Alain Dassargues

Hydrogeological investigations at the Membach station, Belgium, and application to correct long periodic gravity variations

[1] A comprehensive hydrogeological investigation regarding the influence of variations in local and regional water mass on superconducting gravity measurements is presented for observations taken near the geodynamic station of Membach, Belgium. Applying a regional water storage model, the gravity contribution due to the elastic deformation of the Earth was derived. In addition, the Newtonian gravity effect induced by the local water mass variations was calculated, using soil moisture observations taken at the ground surface (about 48 m above the gravimeters). The computation of the gravimetric effect is based on a digital elevation model with spatially discretized rectangular prisms. The obtained results are compared with the observations of a superconducting gravimeter (SG). We find that the seasonal variations can be reasonably well predicted with the regional water storage model and the local Newtonian effects. Shorter-period effects depend on the local changes in hydrology. This result shows the sensitivity of SG observations to very local water storage changes.

Application of Multiple-Point Geostatistics on Modelling Groundwater Flow and Transport in a Cross-Bedded Aquifer

Sedimentological processes often result in complex three-dimensional subsurface heterogeneity of hydrogeological parameter values. Variogram-based stochastic approaches are often not able to describe heterogeneity in such complex geological environments. This work shows how multiple-point geostatistics can be applied in a realistic hydrogeological application to determine the impact of complex geological heterogeneity on groundwater flow and transport. The approach is applied to a real aquifer in Belgium that exhibits a complex sedimentary heterogeneity and anisotropy. A training image is constructed based on geological and hydrogeological field data. Multiple-point statistics are borrowed from this training image to simulate hydrofacies occurrence, while intrafacies permeability variability is simulated using conventional variogram-based geostatistical methods. The simulated hydraulic conductivity realizations are used as input to a groundwater flow and transport model to investigate the effect of small-scale sedimentary heterogeneity on contaminant plume migration. Results show that small-scale sedimentary heterogeneity has a significant effect on contaminant transport in the studied aquifer. The uncertainty on the spatial facies distribution and intrafacies hydraulic conductivity distribution results in a significant uncertainty on the calculated concentration distribution. Comparison with standard variogram-based techniques shows that multiple-point geostatistics allow better reproduction of irregularly shaped low-permeability clay drapes that influence solute transport.RésuméLes processus de sédimentation génèrent fréquemment des hétérogénéités tridimensionnelles dans les paramètres hydrogéologiques en subsurface. Les approches stochastiques basées sur des variogrammes ne sont souvent pas en mesure de décrire cette hétérogénéité de manière satisfaisante dans des environnements géologiques si complexes. La présente étude expose comment des analyses géostatistiques à points multiples peuvent être mises en œuvre pour une application hydrogéologique réaliste, afin de déterminer l’impact des hétérogénéités géologiques complexes sur l’écoulement et le transport souterrains. Cette approche est mise en application sur un aquifère de Belgique, qui démontre une hétérogénéité sédimentaire et une anisotropie complexes. Une image expérimentale est construite sur la base des données géologiques et hydrogéologiques de terrain. Les statistiques à points multiples sont empruntées à cette image afin de simuler l’occurrence des faciès perméables, tandis que la variabilité de la perméabilité entre les faciès est simulée en utilisant les méthodes géostatistiques standard, basées sur les variogrammes. Les transmissivités simulées sont utilisées comme données entrantes dans un modèle d’écoulement et de transport souterrain, afin de simuler l’effet des hétérogénéités sédimentaires à petite échelle sur la migration d’un panache de polluant. Les résultats montrent que les hétérogénéités à petite échelle ont un effet significatif sur le transport de polluant dans l’aquifère étudié. L’incertitude sur la distribution spatiale des faciès et des transmissivités intrafaciès génère une incertitude significative sur la distribution calculée des concentrations. La comparaison avec les techniques standard basées sur les variogrammes démontre que les analyses géostatistiques à points multiples permettent une reproduction plus fidèle des horizons argileux peu perméables et discontinus qui influencent le transport de solutés.ResumenLos procesos sedimentológicos a menudo dan como resultado heterogeneidad tridimensional compleja en el subsuelo de los valores de los parámetros hidrogeológicos. Aproximaciones estocásticas basadas en variogramas no son a menudo capaces de describir la heterogeneidad en tales ambientes geológicos complejos. Este trabajo muestra de que manera la geoestadística de múltiples puntos puede ser aplicada en una aplicación realista para determinar el impacto de una heterogeneidad geológica compleja en el flujo y transporte de aguas subterráneas. La aproximación es aplicada a un acuífero real en Bélgica que exhibe una complejo heterogeneidad y anisotropía sedimentaria. Una imagen de ensayo se construyó basada en datos de hidrogeológicos y geológicos de campo. La estadística de múltiples puntos es tomada de esta imagen de ensayo para simular la ocurrencia de hidrofacies, mientras que la variabilidad de la permeabilidad de intrafacies está simulada usando métodos geoestadísticos convencionales basados en variogramas. Los ensayos simulados de la conductividad hidráulica son usados como entrada al modelo de flujo y transporte para investigar el efecto de la heterogeneidad sedimentaria de pequeña escala en la migración de la pluma contaminante. Los resultados muestran que la heterogeneidad sedimentaria de pequeña escala tiene un efecto significativo sobre el transporte de contaminantes en el acuífero estudiado. La incertidumbre en la distribución espacial de las facies y la distribución de la conductividad hidráulica de intrafacies da como resultado en una significativa incertidumbre sobre el cálculo de la distribución de la concentración. La comparación con las técnicas standard basadas en variogramas muestra que la geoestadística de múltiples puntos permite una mejor reproducción de cortinas de arcilla de forma irregular y baja permeabilidad que influyen en el transporte de soluto.摘要沉积过程往往会导致水文地质参数在地下空间的复杂的非均质性。基于方差图的随机方法往往不能描述这种复杂地质环境的非均质性。本次工作展示了如何将多点统计地质学应用于实际水文地质问题, 确定复杂的地质非均质性对地下水流和溶质运移的影响。该方法被用于比利时的一个实际的具有复杂的沉积非均质性和各向异性含水层中。根据地质和水文地质实测资料绘制了训练图像。从这个训练图像中得出多点统计数值用于模拟水相分布, 亚相渗透率变化率利用传统的基于方差图的地质统计方法进行模拟。模拟出的水力传导度作为地下水流和溶质运移模型的输入值, 用来研究小范围内沉积造成的非均质性对污染物晕运移的影响。结果显示, 小范围的沉积造成的非均质性对研究目的含水层中的污染物运移有重要影响。空间相分配和亚相水力传导系数分配的不确定性导致计算出的浓度分布的显著不确定性。同标准的基于方差图方法对比, 多点统计地质学能够更好地再现影响溶质运移的不规则形态的低渗透率粘土隔层。ResumoOs processos sedimentológicos resultam frequentemente em heterogeneidades subsuperficiais tridimensionais complexas dos valores dos parâmetros hidrogeológicos. Abordagens estocásticas baseadas na análise variográfica são frequentemente incapazes de descrever a heterogeneidade nesses ambientes geológicos complexos. O presente trabalho evidencia como a geoestatística multiponto pode ser utilizada numa aplicação hidrogeológica realista para determinar o impacte da heterogeneidade geológica complexa no fluxo e transporte hídrico subterrâneo. A abordagem é aplicada a um aquífero real na Bélgica que apresenta uma heterogeneidade e anisotropia sedimentar complexa. É construída uma imagem de treino baseada em dados de campo de natureza geológica e hidrogeológica. As estatísticas multiponto são retiradas desta imagem de treino para simular a ocorrência de hidrofácies, enquanto a variabilidade da permeabilidade intrafácies é simulada utilizando métodos geoestatísticos baseados na análise variográfica convencional. Os resultados da simulação da condutividade hidráulica são utilizados como “input” no modelo de simulação de fluxo e de transporte para investigar o efeito de heterogeneidades sedimentares de pequena escala na migração de plumas de contaminação. Os resultados mostram que as heterogeneidades sedimentares de pequena escala têm um efeito significativo sobre o transporte de contaminantes no aquífero estudado. A incerteza na distribuição espacial das fácies e na distribuição da condutividade hidráulica intrafácies resulta numa incerteza significativa na distribuição das concentrações calculadas. A comparação com técnicas clássicas de análise variográfica mostra que a geoestatística multiponto permite uma melhor reprodução das barreiras argilosas irregulares de baixa permeabilidade que influenciam o transporte em solução.

Estimation of hydraulic conductivity and its uncertainty from grain-size data using GLUE and artificial neural networks

Various approaches exist to relate saturated hydraulic conductivity (Ks) to grain-size data. Most methods use a single grain-size parameter and hence omit the information encompassed by the entire grain-size distribution. This study compares two data-driven modelling methods—multiple linear regression and artificial neural networks—that use the entire grain-size distribution data as input for Ks prediction. Besides the predictive capacity of the methods, the uncertainty associated with the model predictions is also evaluated, since such information is important for stochastic groundwater flow and contaminant transport modelling.Artificial neural networks (ANNs) are combined with a generalised likelihood uncertainty estimation (GLUE) approach to predict Ks from grain-size data. The resulting GLUE-ANN hydraulic conductivity predictions and associated uncertainty estimates are compared with those obtained from the multiple linear regression models by a leave-one-out cross-validation. The GLUE-ANN ensemble prediction proved to be slightly better than multiple linear regression. The prediction uncertainty, however, was reduced by half an order of magnitude on average, and decreased at most by an order of magnitude. This demonstrates that the proposed method outperforms classical data-driven modelling techniques. Moreover, a comparison with methods from the literature demonstrates the importance of site-specific calibration. The data set used for this purpose originates mainly from unconsolidated sandy sediments of the Neogene aquifer, northern Belgium. The proposed predictive models are developed for 173 grain-size Ks-pairs. Finally, an application with the optimised models is presented for a borehole lacking Ks data.

Coupling heat and chemical tracer experiments for estimating heat transfer parameters in shallow alluvial aquifers

Geothermal energy systems, closed or open, are increasingly considered for heating and/or cooling buildings. The efficiency of such systems depends on the thermal properties of the subsurface. Therefore, feasibility and impact studies performed prior to their installation should include a field characterization of thermal properties and a heat transfer model using parameter values measured in situ. However, there is a lack of in situ experiments and methodology for performing such a field characterization, especially for open systems. This study presents an in situ experiment designed for estimating heat transfer parameters in shallow alluvial aquifers with focus on the specific heat capacity. This experiment consists in simultaneously injecting hot water and a chemical tracer into the aquifer and monitoring the evolution of groundwater temperature and concentration in the recovery well (and possibly in other piezometers located down gradient). Temperature and concentrations are then used for estimating the specific heat capacity. The first method for estimating this parameter is based on a modeling in series of the chemical tracer and temperature breakthrough curves at the recovery well. The second method is based on an energy balance. The values of specific heat capacity estimated for both methods (2.30 and 2.54MJ/m(3)/K) for the experimental site in the alluvial aquifer of the Meuse River (Belgium) are almost identical and consistent with values found in the literature. Temperature breakthrough curves in other piezometers are not required for estimating the specific heat capacity. However, they highlight that heat transfer in the alluvial aquifer of the Meuse River is complex and contrasted with different dominant process depending on the depth leading to significant vertical heat exchange between upper and lower part of the aquifer. Furthermore, these temperature breakthrough curves could be included in the calibration of a complex heat transfer model for estimating the entire set of heat transfer parameters and their spatial distribution by inverse modeling.

Finite elements modelling of a large water table aquifer in transient conditions

Abstract A chalky aquifer located near Liege (Belgium), is recharged by infiltration through the overlying loess. Wells and collecting tunnels produce a daily flow of 60 000 m3 out of this aquifer when hydrogeological balances have shown that an average yield of 100 000 m3/day should be possible. Finite element modelling has been developed to foresee the evolution of the water table to get some additional information especially about the main drainage axis. The transient flow constitutive laws are recalled in confined and unconfined aquifers. A new law is proposed to model the water table surface in transient conditions and with a fixed meshing network. Using the local flow equilibrium and the virtual power principle, the FEM formulation is set up. The time integration and the iteration technique are shortly discussed. The 3D discretization and the modelling of the entire aquifer has been realized. The problem requires about 3600 DOF and 2670 8-nodes isoparametric brick finite elements. The modelling has been quite delicate because of the geometric complexity of the different geological layers. This complexity justifies fully the use of the finite element method; there is indeed a great diversity of geological characteristics and the range of the different permeabilities is very wide. Sensitivity of the model to permeabilities and storage coefficient variations has been studied and various numerical problems have been notified. The calibration procedure is described in its main steps, and the most significant results are presented.

Geotechnical properties of the Quaternary sediments in Shanghai

Abstract The subsoil of Shanghai is composed of Quaternary sediments of the Yangtse River estuary. The center of the city has undergone a man-induced subsidence due to water pumping in the confined multi-aquifer system located in these sediments. Together with a sedimentological study of the post-Pleistocene conditions of deposition and accumulation, an accurate geotechnical study has been completed on the basis of all the available data from identification, oedometer and triaxial tests. Size analysis, X-ray analysis, compressibility characteristics, void ratio, preconsolidation pressures, permeabilities are considered taking into account the sedimentary sequence. Some relations between parameters are deduced and the hydrodynamic parameters are related to compaction parameters in order to predict their coupled and non-linear effects on the subsidence phenomena. Conclusions consist in the determination of the zones where different sensitivities to compaction are evaluated in these Quaternary formations.

Underground pumped storage hydroelectricity using abandoned works (deep mines or open pits) and the impact on groundwater flow

Underground pumped storage hydroelectricity (UPSH) plants using open-pit or deep mines can be used in flat regions to store the excess of electricity produced during low-demand energy periods. It is essential to consider the interaction between UPSH plants and the surrounding geological media. There has been little work on the assessment of associated groundwater flow impacts. The impacts on groundwater flow are determined numerically using a simplified numerical model which is assumed to be representative of open-pit and deep mines. The main impact consists of oscillation of the piezometric head, and its magnitude depends on the characteristics of the aquifer/geological medium, the mine and the pumping and injection intervals. If an average piezometric head is considered, it drops at early times after the start of the UPSH plant activity and then recovers progressively. The most favorable hydrogeological conditions to minimize impacts are evaluated by comparing several scenarios. The impact magnitude will be lower in geological media with low hydraulic diffusivity; however, the parameter that plays the more important role is the volume of water stored in the mine. Its variation modifies considerably the groundwater flow impacts. Finally, the problem is studied analytically and some solutions are proposed to approximate the impacts, allowing a quick screening of favorable locations for future UPSH plants.RésuméDes centrales hydroélectriques de pompage-turbinage utilisant un réservoir souterrain (UPSH Underground pumped storage hydroelectricity) constitué d’une carrière ou d’une mine profonde permettent de stocker l’excès d’électricité produite au cours des périodes de faible demande dans des régions sans relief. Il est essentiel de prendre en considération l’interaction entre les systèmes UPSH et le milieu géologique encaissant. Peu de travail existe concernant l’évaluation des impacts induits sur les écoulements d’eaux souterraines. Ceux-ci sont déterminés numériquement à l’aide d’un modèle numérique simplifié pour lequel on fait l’hypothèse qu’il est représentatif de considérer un puits ouvert de large diamètre. Le principal impact est une oscillation du niveau piézométrique dont l’amplitude dépend des caractéristiques de l’aquifère/milieu géologique, de la mine et des cycles de pompage et d’injection. Si un niveau piézométrique moyen est considéré, celui-ci diminue au début de la mise en activité du système UPSH et puis revient progressivement à son état initial. Les conditions hydrogéologiques les plus favorables pour minimiser les impacts ont été évaluées en comparant plusieurs scénarios. Comme attendu, l’amplitude de l’impact sera la plus faible dans un milieu géologique de faible diffusivité hydraulique. Cependant, le paramètre qui joue le rôle le plus important est le volume d’eau stocké dans la mine. Sa variation modifie de manière considérable les impacts sur les écoulements d’eaux souterraines. Finalement, le problème est étudié de manière analytique et des solutions sont proposées pour évaluer les impacts, permettant un premier tri rapide des sites favorables pour implanter de futures installations UPSH.ResumenLas centrales hidroeléctricas reversibles utilizando un embalse subterráneo (UPSH Underground Pumped Storage Hydroelectricity) formado por una mina a cielo abierto o subterránea permiten almacenar el exceso de electricidad producida durante períodos de baja demanda en regiones llanas. Es esencial tener en cuenta la interacción entre los sistemas UPSH y el medio geológico circundante. Existen pocos trabajos acerca de la evaluación de los impactos sobre el agua subterránea. Estos son determinados numéricamente utilizando un modelo simplificado que se supone representativo de minas subterráneas y a cielo abierto. El principal impacto consiste en la oscilación del nivel piezométrico, y su magnitud depende de las características del acuífero/medio geológico, la mina y los intervalos de bombeo e inyección. Si se considera un nivel piezométrico promedio, este decrece al inicio de la actividad del sistema UPSH y luego recupera progresivamente hasta su estado inicial. Las condiciones hidrogeológicas más favorables para minimizar los impactos son evaluadas comparando varios escenarios. La magnitud del impacto será menor en medios geológicos con baja difusividad hidráulica. Sin embargo, el volumen de agua almacenada en la mina es el parámetro que desempeña el papel más importante. Su variación modifica considerablemente los impactos sobre el flujo de agua subterránea. Por último, el problema se estudia analíticamente y se proponen algunas soluciones para evaluar los impactos. Estas soluciones permiten seleccionar rápidamente los sitios favorables para la construcción de futuras instalaciones UPSH.摘要在平坦地区可以利用露天矿坑或深矿井建造地下抽水蓄能电站,储存能源需求低的时期多余的电。充分考虑地下抽水蓄能电站和周围地质介质的相互作用必不可少。过去几乎没有对相关的地下水流影响做过评价。利用一个假定能够代表露天矿坑和深矿井的简化数值模型确定了对地下水流的影响。主要影响包括测压水头的振荡,其幅度取决于含水层/地质介质的特征、矿井和抽水注水的间隔。如果考虑平均测压水头,测压水头在地下抽水蓄能电站运行开始后初期下降,然后逐渐恢复。通过比较不同的几个方案,评价了使影响达到最小的最有利水文地质条件。水利扩散系数低的地质介质影响幅度较低。然而,发挥重要作用的参数是矿井储存水量。其变化可大大更改对地下水流的影响。最后,分析研究了出现的问题,提出了粗略估算影响的解决方法,可使人们对未来的地下抽水蓄能电站有利位置做出快速筛选.ResumoUsinas hidrelétricas reversíveis subterrâneas (UHRS) utilizando minas a céu aberto ou minas subterrâneas podem ser utilizadas em regiões planas para armazenamento do excedente da energia elétrica produzida durante períodos de baixa demanda energética. É essencial considerar a interação entre as UHRS e o meio geológico do entorno. Poucos estudos têm sido realizados na avaliação dos impactos associados ao fluxo das águas subterrâneas. Os impactos no fluxo das águas subterrâneas são determinados numericamente utilizando modelos numéricos simplificados, considerados representativos para minas a céu aberto e minas subterrâneas. Os principais impactos consistem na oscilação do nível piezométrico, e sua magnitude depende das características do aquífero/meio geológico, da mina e dos intervalos de bombeamento e injeção. Assumindo um nível piezométrico médio, há um declínio que sucede o início das atividades da UHRS, seguido de uma recuperação progressiva. As condições hidrogeológicas mais favoráveis para minimizar os impactos são avaliadas através da comparação de diversos cenários. A magnitude do impacto será inferior em meios geológicos de baixa difusividade hidráulica. Entretanto, o parâmetro que desempenha o papel mais importante é o volume da água armazenada na mina. Sua variação modifica consideravelmente os impactos no fluxo das águas subterrâneas. Por fim, o problema é estudado analiticamente e algumas soluções são propostas para aproximar os impactos, permitindo a rápida triagem de locais favoráveis para futuras UHRS.

Transport of Dissolved Si from Soil to River: A Conceptual Mechanistic Model

This paper reviews the processes which determine the concentrations of dissolved silicon (DSi) in soil water and proposes a conceptual mechanistic model for understanding the transport of Si through soils to rivers. The net DSi present in natural waters originates from the dissolution of mineral and amorphous Si sources in the soil, as well as precipitation processes. Important controlling factors are soil composition (mineralogy and saturated porosity) and soil water chemistry (pH, concentrations of organic acids, CO2 and electrolytes). Together with production, polymerization and adsorption equations they constitute a mechanistic framework determining DSi concentrations. We discuss how key controls differ across soil horizons and how this can influence the DSi transport. A typical podzol soil profile in a temperate climate is used as an example, but the proposed model is transferrable to other soil types. Additionally, the impact of external forcing factors such as seasonal climatic variations and land use is evaluated. This blueprint for an integrated model is a first step to mechanistic modelling of Si transport processes in soils. Future implementation with numerical methods should validate the model with field measurements.

A regional flux-based risk assessment approach for multiple contaminated sites on groundwater bodies.

In the context of the Water Framework Directive (EP and CEU, 2000), management plans have to be set up to monitor and to maintain water quality in groundwater bodies in the EU. In heavily industrialized and urbanized areas, the cumulative effect of multiple contaminant sources is likely and has to be evaluated. In order to propose adequate measures, the calculated risk should be based on criteria reflecting the risk of groundwater quality deterioration, in a cumulative manner and at the scale of the entire groundwater body. An integrated GIS- and flux-based risk assessment approach for groundwater bodies is described, with a regional scale indicator for evaluating the quality status of the groundwater body. It is based on the SEQ-ESO currently used in the Walloon Region of Belgium which defines, for different water uses and for a detailed list of groundwater contaminants, a set of threshold values reflecting the levels of water quality and degradation with respect to each contaminant. The methodology is illustrated with first results at a regional scale on a groundwater body-scale application to a contaminated alluvial aquifer which has been classified to be at risk of not reaching a good quality status by 2015. These first results show that contaminants resulting from old industrial activities in that area are likely to contribute significantly to the degradation of groundwater quality. However, further investigations are required on the evaluation of the actual polluting pressures before any definitive conclusion be established.

Physically Based Groundwater Vulnerability Assessment Using Sensitivity Analysis Methods

A general physically based method is presented to assess the vulnerability of groundwater to external pressures by numerical simulation of groundwater flow. The concept of groundwater vulnerability assessment considered here is based on the calculation of sensitivity coefficients for a user-defined groundwater state for which we propose several physically based indicators. Two sensitivity analysis methods are presented: the sensitivity equation method and the adjoint operator method. We show how careful selection of a method can significantly minimize the computational effort. An illustration of the general methodology is presented for the Herten aquifer analog (Germany). This application to a simple, yet insightful, case demonstrates the potential use of this general and physically based vulnerability assessment method to complex aquifers.