste de recarga inducida a partir de aguas superficiales. La descarga proveniente de acuíferos regionales ocurre a través de flujo a ríos (76.9%), utilización de agua subterránea (18.4%), y salida subterránea (4.7%).

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Résumé La plateforme Lowllands du Saint-Laurent, Québec, Canada, est une aire densément peuplée, dépendant grandement des ressources en eau souterraine. En 1999, le Service Géologique du Canada a initié une étude hydrogéologique à grande-échelle sur 1500 km² au Nord-Ouest de Montréal. Le sobjectifs ont été de définir la dynamique de lécoulement régional, et de donner des estimations quantitatives des paramètres dynamiques des ressources disponibles en eau souterraine. Lapproche utilisée consista à définir le cadre hydrogéologique de travail, les propriétés hydrauliques des unités aquifères, et les composantes dynamiques des eaux souterraines. Les roches sédimentaires du Paléozoïque Inférieur représentent les unités aquifères régionales. Les sédiments marins fins confinent la plus grande partie des aquifères régionaux. Les informations de base, collectées dans un SIG, ont été synthétisées dans un modèle numérique aux éléments finis. Lécoulement permanent régional, en zone saturée, a été calibré en conditions de stress en assumant une approche de milieu poreu équivalent. Les calculs du bilan hydrologique montrent que lécoulement total des eaux souterraines dans les aquifères régionaux atteind 97,7 Mm³/an. Linfiltration à partir des précipitations apporte 86.6% de leau souterraine exploitée; sachant que 9,6% proviennent découlement de subsurfaces, et que les 3,8% restants proviennent de la recharge via les eaux de surface. Le débit pompé des aquifères régionaux apparaît à travers lécoulement des cours deau, le rabattement des eaux souterraines (18.4%), et lécoulement ascendant (4.7%).

     

Characterization of the shallow groundwater system in an alpine watershed: Handcart Gulch,Colorado, USA

Water-table elevation measurements and aquifer parameter estimates are rare in alpine settings because few wells exist in these environments. Alpine groundwater systems may be a primary source of recharge to regional groundwater flow systems. Handcart Gulch is an alpine watershed in Colorado, USA comprised of highly fractured Proterozoic metamorphic and igneous rocks with wells completed to various depths. Primary study objectives include determining hydrologic properties of shallow bedrock and surficial materials, developing a watershed water budget, and testing the consistency of measured hydrologic properties and water budget by constructing a simple model incorporating groundwater and surface water for water year 2005. Water enters the study area as precipitation and exits as discharge in the trunk stream or potential recharge for the deeper aquifer. Surficial infiltration rates ranged from 0.1–6.2×10^?5 m/s. Discharge was estimated at 1.28×10^?3 km³. Numerical modeling analysis of single-well aquifer tests predicted lower specific storage in crystalline bedrock than in ferricrete and colluvial material (6.7×10^?5–2.0×10^?3 l/m). Hydraulic conductivity in crystalline bedrock was significantly lower than in colluvial and alluvial material (4.3×10^?9–2.0×10^?4 m/s). Water budget results suggest that during normal precipitation and temperatures water is available to recharge the deeper groundwater flow system.     

Numerical modeling of stress effects on solute transport in fractured rocks

The effects of stress/deformation on fluid flow and contaminant transport in fractured rocks is one of the major concerns for performance and safety assessments of many subsurface engineering problems, especially radioactive waste disposal and oil/gas reservoir fields. However, very little progress has been made to study this issue due to difficulties in both experiments and numerical modeling. The objective of this study is to systematically investigate the influence of stress on solute transport in fractured rocks for the first time, considering different stress and hydraulic pressure conditions. A hybrid approach combining discrete element method (DEM) for stress-flow simulations and a particle tracking algorithm is developed. The impact of matrix diffusion (diffusion of molecular size solutes in and out of the rock matrix, and sorption onto the surface of micropores in rock matrix) is also included. The numerical results show that stress not only significantly changes the solute residence time through the fracture networks, but also changes the solute travel paths. Matrix diffusion plays a dominant role in solute transport when the hydraulic gradient is small, which is often encountered in practice.     

Sensitivity analysis of hydrological parameters in modeling flow and transport in the unsaturated zone of Yucca Mountain, Nevada, USA

The unsaturated fractured volcanic deposits at Yucca Mountain in Nevada, USA, have been intensively investigated as a possible repository site for storing high-level radioactive waste. Field studies at the site have revealed that there exist large variabilities in hydrological parameters over the spatial domain of the mountain. Systematic analyses of hydrological parameters using a site-scale three-dimensional unsaturated zone (UZ) flow model have been undertaken. The main objective of the sensitivity analyses was to evaluate the effects of uncertainties in hydrologic parameters on modeled UZ flow and contaminant transport results. Sensitivity analyses were carried out relative to fracture and matrix permeability and capillary strength (van Genuchten α) through variation of these parameter values by one standard deviation from the base-case values. The parameter variation resulted in eight parameter sets. Modeling results for the eight UZ flow sensitivity cases have been compared with field observed data and simulation results from the base-case model. The effects of parameter uncertainties on the flow fields were evaluated through comparison of results for flow and transport. In general, this study shows that uncertainties in matrix parameters cause larger uncertainty in simulated moisture flux than corresponding uncertainties in fracture properties for unsaturated flow through heterogeneous fractured rock.     

In situ estimation of the effective chemical diffusion coefficient of a rock matrix in a fractured aquifer

An in situ method of estimating the effective diffusion coefficient for a chemical constituent that diffuses into the primary porosity of a rock is developed by abruptly changing the concentration of the dissolved constituent in a borehole in contact with the rock matrix and monitoring the time-varying concentration. The experiment was conducted in a borehole completed in mudstone on the campus of the University of the Free State in Bloemfontein, South Africa. Numerous tracer tests were conducted at this site, which left a residual concentration of sodium chloride in boreholes that diffused into the rock matrix over a period of years. Fresh water was introduced into a borehole in contact with the mudstone, and the time-varying increase of chloride was observed by monitoring the electrical conductivity (EC) at various depths in the borehole. Estimates of the effective diffusion coefficient were obtained by interpreting measurements of EC over 34 d. The effective diffusion coefficient at a depth of 36 m was approximately 7.8×10^?6 m²/d, but was sensitive to the assumed matrix porosity. The formation factor and mass flux for the mudstone were also estimated from the experiment.     

Examining the utility of epoxy replicas of single,natural fractures in dolostone for visualization experiments

Epoxy casts were created of five naturally occurring fractures in dolostone and aperture distribution was measured using a light attenuation method. The optically determined aperture matrices showed a skew towards smaller apertures, with the larger models demonstrating a lognormal distribution and the smaller models a normal distribution. Surface variograms of each display a significant degree of anisotropy. The models had spatial correlation ranges between 1.8 and 5 cm. The optical mean aperture, mean hydraulic aperture and mass-balance aperture were all determined. In addition, the coefficient of variation (C _v) and anisotropy ratio (AR) for each model was determined to compare their predictions with the actual tracer behavior in each of the models. In all cases, the hydraulically determined mean aperture was the same order of magnitude as the optically determined mean aperture. The mass-balance-derived mean aperture differed significantly from the optical mean aperture based on the degree of channeling in the model. However, neither the C _v nor the AR correlated well to the actual behavior of the tracers, suggesting they are insufficient to predict void connectivity. Overall the epoxy replicas performed moderately well and can be well characterized for use in studies where visualization is of primary importance.     

Hybrid analytical and finite element numerical modeling of mass and heat transport in fractured rocks with matrix diffusion

Quantification of mass and heat transport in fractured porous rocks is important to areas such as contaminant transport, storage and release in fractured rock aquifers, the migration and sorption of radioactive nuclides from waste depositories, and the characterization of engineered heat exchangers in the context of enhanced geothermal systems. The large difference between flow and transport characteristics in fractures and in the surrounding matrix rock means models of such systems are forced to make a number of simplifications. Analytical approaches assume a homogeneous system, numerical approaches address the scale at which a process is operating, but may lose individual important processes due to averaging considerations. Numerical stability criteria limit the contrasts possible in defining material properties. Here, a hybrid analytical–numerical method for transport modeling in fractured media is presented. This method combines a numerical model for flow and transport in a heterogeneous fracture and an analytical solution for matrix diffusion. By linking the two types of model, the advantages of both methods can be combined. The methodology as well as the mathematical background are developed, verified for simple geometries, and applied to fractures representing experimental field conditions in the Grimsel rock laboratory.     

Numerical modeling of contaminant transport resulting from dissolution of a coal-tar pool in an experimental aquifer

A previously developed two-dimensional numerical model is further developed for simulating the transport of dissolved contaminants originating from dissolution of a coal tar pool in a stratified, saturated porous medium. The model is used to simulate contaminant transport resulting from a rectangular-shaped coal-tar-pool dissolution experiment conducted in a large-scale experimental aquifer. The experimental porous medium consists of two sand strata, a high-hydraulic-conductivity upper stratum and a low-hydraulic-conductivity bottom stratum. The experiment was conducted to a time of 354 days and the groundwater velocity was changed several times within this duration. Model simulations show good agreement against observed contaminant concentrations, and simulations show that dissolved solute below the pool migrated deeper into the bottom stratum as compared to the upper stratum. Furthermore, simulations also suggest that contaminant concentrations in the lower stratum never reached quasi steady-state during the experimental time frame.     

Evaluation and numerical modeling of seawater intrusion in the Gaza aquifer (Palestine)

A numerical assessment of seawater intrusion in Gaza, Palestine, has been achieved applying a 3-D variable density groundwater flow model. A two-stage finite difference simulation algorithm was used in steady state and transient models. SEAWAT computer code was used for simulating the spatial and temporal evolution of hydraulic heads and solute concentrations of groundwater. A regular finite difference grid with a 400 m² cell in the horizontal plane, in addition to a 12-layer model were chosen. The model has been calibrated under steady state and transient conditions. Simulation results indicate that the proposed schemes successfully simulate the intrusion mechanism. Two pumpage schemes were designed to use the calibrated model for prediction of future changes in water levels and solute concentrations in the groundwater for a planning period of 17 years. The results show that seawater intrusion would worsen in the aquifer if the current rates of groundwater pumpage continue. The alternative, to eliminate pumpage in the intruded area, to moderate pumpage rates from water supply wells far from the seashore and to increase the aquifer replenishment by encouraging the implementation of suitable solutions like artificial recharge, may limit significantly seawater intrusion and reduce the current rate of decline of the water levels.     

Numerical simulation of groundwater and early warnings from the simulated dynamic evolution trend in the plain area of Shenyang,Liaoning Province (P.R. China)

Groundwater level is the most direct factor reflecting whether groundwater is in a virtuous cycle. It is the most important benchmark for deciding whether a balance can be struck between groundwater discharge and recharge and whether groundwater exploitation will trigger problems pertinent to environment, ecology and environmental geology. According to the borehole and long-term monitoring wells data in the plain area of Shenyang, a numerical groundwater model is established and used to identify and verify the hydrogeological parameters and balanced items of groundwater. Then the concept of red line levels, the control levels of groundwater is proposed, the dynamic evolution trend of groundwater under different scenarios is analyzed and predicted and groundwater alerts are given when groundwater tables are not between the lower limit and the upper limit. Results indicated: (1) The results of identification and verification period fitted well, and the calculation accuracy of balanced items was high; (2) with the implementation of shutting wells, groundwater levels in urban areas of Shenyang would exceed the upper limit water level after 2020 and incur some secondary disasters; (3) under the recommended scenario of water resources allocation, early-warnings for groundwater tables outside the range would occur in the year of 2020, 2023, 2025 respectively for successive wet, normal and dry years. It was imperative to reopen some groundwater sources and enhance real-time supervision and early-warning to prevent the occurrence of potential problems.     

Evaluation of distributed recharge in an upland semi-arid karst system: the West Bank Mountain Aquifer,Middle East

Assessment of recharge in a structurally complex upland karst limestone aquifer situated in a semi-arid environment is difficult. Resort to surrogate indicators such as measurement of spring outflow and borehole discharge, is a common alternative, and attempts to apply conventional soil moisture deficit analysis may not adequately account for the intermittent spate conditions that arise in such environments. A modelling approach has been made using the West Bank Mountain Aquifer system in the Middle East as a trial. The model uses object oriented software which allows various objects to be switched on and off. Each of the main recharge processes identified in the West Bank is incorporated. The model allows either conventional soil moisture deficit analysis calculations or wetting threshold calculations to be made as appropriate, and accommodates both direct recharge and secondary recharge. Daily time steps enable recharge and runoff routing to be calculated for each node. Model runs have enabled a series of simulations for each of the three aquifer basins in the West Bank and for the whole of the West Bank. These provide recharge estimates comparable to those prepared by earlier workers by conventional means. The model is adaptable and has been successfully used in other environments.     

Effect of advective mass transfer on field scale fluid and solute movement: Field and modeling studies at a waste disposal site in fractured rock at Oak Ridge National Laboratory, Tennessee, USA

Advective mass transfer is a pore scale mass-transfer process that affects fluid and solute movement between pore domains such as fracture and matrix in a structured porous medium. Mechanistically similar to advection in the advection-dispersion of solutes in non-structured porous medium, it redistributes solutes by moving solute and solvent simultaneously between pore domains. While there is much research on diffusive mass transfer that is often referred to as matrix diffusion, there is a lack of information and study for advective mass transfer in the literature. The objective of this research is to study the effects of advective mass transfer on fluid and solute movement between pore domains. First, field hydraulic measurements at a waste disposal site in fractured rock at Oak Ridge National Laboratory (ORNL), Tennessee, USA, are used to calibrate a fracture-matrix, two-pore-domain groundwater flow model. Latin-hypercube sensitivity analysis suggests that the uncertainty of the calibrated model parameters is small and the calibrated flow model is nearly the optimal. Fracture spacing thus obtained is used to calculate diffusive mass transfer coefficients. The individual effects of advective and diffusive mass transfer on solute movement are then quantitatively evaluated. The calculations indicate that pore structure conceptual models may significantly affect the role of advective mass transfer in field and pore-scale mass transfer. In the particular ORNL field site and with a fracture-matrix pore structure model, contribution of advective mass transfer to solute mass movement is about three to eight orders of magnitude smaller than that of diffusive mass transfer.

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Resumen La transferencia de masa advectiva es un proceso de transferencia de masa en escala intersticial que afecta el movimiento de fluido y soluto entre ámbitos porosos tal como fractura y matriz en un medio estructurado poroso. Este proceso, mecánicamente similar a la advección en la dispersión-advección de solutos de medios porosos no estructurados, redistribuye los solutos mediante el movimiento simultáneo de soluto y solvente entre ámbitos porosos. Mientras que existe bastante investigación en transferencia difusiva de masa que frecuentemente se conoce como difusión en matriz, existe falta de información y estudio de transferencia advectiva de masa en la literatura. El objetivo de esta investigación es estudiar los efectos de la transferencia advectiva de masa en el movimiento de fluido y soluto entre ámbitos porosos. Primero se utilizaron mediciones hidráulicas de campo en un sitio de depósito de residuos en roca fracturada en el Laboratorio Nacional Oak Ridge (ORNL), Tennessee, USA, para calibrar un modelo de flujo de agua subterránea de ámbito de dos poros fractura-matriz. Análisis de sensitividad hipercúbico-latino sugieren que la incertidumbre de los parámetros del modelo calibrado es pequeña y que el modelo de flujo calibrado es aproximadamente el óptimo. El espaciamiento de fracturas así obtenido se utiliza para calcular los coeficientes de transferencia de masa difusiva. Luego se evalúa cuantitativamente los efectos individuales de transferencia de masa advectiva y difusiva en el movimiento de soluto. Los cálculos indican que los modelos conceptuales de estructura porosa pueden afectar significativamente el papel de transferencia de masa advectiva en escalas de campo e intersticial de transferencia de masa. En el sitio de campo específico ORNL y con un modelo de estructura porosa de matriz-fractura, la contribución de transferencia de masa advectiva al movimiento de masa soluto es aproximadamente tres a ocho órdenes de magnitud más pequeño que la contribución por transferencia de masa difusiva.

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Résumé Résumé Le transfert advectif de masse est un processus de transfert de masse à léchelle du pore qui affecte les mouvements du fluide et du soluté entre les différents domaines de pores, tel les fractures et la matrice dans un milieu poreux structuré. Mécaniquement similaire à ladvection dans le concept advection-dispersion de solutés dans les milieux non structurés, ce transfert redistribue les solutés simultanément avec le fluide entre les différents domaines poreux. Alors que de nombreuses recherches portent sur les transferts de masse par diffusion, se référant généralement à une diffusion par la matrice, il y a un grand manquement dinformations et détudes sur les transferts de masse par advection dans la littérature. Lobjectif de cette recherche est détudier leffet du transfert advectif de masse de fluide et de soluté entre les différents domaines poreux. Premièrement, les mesures hydrauliques de terrain sur la décharge en milieu fracturé du laboratoire national dOak Ridge ORNL, Tennessee, USA, sont utilisées pour calibrer un modèle hydrogéologique à double porosité fracture-matrice. Lanalyse de sensibilité latin-hypercube suggère que lincertitude sur les paramètres du modèle est faible et que le calibrage est pratiquement optimal. Lespace de fracture résultant permet de calculer les coefficients de transfert de masse par diffusion. Les effets individuels de ladvection et de la diffusion sur les mouvements de solutés sont dés lors évalués. Les calculs indiquent que le modèle conceptuel de la structure des pores peuvent significativement affectés le rôle du transfert advectif de masse à léchelle du pore et du terrain. Dans le cas du site de lORNL et avec un modèle structuré fracture-matrice, la contribution de ladvection au transfert de masse est de lordre de trois-huitième du transfert de masse par diffusion.

     

Numerical modeling of Late Miocene tectonic inversion in the Xihu Sag,East China Sea Shelf Basin,China

The East China Sea Shelf Basin is an important oil- and gas-bearing basin in the West Pacific continental margin. This region was affected by subduction of the Pacific Plate and the Philippine Plate in Cenozoic and experienced multi-stage tectonic inversions. This paper presents results from a numerical simulation by finite element method to the Xihu Sag in the East China Sea Shelf Basin and neighboring areas in an attempt to evaluate the WNW-directed compression on the sag during Late Miocene. Based on comprehensive structural analysis of a large number of seismic profiles, we determine the structural geometry of the sag, including the basement of the basin, the sedimentary cover, and 29 major faults in the Xihu Sag. Simulation results show that under continuous WNW-directed compression, tectonic inversion occurred firstly in the Longjing and Yuquan tectonic zones in the sag. Based on quantitative analysis of vertical displacement field of the Xihu Sag and peripheral areas and its stress intensity evolution, we identify a compressional regime in the Longjing Anticline Zone with a gradually propagated uplifting from south to north; whereas the propagation of uplifting in the Yuquan Anticline Zone is from north to south. The inversion intensity decreases from north to south. The formation of the tectonic inversion zone in the Xihu Sag is not only correlated to the direction of compression and fault patterns in the basin, but also closely related to the spatial configuration of fault surfaces of the Xihu–Jilong Fault in the Xihu Sag.     

青藏高原移动冰丘引起输油管道拱曲的数值模拟

青藏高原北部常年冻土区沿活动断裂发育有移动冰丘,其对输油管道、桥梁、涵洞等工程设施具有破坏作用。应用ANSYS有限元通用程序,对移动冰丘引起的输油管道的破坏进行了三维非线性有限元数值模拟计算,揭示了移动冰丘冻胀产生的应力场、位移场与塑性变形,给出了移动冰丘冻胀导致输油管道拱曲变形及应力分布。移动冰丘导致输油管道拱曲变形,在管道内部产生的应力超过管道的强度,使管道产生塑性弯曲变形和破坏,导致输油管道局部报废。移动冰丘导致输油管道拱曲变形的三维有限元数值模拟能够为管道工程设计和地质灾害防治提供依据。在数值模拟的基础上,提出了灾害防治措施。     

Rates of salinization by free convection in high-permeability sediments: insights from numerical modeling and application to the Dutch coastal area

Numerical modeling and dimensional analysis is used to study the salinization of thick, high-permeability aquifers by free convection from a salt source at the surface. Current understanding of this process mainly concerns the initial stages of salinization only (boundary-layer development, break-up into fingers and initial phase of finger descent). In the modeling, special attention is paid to the role of two processes in the long-term salinization rate: (1) the progressive loss of salt from fingers by lateral diffusion, and (2) the coalescence of fingers during their descent. From the numerical simulations a relationship is derived that describes the development of the horizontally averaged salinity with depth and time as a function of permeability and initial-density contrast for aquifer Rayleigh numbers up to Ra =6,000. This relationship is consistent with and provides an extension to previous generalized relationships of the rate of finger descent. Its applicability to real-world aquifers (Ra >10⁵) that include complexities due to anisotropy, heterogeneity, and mechanical dispersion is discussed. Application to the Pleistocene coastal aquifer of the Netherlands (thickness 200 m, permeability 10^-11 m²) suggests that salinization of the aquifer during historic episodes of inundation by seawater occurred within decades.

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Resumen Se utiliza modelos numéricos y un análisis dimensional para estudiar la salinización de acuíferos potentes de alta permeabilidad por convección libre a partir de una fuente salina superficial. El conocimiento acutal de este proceso se limita a las fases iniciales de la salinización (desarrollo de la capa de contorno, creación de digitaciones y fase inicial de la progresión de éstas). En la modelación, se presta atención especial al papel desempeñado por dos procesos de salinización a largo plazo: (1) la pérdida progresiva de sal por difusión lateral desde las digitaciones, y (2) la coalescencia de las digitaciones durante su avance. A partir de las simulaciones numéricas, se obtiene una relación que describe el desarrollo de la salinidad con la profundidad y el tiempo, promediada horizontalmente, el cual depende de la permeabilidad y del contraste inicial de densidad para números de Rayleigh inferiores a 6.000. Esta relación es coherente con índices previos generalizados del avance de las digitaciones, y representa una extensión a estos. Se discute su aplicabilidad a acuíferos reales (con números de Rayleigh superiores a 105), que tienen complejidades asociadas a la anisotropía, la heterogeneidad y la dispersión mecánica. La aplicación al acuífero costero Pleistoceno de los Países Bajos (20 m de potencia y 10-11 m2 de permeabilidad) sugiere que la salinización tuvo lugar en décadas, debido a episodios históricos de inundación por aguas marinas.

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Résumé Une modélisation numérique et une analyse dimensionnelle ont été mises en oeuvre pour étudier la salinisation d'aquifères épais et à forte perméabilité par convection libre d'une source de sel en surface. La compréhension habituelle de ce processus concerne principalement les étapes initiales de la salinisation seule (développement d'une couche limite, partition en digitations et phase initiale de développement des digitation). Dans la modélisation, une attention particulière a été portée au rôle de deux processus du taux de salinisation à long terme: (1) la perte progressive de sel dans les digitations par diffusion latérale et (2) la coalescence des digitations au cours de leur développement. À partir de simulations numériques, une relation a été obtenue qui permet de décrire l'extension de la salinité horizontalement en profondeur et au cours du temps en fonction de la perméabilité et du contraste initial de densité pour des nombres de Rayleigh de l'aquifère jusqu'à Ra =6,000. Cette relation est compatible avec ces résultats et fournit une extension des relations précédemment généralisées du taux de développement des digitations. On discute son applicabilité à des aquifères réels (Ra >105) incluant des complexités liées à l'anisotropie, l'hétérogénéité et la dispersion mécanique. L'application à l'aquifère côtier du Pléistocène des Pays-Bas (épaisseur environ 200 m, perméabilité environ10–11 m²) laisse penser que la salinisation de cet aquifère au cours d'épisodes historiques d'inondation par la mer s'est produit durant des décennies.

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