Alexander Vandenbohede

Effects of Global Change on Heterogeneous Coastal Aquifers: A Case Study in Belgium

Abstract Coastal plains are in the frontline of climate change. Predicted increase in recharge and sea level rise will alter groundwater flow, water quality distribution, recharge, and discharge considerably. This is simulated here in the Belgian western coastal plain. It consists of a shore, dunes, and polder (low-lying area) with a heterogeneous ground-water reservoir of quaternary age. A three-dimensional density-dependent groundwater flow model based on numerous (hydro)geologic observations was made. First the current groundwater flow and distribution between fresh and salt water was simulated. Then the effects of a 15% recharge increase and 0.4 m of sea level rise in the next 100 years were modelled. Sea level rise results in an increased flow of fresh water toward the polder and a decreased flow toward the sea. An increase in recharge results in more water flowing toward both the polder and the sea. Brackish water present in the polder will be pushed back as is a current saltwater intrusion from the polder in the dunes. The simulations also show that groundwater levels will rise. This will put strain on the ecologically valuable dunes and the drainage system in the polders.

Combined interpretation of pumping and tracer tests: theoretical considerations and illustration with a field test

Pumping and tracer tests are two well-established ground water reservoir tests. Hydraulic conductivity can be derived from both. Conductivities are studied in a mainly layered heterogeneous coastal aquifer in Belgium with a pumping and tracer test and results of both are compared. Characteristics and important points in performance and interpretation of the tests are discussed. The interpretation method of pumping tests must be chosen very carefully. Application of interpretation methods based on models underestimating or even neglecting the leakage towards the pumped permeable layer results in an overestimation of the horizontal conductivity of this layer. The larger the distance of the used observation well(s), the larger this overestimation becomes. This is illustrated here with the interpretation of the drawdown data obtained during the pumping test using different analytical models and a numerical model. With an inverse axi-symmetric numerical model incorporating layered heterogeneity, results of the pumping test are in good agreement with results from the tracer test. Many discrepancies between pumping and tracer test data performed in layered heterogeneous media can be explained when realistic pumping test interpretation methods are applied and when leakage is not underestimated.

Simulating MODFLOW-based reactive transport under radially symmetric flow conditions.

Radially symmetric flow and solute transport around point sources and sinks is an important specialized topic of groundwater hydraulics. Analysis of radial flow fields is routinely used to determine heads and flows in the vicinity of point sources or sinks. Increasingly, studies also consider solute transport, biogeochemical processes, and thermal changes that occur in the vicinity of point sources/sinks. Commonly, the analysis of hydraulic processes involves numerical or (semi-) analytical modeling methods. For the description of solute transport, analytical solutions are only available for the most basic transport phenomena. Solving advanced transport problems numerically is often associated with a significant computational burden. However, where axis-symmetry applies, computational cost can be decreased substantially in comparison with full three-dimensional (3D) solutions. In this study, we explore several techniques of simulating conservative and reactive transport within radial flow fields using MODFLOW as the flow simulator, based on its widespread use and ability to be coupled with multiple solute and reactive transport codes of different complexity. The selected transport simulators are MT3DMS and PHT3D. Computational efficiency and accuracy of the approaches are evaluated through comparisons with full 2D/3D model simulations, analytical solutions, and benchmark problems. We demonstrate that radial transport models are capable of accurately reproducing a wide variety of conservative and reactive transport problems provided that an adequate spatial discretization and advection scheme is selected. For the investigated test problems, the computational load was substantially reduced, with the improvement varying, depending on the complexity of the considered reaction network.

Seasonal dynamic of a shallow freshwater lens due to irrigation in the coastal plain of Ravenna, Italy

Irrigation in low-lying coastal plains may enhance the formation of fresh groundwater lenses, which counteract salinization of groundwater and soil. This study presents seasonal dynamics of such a freshwater lens and discusses its influence on the salinity distribution of the unconfined aquifer in the coastal plain of Ravenna, Italy, combining field observations with numerical modeling (SEAWAT). The lens originates from an irrigation ditch used as a water reservoir for spray irrigation. The geometry of the freshwater lens shows seasonal differences because of freshwater infiltration during the irrigation season and upconing of deeper saltwater for the remainder of the year. The extent of the freshwater lens is controlled by the presence of nearby drainage ditches. Irrigation also results in a temperature anomaly in the aquifer because of the infiltration of warm water during the irrigation season. The surficial zone in the vicinity of the irrigation ditch is increased considerably in thickness. Finally, different irrigation alternatives and the influence of sea-level rise are simulated. This shows that it is necessary to integrate irrigation planning into the water management strategy of the coastal zone to have maximum benefits for freshening of the aquifer and to make optimal use of the existing infrastructure.RésuméL’irrigation dans les basses plaines côtières est susceptible de favoriser la formation de lentilles d’eaudouce, qui s’opposent à la salinisation de l’eau souterraine et des sols. Cette étude présente ladynamique saisonnière d’une telle lentille d’eau douce et discute de son influence sur la distribution desalinité dans l’aquifère à nappe libre de la plaine côtière de Ravenne en Italie, combinant desobservations de terrain avec une modélisation numérique (SEAWAT). La lentille provient d’un fosséd’irrigation utilisé comme réservoir d’eau pour l’irrigation par aspersion. La géométrie de la lentilled’eau douce montre des différences saisonnières à cause de l’infiltration d’eau douce durant la saisond’irrigation et des remontées d’eau salées pendant le reste de l’année. L’extension de la lentille d’eaudouce est contrôlée par la présence des fossés voisins. L’irrigation a aussi comme conséquence uneanomalie thermique dans l’aquifère du fait de l’infiltration d’eau à température ambiante pendant lasaison d’irrigation. L’épaisseur de la zone superficielle augmente considérablement au voisinage dufosse d’irrigation Enfin, diverses alternatives d’irrigation et l’influence de l’augmentation du niveau dela mer sont simulées. Elles montrent que l’intégration d’un schéma d’irrigation dans la stratégie degestion de l’eau en zone côtière est nécessaire pour obtenir les avantages maximum pour adoucir lanappe et faire une utilisation optimale de l’infrastructure existante.ResumenLa irrigación en planicies costeras bajas puede favorecer la formación de lentes de agua subterráneadulce, las cuales contrarrestan la salinización del agua subterránea y del suelo. Este estudio presenta ladinámica estacional de una de tales lentes de agua dulce y discute su influencia en la distribución de lasalinidad del acuífero no confinado en la planicie costera de Ravenna, Italia, combinandoobservaciones de campo con modelado numerico (SEAWAT). La lente se origina a partir de unaacequia usada como un reservorio de agua para la irrigación por aspersión. La geometría de la lente deagua dulce muestra diferencias estacionales debido a que la infiltración de agua dulce durante de laestación de irrigación y el desplazamiento vertical de agua salina más profunda durante el resto delaño. La extensión de la lente de agua dulce está controlada por la presencia de acequias de drenajecercanas. La irrigación también produce una anomalía en la temperatura del acuífero debido a lainfiltración de agua cálida durante la estación de irrigación. La zona superficial en la vecindad de laacequia de irrigación incrementa considerablemente su espesor. Finalmente, se simulan las diferentesalternativas de irrigación y la influencia del ascenso del nivel del mar. Esto muestra que la integraciónde la planificación de la irrigación dentro de la estrategia de manejo del agua de la zona costera esnecesaria para obtener los máximos beneficios y para proveer de agua dulce al acuífero y hacer un usoóptimo de la infraestructura existente.摘要低洼沿海平原灌溉可增加地下淡水透镜体的形成, 从而抵消地下水和土壤的盐度。结合野外观测和数值模拟, 本研究展示了这样的淡水透镜体的季节性动态变化, 论述了淡水透镜体对意大利Ravenna (拉文那) 沿海平原非承压含水层盐度分布的影响。透镜体源自作为喷灌储水池的灌溉渠沟。由于灌溉期间的淡水渗透及其他时间内出现的深部咸水倒锥, 透镜体的几何结构显示出季节性差异。淡水透镜体的范围受附近排水沟的控制。由于灌溉季节温水的渗透, 灌溉也导致含水层温度异常。灌溉沟渠附近的表层带厚度增加非常大。最后, 模拟了不同的灌溉替代选择及海平面上升的影响。表明, 有必要制定综合的沿海地区水管理战略灌溉规划, 以获取含水层淡化的最 大效益及优化利用现有的基础设施。ResumoA rega em planícies costeiras de zonas baixas pode induzir a formação de lentes de água doce queimpedem a salinização da água subterrânea e do solo. Este estudo apresenta a dinâmica sazonal deuma destas lentes de água doce e analisa a sua influência na distribuição da salinidade do aquíferofreático na planície costeira de Ravenna, Itália, combinando campos de observação com modelaçãomatemática (SEAWAT). A lente forma-se a partir de um açude que é usado como reservatório de águapara rega por aspersão. A geometria da lente de água doce mostra diferenças sazonais devido àinfiltração de água doce durante o período de rega e ascensão de água salgada mais profunda durante oresto do ano. A dimensão da lente de água doce é controlada pela presença de valas de drenagempróximas. A rega também induz uma anomalia de temperatura no aquífero devido à infiltração de águaquente durante o período de rega. A zona superficial nas proximidades do açude de rega aumentousignificativamente de espessura. Finalmente, foram simuladas alternativas distintas de rega e ainfluência do aumento do nível do mar. Isto demonstra que a integração do planeamento da rega noâmbito da estratégia de gestão de água da zona costeira é necessária para obter o máximo rendimentoda introdução de água doce no aquífero e para fazer o melhor uso da infraestrutura existente.

Study of the feasibility of an aquifer storage and recovery system in a deep aquifer in Belgium

Abstract The feasibility of aquifer storage and recovery (ASR) was tested in a deep aquifer near Koksijde, Belgium. To achieve this, oxic drinking water was injected into a deep aquifer (the Tienen Formation) that contains anoxic brackish water. The hydraulic properties of the aquifer were determined using a step-drawdown test. Chemical processes caused by the injection of the water were studied by two push—pull tests. The step-drawdown test was interpreted by means of an inverse numerical model, resulting in a transmissivity of 3.38 m2/d and a well loss coefficient of 0.00038 d2/m5. The push—pull tests identified mixing between the injection and pristine waters, and cation exchange, as the major processes determining the quality of the recovered water. Mobilization of DOC, aerobic respiration, denitrification and mobilization of phosphate were also observed.

Recharge assessment by means of vertical temperature profiles: analysis of possible influences

Abstract Temperature profiles (temperature as function of depth) can be used to derive vertical flow velocities or recharge rates. In many cases, solutions to the one-dimensional (1-D) heat transport equation are used, considering steady-state boundary conditions. Factors which can influence the derivation of the mean vertical flow velocity are studied here. Therefore, an explicit finite-difference approximation to the 1-D heat transport equation coupled with an inverse scheme was used to interpret temperature profiles. Measurement error (larger than 0.05°C) can result in important deviation of the derived mean flow velocity. Variation of vertical flow velocity as a function of time leads to asymmetric temperature envelopes. Yearly variation in vertical flow velocities, or temperature variations of the recharge water, also results in asymmetric temperature envelopes. Interpretation of these asymmetric envelopes leads to important differences between derived and actual mean vertical flow velocities. Citation Vandenbohede, A. & Lebbe, L. (2010) Recharge assessment by means of vertical temperature profiles: analysis of possible influences. Hydrol. Sci. J. 55(5), 792–804.

Simulation of axi-symmetric flow towards wells: A finite-difference approach

A detailed finite-difference approach is presented for the simulation of transient radial flow in multi-layer systems. The proposed discretization scheme simulates drawdown within the well more accurately than commonly applied schemes. The solution is compared to existing (semi) analytical models for the simulation of slug tests and pumping tests with constant discharge in single- and multi-layer systems. For all cases, it is concluded that the finite-difference model approximates drawdown to acceptable accuracy. The main advantage of finite-difference approaches is the ability to account for the varying saturated thickness in unconfined top layers. Additionally, it is straightforward to include radial variation of hydraulic parameters, which is useful to simulate the effect of a finite-thickness well skin. Aquifer tests with variable pumping rate and/or multiple wells may be simulated by superposition. The finite-difference solution is implemented in MAxSym, a MATLAB tool which is designed specifically to simulate axi-symmetric flow. Alternatively, the presented equations can be solved using a standard finite-difference model. A procedure is outlined to apply the same approach with MODFLOW. The required modifications to the input parameters are much larger for MODFLOW than for MAxSym, but the results are virtually identical. The presented finite-difference solution may be used, for example, as a forward model in parameter estimation algorithms. Since it is applicable to multi-layer systems, its use is not limited to the simulation of traditional pumping and slug tests, but also includes advanced aquifer tests, such as multiple pumping tests or multi-level slug tests.

Global sampling to assess the value of diverse observations in conditioning a real‐world groundwater flow and transport model

The use of additional types of observational data has often been suggested to alleviate the ill-posedness inherent to parameter estimation of groundwater models and constrain model uncertainty. Disinformation in observational data caused by errors in either the observations or the chosen model structure may, however, confound the value of adding observational data in model conditioning. This paper uses the global generalized likelihood uncertainty estimation methodology to investigate the value of different observational data types (heads, fluxes, salinity, and temperature) in conditioning a groundwater flow and transport model of an extensively monitored field site in the Netherlands. We compared model conditioning using the real observations to a synthetic model experiment, to demonstrate the possible influence of disinformation in observational data in model conditioning. Results showed that the value of different conditioning targets was less evident when conditioning to real measurements than in a measurement error-only synthetic model experiment. While in the synthetic experiment, all conditioning targets clearly improved model outcomes, minor improvements or even worsening of model outcomes was observed for the real measurements. This result was caused by errors in both the model structure and the observations, resulting in disinformation in the observational data. The observed impact of disinformation in the observational data reiterates the necessity of thorough data validation and the need for accounting for both model structural and observational errors in model conditioning. It further suggests caution when translating results of synthetic modeling examples to real-world applications. Still, applying diverse conditioning data types was found to be essential to constrain uncertainty, especially in the transport of solutes in the model.

SEAWAT-Based Simulation of Axisymmetric Heat Transport

Simulation of heat transport has its applications in geothermal exploitation of aquifers and the analysis of temperature dependent chemical reactions. Under homogeneous conditions and in the absence of a regional hydraulic gradient, groundwater flow and heat transport from or to a well exhibit radial symmetry, and governing equations are reduced by one dimension (1D) which increases computational efficiency importantly. Solute transport codes can simulate heat transport and input parameters may be modified such that the Cartesian geometry can handle radial flow. In this article, SEAWAT is evaluated as simulator for heat transport under radial flow conditions. The 1971, 1D analytical solution of Gelhar and Collins is used to compare axisymmetric transport with retardation (i.e., as a result of thermal equilibrium between fluid and solid) and a large diffusion (conduction). It is shown that an axisymmetric simulation compares well with a fully three dimensional (3D) simulation of an aquifer thermal energy storage systems. The influence of grid discretization, solver parameters, and advection solution is illustrated. Because of the high diffusion to simulate conduction, convergence criterion for heat transport must be set much smaller (10(-10) ) than for solute transport (10(-6) ). Grid discretization should be considered carefully, in particular the subdivision of the screen interval. On the other hand, different methods to calculate the pumping or injection rate distribution over different nodes of a multilayer well lead to small differences only.

The hydrogeology of the military inundation at the 1914–1918 Yser front (Belgium)

Protection against flooding by the sea, drainage of rainwater and integrated management of groundwater and surface-water resources are key issues in low-lying coastal areas. However, under exceptional circumstances, knowledge to keep coastal areas dry and habitable can be used otherwise. Inundation for military purposes is such an example. The hydrogeology of the inundation at the Yser River, Belgium, during the Great War is studied. The inundation started in October 1914 to stop the German advance and lasted until 1918. A water balance and groundwater model are combined to derive the water balance before and during the inundation and to study the impact on the groundwater system. It is concluded that a number of hydrogeological factors contributed to the effectiveness of the inundation. Most importantly, the low-permeability subsoil facilitated loss of inundation water mainly by evaporation. Further, the normal water management strategy of the area (aimed at evacuating excess water towards the sea) was reversed to keep water between the opposing armies. However, the duration of the inundation meant a reorganization of the drainage of areas not inundated; truly an exercise in integrated water management.RésuméLa protection contre la submersion marine, le drainage de l’eau de pluie et la gestion intégrée des ressources en eau souterraines et de surface sont des dispositions essentielles dans les zones côtières basses. Toutefois, dans des circonstances exceptionnelles, la connaissance des dispositions pour maintenir les zones côtières hors d’eau et habitables peut être utilisée autrement. L’inondation à des fins militaires est un tel exemple. L’hydrogéologie de l’inondation par la rivière Yser, Belgique, durant la Grande Guerre est étudiée. L’inondation débuta en octobre 1914 pour ralentir l’avance allemande et dura jusqu’en 1918. Un bilan d’eau et un modèle hydrogéologique sont combinés pour établir les bilans d’eau avant et durant l’inondation et pour étudier son impact sur le système aquifère. Il a été conclu que plusieurs facteurs hydrogéologiques ont contribué à rendre l’inondation effective. Pour une grande part, la faible perméabilité de la sub-surface a entraîné la perte d’eau d’inondation principalement par évaporation. De plus, la stratégie de gestion normale de l’eau dans la zone (sensée évacuer l’excédent d’eau vers la mer) a été inversée pour conserver l’eau entre les armées adverses. Cependant, la durée de l’inondation signifiait la réorganisation des aires de drainages non inondées; véritablement un exercice de gestion intégrée de l’eau.ResumenLas protecciones contra la inundación a orillas del mar, el drenaje del agua de lluvia y la gestión integrada de los recursos hídricos subterráneas y superficiales son temas claves en las áreas costeras bajas. Sin embargo, en circunstancias excepcionales, el conocimiento para mantener las zonas costeras secas y habitables puede ser utilizado de otra manera. La inundación voluntaria para fines militares es un ejemplo. Se estudia la hidrogeología de la inundación en el río Yser, Bélgica, durante la Primera Guerra. La inundación se inició en octubre de 1914 para detener el avance alemán y duró hasta 1918. Un balance hídrico y un modelo de agua subterránea se combinan para obtener el balance de agua antes y durante la inundación y para estudiar el impacto sobre el sistema de agua subterránea. Se concluye que una serie de factores hidrogeológicos contribuyó a la eficacia de la inundación. El más importante, es la baja permeabilidad del subsuelo de facilitó la pérdida del agua de la inundación principalmente por evaporación. Además, la estrategia de gestión del agua habitual de la zona (destinado a evacuar el exceso de agua hacia el mar) se invirtió para mantener el agua entre los ejércitos enemigos. Sin embargo, la duración de la inundación significó una reorganización del drenaje de las zonas no inundadas, verdaderamente un ejercicio de la gestión integrada del agua.摘要防海水淹没、雨水排泄和地下水和地表水资源的综合管理是低洼沿海地区的关键问题。然而,在特殊情况下,保持沿海地区干燥及可居住的知识可以在用在其他方面。军事目的的洪水淹没就是这样的例子。本文研究了世界大战期间比利时伊瑟河前缘被淹没时的水文地质状况。洪水淹没始于1914年10月,以阻止德军的前进步伐,一直持续到1918年。水平衡模型和地下水模型结合起来得出了淹没前和淹没期间的水平衡状况,并且研究了对地下水系统的影响。结论是,许多水文地质因素对淹没的效果产生影响。最重要的是,低渗透性下层土促进了洪水的损耗,损耗主要是蒸发。此外,本地区正常的水管理战略(目的是向海里排放多余的水)被颠倒,以在两军之间保持有水。然而,洪水淹没的持续时间意味着未被淹没地区排水的重新组织;这真是综合水管理的一个实践。ResumoProteção contra inundações pelo mar, drenagem de água da chuva e gestão integrada de recursos hídricos superficiais e subterrâneos são questões chave em áreas costeiras de baixa altitude. Entretanto, sob circunstâncias excepcionais, o conhecimento de manter áreas costeiras secas e habitáveis pode ser utilizado de outra forma. Inundação para fins militares é um exemplo. Nesse trabalho, estudou-se a hidrogeologia da inundação no Rio Yser, Bélgica, durante a Primeira Guerra Mundial. A inundação começou em Outubro de 1914 para conter o avanço alemão e permaneceu até 1918. Um balanço hídrico e um modelo de águas subterrâneas são combinados para extrair o balanço hídrico antes e durante a inundação, e para estudar seu impacto no sistema de águas subterrâneas. Conclui-se que uma gama de fatores hidrogeológicos contribuiu para a efetividade da inundação. Mais importante, o subsolo de baixa permeabilidade facilitou a perda de água de inundação principalmente por evaporação. Além disso, a estratégia normal de gestão da água na área (destinada a escoar a água em excesso em direção ao mar) foi revertida para manter a água entre os exércitos opostos. Entretanto, a duração da inundação significou uma reorganização da drenagem em áreas não inundadas; um verdadeiro exercício de gestão integrada da água.