References of "Dassargues, Alain"
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See detailGeo-electrical data fusion by stochastic co-conditioning simulations for delineating groundwater protection zones
Dassargues, Alain ULg

in CMWR XVI - Computational Methods in Water Resources (2006)

In hydrogeology, advances in the delimitation of protection zones are made by the use of stochastic simulations integrating all available data. In practice, due to the few available measurements of the ... [more ▼]

In hydrogeology, advances in the delimitation of protection zones are made by the use of stochastic simulations integrating all available data. In practice, due to the few available measurements of the main parameters (hard data), it is very useful to integrate several secondary properties of the media as indirect data (soft data) to reduce the uncertainty of the results. In aquifers, most of the solute spreading is governed by the hydraulic conductivity (K) spatial variability, which is generally considered as the main uncertain parameter. A stochastic approach integrating hydraulic conductivity measurements (hard data), head observations and shallow electrical resistivity tomography (soft data) is presented. Results are discussed on a synthetic and on a practical case. It is shown in practice how the uncertainty of the well capture zone probability distribution (CaPD) can be reduced. Since geophysical data and head observations are easier to collect on the field then hydraulic conductivity measurements, they are generally more abundant. The methodology presented can even be used in real applications when little or no information is available about the hydraulic properties, through the conditioning on geophysical data and/or head observations. [less ▲]

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See detailEffect of excavation induced fractures on radionuclide migration through the Boom Clay (Belgium)
Huysmans, Marijke; Berckmans, Arne; Dassargues, Alain ULg

in Applied Clay Science (2006), 33(3-4), 207-218

In Belgium, the Boom Clay is being evaluated as a potential host formation for the disposal of high-level nuclear waste. In order to investigate this option, an underground research facility composed of ... [more ▼]

In Belgium, the Boom Clay is being evaluated as a potential host formation for the disposal of high-level nuclear waste. In order to investigate this option, an underground research facility composed of two access shafts and 200m of galleries was excavated. Excavation induced fractures were observed in a zone of approximately 1 m around the galleries. In this study, the potential effect of these fractures on radionuclide migration in the Boom Clay is investigated. Therefore, a hydrogeological model of the clay is built with a radionuclide source in the middle of the clay layer surrounded by different fracture configurations. Two types of fracture configurations are inserted in the model. The first type of fracture properties is drawn stochastically from the probability distributions of the properties of the fractures observed around previously excavated galleries. These fracture patterns are considered to be realistic although in this study it is conservatively assumed that no self-sealing occurs. The model is run for a large number of stochastically drawn fracture configurations and the results are compared to a model without fractures. These calculations show that the radionuclide fluxes through the clay are not significantly influenced by these fractures. For the second type of fracture configurations, the fracture properties are varied over a much larger range. Hypothetical fractures with much higher values of fracture extent, aperture, dip and frequency than observed are modeled, With these hypothetical fracture configurations, the critical values of the fracture parameters are determined that must be exceeded to have a significant effect on the radionuclide fluxes through the clay. These calculations show that the extent of the fractured zone has the largest effect on radionuclide migration. The other fracture parameters (aperture, spacing and dip) have a limited effect on the radionuclide fluxes. To obtain a total radionuclide flux through the lower clay boundary that is respectively 10%, 50% and 100% larger than without a fractured zone, the extent of the hypothetical fractured zone should be respectively 27.97 m, 43.86 m and 46.92 m. Such fractures are not expected to be generated by mechanical excavation in the Boom Clay. The obtained critical extent values of the hypothetical fractures are a factor 30 to 40 higher than the measured values of the extent of the excavation disturbed zone. These calculations thus indicate that it is very unlikely that the extent of the fractured zone around the galleries will be large enough to have a significant effect on the radionuclide fluxes through the Boom Clay. This conclusion is further supported by the conservative assumption that no self-sealing occurs. These calculations can function as a preliminary robustness test in ongoing safety analysis studies. (c) 2006 Elsevier B.V. All rights reserved. [less ▲]

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See detailA framework for an optimised groundwater monitoring network and aggregated indicators
Rentier, Céline; Delloye, Francis; Brouyère, Serge ULg et al

in Environmental Geology (2006), 50(2), 194-201

The implementation of the Water Framework Directive (EU 2000) requires a groundwater quality monitoring. It is used for characterisation of the 'good' chemical status of each groundwater body and for the ... [more ▼]

The implementation of the Water Framework Directive (EU 2000) requires a groundwater quality monitoring. It is used for characterisation of the 'good' chemical status of each groundwater body and for the restoration or protection purposes of those bodies already at 'good' status. Interpretative aspects are lying in the design of monitoring network and in the way of building global indicators. Attention is given here to the global chemical status of the groundwater bodies and to the role of diffuse pollution, much of which is brought via groundwater to surface water. Monitoring 'local' pollution associated with individual sites is not addressed. Groundwater bodies with different contrasted hydrogeology conditions, land use and topography have been considered to establish an approach for choosing an optimised monitoring network. Then, a quality assessment system has been developed and applied for qualifying the general status of each groundwater body. The use of non-dimensional indexes allows us to process with all kinds of chemical parameters in a normalised way and, by means of adequate aggregation rules, to qualify the general quality status of a groundwater body. The obtained diagnostic, even if not fully validated, is closely linked to the pragmatic objectives contained in the EU Water Directive. [less ▲]

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See detailHydrogeological modeling of radionuclide transport in low permeability media: a comparison between Boom Clay and Ypresian Clay
Huysmans, Marijke; Dassargues, Alain ULg

in Environmental Geology (2006), 50(1), 122-131

Deep low-permeability clay layers are considered as suitable environments for disposal of high-level radioactive waste. In Belgium, the Boom Clay is the reference host formation and the Ypresian Clay an ... [more ▼]

Deep low-permeability clay layers are considered as suitable environments for disposal of high-level radioactive waste. In Belgium, the Boom Clay is the reference host formation and the Ypresian Clay an alternative host formation for research and safety and feasibility assessment of deep disposal of nuclear waste. In this study, two hydrogeological models are built to calculate the radionuclide fluxes that would migrate from a potential repository through these two clay formations. Transport parameter heterogeneity is incorporated in the models using geostatistical co-simulations of hydraulic conductivity, diffusion coefficient and diffusion accessible porosity. The calculated radionuclide fluxes in the two clay formations are compared. The results show that in the Ypresian Clay larger differences between the fluxes through the lower and the upper clay boundary occur, larger total output radionuclide amounts are calculated and a larger effect of parameter heterogeneity on the calculated fluxes is observed, compared to the Boom Clay. [less ▲]

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See detailCharacterisation of fissured aquifers in the semi-arid region of the mid-Atlas plateau (Oulmès, Morocco)
Orban, Philippe ULg; Ruthy, Ingrid ULg; Elbatloussi, D. et al

in GIRE3D: International Congress on Integrated Water Resources management and Challenges of the Sustainable Development (2006)

The Oulmès plateau (mid-Atlas in Morocco) is known for its groundwater resources in this semi-arid region. In the studied zone (approximately 80 km2), the mineral water of ‘Sidi Ali’ is exploited as well ... [more ▼]

The Oulmès plateau (mid-Atlas in Morocco) is known for its groundwater resources in this semi-arid region. In the studied zone (approximately 80 km2), the mineral water of ‘Sidi Ali’ is exploited as well as the naturally semi-sparkling ‘mineral water of Oulmès’ which are main mineral waters marketed in Morocco. Their sources are located at less than four kilometers from each other. In the same time, the drinking water supply to the local rural populations is far from being sufficient, and even in some places non-existent. In addition the plateau is also the place where many fruit-bearing cultures have been undertaken for a few years. Irrigation and the use of pesticide and herbicide compounds as well as fertilizers could constitute threats for the groundwater quantity and quality and unpleasant consequences are expected for all dependent groundwater end-users. Thus, in this semi-arid zone, an acute problem of sustainable development arises in terms of groundwater quantity and quality. An extended study has involved data collection, measurement campaigns (piezometric levels, sampling & analysis of groundwater), shallow geophysical prospecting, pumping tests, building of a data base, estimation of the recharge spatial distribution and hydrogeological mapping. On the basis of this study, a better understanding is possible about the state and the particular hydrodynamic behaviour of groundwater in the fissured hard-rocks of this plateau. Results are expressed in maps providing explicit and useful information allowing future decisions which can be taken for the welfare of everyone and particularly for the local populations. [less ▲]

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See detailHydrogeological modeling of radionuclide transport in heterogeneous low-permeability media: a comparison between Boom Clay and Ieper Clay
Huysmans, Marijke; Dassargues, Alain ULg

in Soares, A.; Pereira, M. J.; Dimitrakopoulos, R. (Eds.) GeoENV VI: Geostatistics for Environmental Applications (2006)

Deep low-permeability clay layers are considered as possible suitable environments for disposal of high-level radioactive waste. In Belgium, the Boom Clay is the reference host formation and the Ieper ... [more ▼]

Deep low-permeability clay layers are considered as possible suitable environments for disposal of high-level radioactive waste. In Belgium, the Boom Clay is the reference host formation and the Ieper Clay an alternative host formation for research and safety and feasibility assessment of deep disposal of nuclear waste. In this study, two hydrogeological models are built to calculate the radionuclide fluxes that would migrate from a potential repository through these two clay formations. Transport parameters heterogeneity is incorporated in the models using stochastic sequential simulation of hydraulic conductivity, diffusion coefficient and diffusion accessible porosity, using primary information and several types of secondary information, i.e. resistivity, gamma ray and grain size. The calculated radionuclide fluxes in the two clay formations are compared. Results show that in the Ieper Clay larger differences between the fluxes through the lower and the upper clay boundary occur, larger total output radionuclide amounts are calculated, and a larger effect of parameter heterogeneity on the calculated fluxes is observed, compared to the Boom Clay. [less ▲]

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See detailStochastic analysis of the effect of spatial variability of diffusion parameters on radionuclide transport in low permeability clay layer
Huysmans, Marijke; Dassargues, Alain ULg

in Calibration and Reliability in Groundwater Modelling: From Uncertainty to Decision Making (2006)

Most studies that incorporate subsurface heterogeneity in groundwater flow and transport models only analyze and simulate the spatial variability of hydraulic conductivity. Heterogeneity of the other flow ... [more ▼]

Most studies that incorporate subsurface heterogeneity in groundwater flow and transport models only analyze and simulate the spatial variability of hydraulic conductivity. Heterogeneity of the other flow and transport parameters is usually neglected. This approach is often justified, but there are however cases in which disregarding the heterogeneity of the other flow and transport parameters can be questionable. In low permeability media, for instance, diffusion is often the dominant transport mechanism. It therefore seems logical to incorporate the spatial variability of the diffusion parameters in the transport model. This study therefore analyzes and simulates the spatial variability of the effective diffusion coefficient and the diffusion accessible porosity with geostatistical techniques and incorporates their heterogeneity in the transport model of a low permeability formation. The calculated output radionuclide fluxes of this model are compared with the fluxes calculated with a homogeneous model and a model with a heterogeneous hydraulic conductivity distribution. This analysis shows that the heterogeneity of the diffusion parameters has a much larger effect on the calculated output radionuclide fluxes than the heterogeneity of hydraulic conductivity. [less ▲]

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See detailStochastic analysis of the effect of spatial variability of diffusion parameters on radionuclide transport in a low permeability clay layer
Huysmans, Marijke; Dassargues, Alain ULg

in Hydrogeology Journal (2006), 14(7), 1094-1106

Most studies that incorporate subsurface heterogeneity in groundwater flow and transport models only analyze and simulate the spatial variability of hydraulic conductivity. Heterogeneity of the other flow ... [more ▼]

Most studies that incorporate subsurface heterogeneity in groundwater flow and transport models only analyze and simulate the spatial variability of hydraulic conductivity. Heterogeneity of the other flow and transport parameters are usually neglected. This approach is often justified, but there are, however, cases in which disregarding the heterogeneity of the other flow and transport parameters can be questionable. In low permeability media, for instance, diffusion is often the dominant transport mechanism. It therefore seems logical to incorporate the spatial variability of the diffusion parameters in the transport model. This study therefore analyses and simulates the spatial variability of the effective diffusion coefficient and the diffusion accessible porosity with geostatistical techniques and incorporates their heterogeneity in the transport model of a low permeability formation. The formation studied was Boom clay (Belgium), a candidate host rock for the deep geological disposal of high-level radioactive waste. The calculated output radionuclide fluxes of this model are compared with the fluxes calculated with a homogeneous model and a model with a heterogeneous hydraulic conductivity distribution. This analysis shows that the heterogeneity of the diffusion parameters has a much larger effect on the calculated output radionuclide fluxes than the heterogeneity of hydraulic conductivity in the low permeability medium under study. [less ▲]

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See detailHydrogeological investigations at the Membach station, Belgium, and application to correct long periodic gravity variations
Van Camp, Michel; Vanclooster, Marnik; Crommen, O. et al

in Journal of Geophysical Research (2006), 111(B10),

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 ... [more ▼]

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. [less ▲]

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See detailDeveloping tools for managing hydrogeological data in a semi-arid region: the case study of Oulmès (Morocco)
Ruthy, Ingrid ULg; Orban, Philippe ULg; Elbatloussi, D. et al

in GIRE3D: International Congress on Integrated Water Resources management and Challenges of the Sustainable Development (2006)

Data and information required by hydrogeological studies are various and complex such as geology, topography, climate, hydrological and hydrogeological raw or pre-processed data. All these data need to be ... [more ▼]

Data and information required by hydrogeological studies are various and complex such as geology, topography, climate, hydrological and hydrogeological raw or pre-processed data. All these data need to be managed, and this can be done in a structured database. A (GIS-managed) hydrogeological database has been developed previously by the Hydrogeology Group of the University of Liège (Gogu et al., 2001) in order to capture, store, edit, query, update and display geographically referenced hydrogeological data. The development of this geo-relational database was constrained by a maximum storage of data with a minimum data redundancy, reduction of storage memory capacity and optimum ‘retrievability’ of information for further analysis. In recent years, the use of Geographical Information System (GIS) has grown rapidly in groundwater management and research. GIS is now widely used to create digital geographic databases, to manipulate and prepare data as input for various model parameters, and to display model output. Coupling GIS with this hydrogeological database provides a powerful tool. This GIS-managed hydrogeological database has been also developed for vulnerability-assessment techniques and numerical modelling for groundwater flow and contaminant transport studies. As well as linking between database and GIS, the design of coupling database with process-based numerical models was also performed. This methodology has been applied, in a semi-arid region: the "Oulmès plateau" located in the Mid-Atlas (Morocco). This study has led to a detailed hydrogeological map at the scale of 1/25000. First, data were collected from existing databases, studies and maps as well as through new field measurements. These data came from numerous and various sources and in different formats (paper or digital diagrams, images, spreadsheets…). After validation, it has been encoded in the hydrogeological database. Because data are geo-referenced, it can be easily represented on the map within GIS-software. The hydrogeological map displays several layers of information as topography, hydrogeological units, hydrographic network, wells, piezometers, isoline of piezometric heads. [less ▲]

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See detailLarge-scale groundwater flow and transport modelling : Methodology and application to the Geer basin, Belgium
Orban, Philippe ULg; Dassargues, Alain ULg; Brouyère, Serge ULg

in Stauffer, Fr.; Dassargues, Alain (Eds.) Quantitative Geology from Multiple Sources: S10 Use of multiple sources in conditioning/calibrating groundwater flow and transport models (2006)

Efficient and sustainable management of water resources should be considered from an integrated way, at the basin scale. For that purpose, physically-based, spatially distributed modelling tools has ... [more ▼]

Efficient and sustainable management of water resources should be considered from an integrated way, at the basin scale. For that purpose, physically-based, spatially distributed modelling tools has turned to be very useful. If regional groundwater flow model have been developed for years now, large scale contaminant transport models are still almost inexistent. The objective of this extended abstract is to present briefly new concepts for large transport modelling, more particularly a modelling approach, the Hybrid Finite Element Mixing Cell (HFEMC) developed by the Hydrogeology Group of the University of Liège and implemented in the 3D simulator SUFT3D. First steps for the application to the Geer basin are also presented. [less ▲]

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See detailIdentification of groundwater quality trends in a chalky aquifer threatened by intensive agriculture
Batlle-Aguilar, Jordi; Orban, Philippe ULg; Dassargues, Alain ULg et al

in Stauffer, Fr.; Dassargues, Alain (Eds.) Quantitative Geology from Multiple Sources: S10 Use of multiple sources in conditioning/calibrating groundwater flow and transport models (2006)

Diffuse groundwater contamination related to agricultural practices is a worldwide environmental problem, particularly the continuous increase in nitrate concentrations. As a response to this threat, the ... [more ▼]

Diffuse groundwater contamination related to agricultural practices is a worldwide environmental problem, particularly the continuous increase in nitrate concentrations. As a response to this threat, the European Union has adopted directives requiring that Member States take measures to reduce agricultural nitrate sources and stating that a “good” status of groundwater is required for all EU members. In order to achieve the environmental objectives for groundwater, the identification and reversal of any significant upward trend in the pollutant concentrations are required. Conclusions are drawn about the evolution of groundwater contamination by nitrates in the following decades with respect to the EU Water directive prescriptions. Measures have to be urgently taken in order to avoid major degradation of groundwater in 10 to 70 years. However, a good groundwater quality status cannot be expected for the 2015 EU Water Framework Directive (WFD). [less ▲]

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See detailCaractérisation isotopique des eaux du granite et de l’auréole métamorphique d’Oulmès (Maroc central)
Olive, Philippe; Dassargues, Alain ULg; Griere, Olivier et al

in Integrated Water Resources management and Challenges of the Sustainable Development (2006)

Dans le bassin d’Oulmès sont exploitées et commercialisées par la société des Eaux Minérales d’Oulmès qui constitue le plus gros embouteilleur d’eaux minérales du Maroc :Sidi Ali Lala Haya. Le pluton ... [more ▼]

Dans le bassin d’Oulmès sont exploitées et commercialisées par la société des Eaux Minérales d’Oulmès qui constitue le plus gros embouteilleur d’eaux minérales du Maroc :Sidi Ali Lala Haya. Le pluton granitique, d’âge hercynien, d’Oulmès est un granite à biotite et muscovite. Il présente une fracturation très marquée et un système filonien très développé. L’altération, très poussée, se manifeste par une arénitisation généralisée qui s’étend en profondeur. Le kaolin caractérise cette altération d’origine hydrothermale. L’eau de Lalla Haya émerge dans les granites à la faveur de fissures à une température de 42°C, il s’agit d’une eau carbo-gazeuse, bicarbonatée sodique. L’auréole métamorphique générée par ce granite englobe la formation des schistes en dalles, roches métapélitiques cambro-ordoviciennes homogènes. L’eau de Sidi Ali est faiblement minéralisée, elle aussi bicarbonatée sodique avec une pression partielle de CO2 non négligeable L’utilisation de l’outil isotopique a permis de préciser un certain nombre de points concernant la recharge et l’écoulement des eaux souterraines dans le bassin d’Oulmès. Les teneurs en 18O d’une vingtaine de sources situées autour du site et étagées entre 300 et 2000 m s’alignent suivant un gradient de -0,21 ‰ / 100 m. La zone de recharge commune des eaux du granite et des schistes en dalle se situe vers 1100/1300 m. De plus le rapport 18O/D pour ces deux types d’eau indique un appauvrissement en 18O de près de 1 ‰ dû à un échange avec le CO2 magmatique. Le temps de séjour moyen des eaux dans l’auréole métamorphique est de quelques dizaines d années (présence de 3H thermonucléaire) et, au moins, de plusieurs centaines d’années dans les eaux du massif granitique (absence de 3H thermonucléaire). De la remontée de CO2 magmatique, à 14C mort, il résulte que la teneur en 14C du carbone minéral dissous est quasiment nulle et qu’il vieillit, plus ou moins, les activités en 14C du carbone minéral dissous des eaux de l’auréole. En conclusion l’eau qui s’infiltre dans les fractures du massif granitique où elle se réchauffe suivant un gradient géothermique d’environ 40°C/km. Comme le géothermomètre Na/K indique une température de 165 °C, elle atteint une profondeur de l’ordre de 2 à 3 km. Elle s’est enrichie en CO2 magmatique et remonte par effet de gaz lift et donne naissance à Lalla Haya. Cette remontée ne se limite pas au seul massif granitique mais s’étend à l’auréole métamorphique en profitant des fractures du massif et des filons de quartz en provenance du batholite où elle se mélange, en proportions variables, aux eaux récentes qui se sont infiltrées directement sur les schistes en dalles. Ce sont ces fluides hydrothermaux qui ont donné naissance aux dépôts hydrothermaux autrefois exploités en mines (Sn). [less ▲]

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See detailReducing the uncertainty of hydrogeological parameters by co-conditional simulations: lessons from practical applications in aquifers and in low permeability layers
Dassargues, Alain ULg; Rentier, Céline; Huysmans, Marijke

in Calibration and Reliability in Groundwater Modelling: From Uncertainty to Decision Making (2006)

Stochastic simulation of aquifer heterogeneity is now often performed to provide a confidence interval of the modelled results for flow and solute transport problems. In practice, due to the few available ... [more ▼]

Stochastic simulation of aquifer heterogeneity is now often performed to provide a confidence interval of the modelled results for flow and solute transport problems. In practice, due to the few available measurements of the hydraulic conductivity (hard data), it is useful to integrate several other properties of the medium as indirect data (soft data). The additional conditioning obtained from the use of these secondary data allows reduction of the variance of the distribution and consequently decrease of the uncertainty of the results. This practice can also be extended to low permeability clay layers. For example, stochastic sequential simulation can be performed involving hydraulic conductivity values as hard data, and grain size measurements, electrical resistivity log, gamma ray log and a description of the lithology variation as soft data. However, other important properties can also be considered. The possible fracturing of clay strongly influences the flow and solute transport. On the other hand, in very low permeability media, diffusion can be considered as the dominant transport mechanism, so that heterogeneity in terms of the effective diffusion coefficient becomes important. Examples of application are summarized considering aquifers and low permeability clay layers. It clearly shows the great advantage of collecting multiple data sets of inter-correlated data on the same geological medium to be modelled. In high conductivity aquifers as well as in low permeability layers, this kind of additional conditioning obtained from various data is always useful when considering applications such as, among many others, well capture zones delineation, impact studies and geological confinement of wastes. [less ▲]

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See detailComparison of Kohonen's Self-Organizing Map algorithm and principal component analysis in the exploratory data analysis of a groundwater quality dataset
Peeters, Luk; Dassargues, Alain ULg

(2006)

Groundwater monitoring networks typically yield large, multivariate datasets. Analysis and interpretation of these datasets starts with an exploratory data analysis in order to summarize the available ... [more ▼]

Groundwater monitoring networks typically yield large, multivariate datasets. Analysis and interpretation of these datasets starts with an exploratory data analysis in order to summarize the available data, extract useful information and formulate hypotheses for further research. Exploratory data analysis is mostly focussed on finding related variables and groupings of similar observations. Traditionally multivariate statistical techniques like principal component analysis (PCA) are used for this purpose. In PCA a linear dimensionality reduction of the original, high dimensional dataset is carried out in order to identify orthogonal directions (principal components) of maximum variance in the dataset based on linear combinations of correlated variables. Projections of the original data in the subspace defined by the principal components can be used to identify groups in the data and to reveal relationships between variables (Davis, 1986). In this study, principal component analysis is compared to Kohonen's self-organizing map (SOM) algorithm. The SOM-algorithm is an artificial neural network technique designed to carry out a non-parametric regression process that is mainly used to represent high-dimensional, nonlinearly related data items in a topology-preserving, often two-dimensional display, and to perform unsupervised classification and clustering (Kohonen, 1995). Both PCA and SOM are applied to a hydrochemical dataset from a monitoring network in two sandy, phreatic aquifers in Central Belgium. The monitoring network consists of 47 monitoring wells each equipped with three filters at different depths, in which 14 variables are measured. The first aquifer, the Diest sands aquifer is of Late Miocene age and consists of coarse, glauconiferous sands and sandstones (Laga et al., 2001). The second aquifer, the Brussels sands aquifer, is of Middle Eocene age and is an heterogeneous formation consisting of an alteration of highly and poorly calcareous sands, locally silicified (Laga et al., 2001). Both techniques succeed in distinguishing between both aquifers and reveal the relationships between variables. The main advantage of PCA is the mathematical quantification of correlation between variables and the expression of the original data in the subspace defined by the principal components. The visualization of the SOM-analysis on the other hand allows a straightforward interpretation of the dataset structure in which even non-linear relationships between variables can be identified. Additionally, the SOM-algorithm can handle a limited amount of missing values in the dataset, contrary to PCA. [less ▲]

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See detailStochastic analysis of the recharge uncertainty of a regional aquifer in extreme arid conditions
Rojas, Rodrigo; Dassargues, Alain ULg

in Stauffer, Fr.; Dassargues, Alain (Eds.) Quantitative Geology from Multiple Sources: S10 Use of multiple sources in conditioning/calibrating groundwater flow and transport models (2006)

The Pampa del Tamarugal Aquifer (PTA) is an important source of groundwater in northern Chile. Since the study area is situated in the Atacama Desert, the estimation of groundwater recharge based on ... [more ▼]

The Pampa del Tamarugal Aquifer (PTA) is an important source of groundwater in northern Chile. Since the study area is situated in the Atacama Desert, the estimation of groundwater recharge based on conventional hydrological methods is subject to large uncertainties. To account for variations in the groundwater balance, caused by uncertainties in the average recharge rates, randomly generated recharge values with different levels of uncertainty are simulated using a groundwater flow model. Results show that evaporation and groundwater outflows are insensitive to the recharge uncertainty, while the storage terms can vary considerably. Considering current groundwater abstraction and random recharge rates, it is unlikely that the cumulative discharged volume from the aquifer, after a 45 years simulation period, will be larger than 12% of the estimated groundwater reserve. Simulated groundwater heads fluctuations due to uncertainties in the average recharge rates are more noticeable in certain areas. These fluctuations could explain anomalies in the observed groundwater heads in these areas. [less ▲]

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See detailClassification of groundwater samples in wetlands using selforganizing maps
Peeters, Luk; Dassargues, Alain ULg

in Stauffer, Fr; Dassargues, Alain (Eds.) Quantitative Geology from Multiple Sources: S10 Use of multiple sources in conditioning/calibrating groundwater flow and transport models (2006)

Groundwater chemistry in groundwater-fed wetlands often is the result of mixing of discharging groundwater and rainfall in combination with chemical reactions altering the chemical composition, mostly due ... [more ▼]

Groundwater chemistry in groundwater-fed wetlands often is the result of mixing of discharging groundwater and rainfall in combination with chemical reactions altering the chemical composition, mostly due to changes in redox conditions. In this study, a Self-Organizing Map is used to classify chemical groundwater samples of three groundwaterfed wetlands in Belgium in order to identify the origin of groundwater and to deduce redox conditions in the wetlands. The Self-Organizing Map (SOM) algorithm is an unsupervised neural network technique to represent a multidimensional dataset on a two-dimensional grid in a topology-preserving way, allowing investigation of non-linear, complex relationships between variables and grouping of the data (Kohonen, 1995). The SOM is trained with data from a regional groundwater monitoring network and rainfall data. The resulting SOM is able to distinguish between samples of different origin or redox conditions within the regional aquifers. Subsequently, samples of the three wetlands are shown to the SOM and each sample is classified as having a chemical composition comparable to rainfall or to one of the regional aquifers. [less ▲]

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See detailSimultaneous use of hydrogeological and geophysical data for groundwater protection zone delineation by co-conditional stochastic simulations
Rentier, Céline; Dassargues, Alain ULg

in Stauffer, Fritz; Dassargues, Alain (Eds.) Quantitative Geology from Multiple Sources: S10 Use of multiple sources in conditioning/calibrating groundwater flow and transport models (2006)

Delineation of protection zones is performed by the use of stochastic simulations integrating all available data. In practice, due to the few available measurements of the main parameters, it can be very ... [more ▼]

Delineation of protection zones is performed by the use of stochastic simulations integrating all available data. In practice, due to the few available measurements of the main parameters, it can be very useful to integrate other data to reduce the uncertainty of the results. Most of the solute spreading is governed by the hydraulic conductivity (K) spatial variability at different scales. A stochastic approach adding measured piezometric heads and electrical resistivity data is presented. Results are discussed on a synthetic and on a practical case. Delineating the ‘Capture zone Probability Distribution’ (CaPD) for a given time, it is shown how the uncertainty can be reduced. The methodology can be used in real applications when little or no information is available about the hydraulic properties, through the conditioning on other data sets. [less ▲]

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See detailSecondary data value for geostatistical estimation of flow and transport parameters in low-permeability media
Huysmans; Dassargues, Alain ULg

in Stauffer, Fr.; Dassargues, Alain (Eds.) Quantitative Geology from Multiple Sources: S10 Use of multiple sources in conditioning/calibrating groundwater flow and transport models (2006)

This study investigates the relative data value of different types of secondaryinformation to estimate the flow and transport parameters in low-permeability media. Data from four boreholes in two ... [more ▼]

This study investigates the relative data value of different types of secondaryinformation to estimate the flow and transport parameters in low-permeability media. Data from four boreholes in two different clay formations are analyzed. Cross-validation is used to estimate the relevance of the different types of secondary data. The flow and transport parameters (hydraulic conductivity, diffusion coefficient and diffusion accessible porosity) are first estimated by kriging using primary information only and then estimated by co-kriging using primary information and one type of secondary information (grain size, gamma ray and resistivity). The kriging residuals, i.e. the difference between the estimated and the measuredvalues, of the different kriging and co-kriging variants are compared to evaluate the relevance of each type of secondary information. This analysis shows that in the different clays and the different boreholes, different types of secondary information result in the largest improvement of the estimates of the flow and transport parameters. It is not possible to determine the "best" type of secondary information for improving the estimation of the flow and transport parameters of low-permeability media. This probably depends on local factors such as the quality of the loggings and the amount of nearby primary and secondary data. [less ▲]

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