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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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See detailReactive transport modelling of a groundwater contamination by ammoniacal liquor
Haerens, Bruno; Prommer, Henning; Lerner, David N. et al

(2006)

A reactive transport modeling study was carried out to assess the fate of a groundwater contamination by ammoniacal liquor from a former coking plant and the associated geochemical response. The ... [more ▼]

A reactive transport modeling study was carried out to assess the fate of a groundwater contamination by ammoniacal liquor from a former coking plant and the associated geochemical response. The simulations over a 45-year period provide a conclusive explanation and quantitative description of all measured data from observation wells down gradient of the contaminant source. It is shown that cation exchange exerts the main control on the fate of the ammonium plume as it strongly retards the migration of dissolved ammonium. The sorption of ammonium is accompanied by the elution of native cations, an effect that can be seen in some observation wells where ammonium is absent. While phenol has not been detected in the observation wells in recent years, the modeling results suggest that it has completely degraded in the aquifer, which is inferred from the agreement between the simulated and the observed geochemical fingerprint that the degradation of phenol imposes on groundwater composition. [less ▲]

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See detailRessources en eaux souterraines: caractérisation pour une gestion durable en liaison avec l’aménagement du territoire
Dassargues, Alain ULg

in Les Risques majeurs en Région Wallonne / Prévenir en aménageant (2006)

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See detailRisk assessment of groundwater pollution using sensitivity analysis and a worst-case scenario analysis
Huysmans, Marijke; Madarasz, Tamas; Dassargues, Alain ULg

in Environmental Geology (2006), 50(2), 180-193

This paper illustrates how sensitivity analysis and a worst-case scenario analysis can be useful tools in risk assessment of groundwater pollution. The approach is applied to a study area in Hungary with ... [more ▼]

This paper illustrates how sensitivity analysis and a worst-case scenario analysis can be useful tools in risk assessment of groundwater pollution. The approach is applied to a study area in Hungary with several known groundwater pollution sources and nearby drinking water production wells. The main concern is whether the contamination sources threaten the drinking water wells of the area. A groundwater flow and transport model is set up to answer this question. Due to limited data availability, the results of this model are associated with large uncertainty. Sensitivity analysis and a worst-case scenario analysis are applied to estimate this uncertainty and build confidence in the model results. [less ▲]

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See detailDétermination des zones de prévention du site de captage d'Onoz
Henrion, Vincent; Dassargues, Alain ULg

Report (2005)

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See detailVulnérabilités et protection des eaux souterraines
Dassargues, Alain ULg

Conference given outside the academic context (2005)

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See detailReview of the use of Peclet numbers to determine the relative importance of advection and diffusion in low permeability environments
Huysmans, Marijke; Dassargues, Alain ULg

in Hydrogeology Journal (2005), 13(5-6), 895-904

In low permeability environments, transport by advection is often neglected based on a Peclet number criterion. Such a criterion usually states that if the Peclet number (Pe) is much smaller than 1 ... [more ▼]

In low permeability environments, transport by advection is often neglected based on a Peclet number criterion. Such a criterion usually states that if the Peclet number (Pe) is much smaller than 1, diffusion dominates over advection and transport may be modeled considering diffusion only. Unfortunately, up to 10 different Peclet number definitions exist and for a particular case these different definitions lead to very diverse Peclet number values, differing several orders of magnitude from each other. In this paper, the different Peclet number definitions are therefore evaluated on their ability to determine the relative importance of transport by advection and by diffusion in low permeability environments. This is done by comparing the results of the analytical solution for pure diffusion with the analytical solution for diffusion, advection and dispersion for a large number of different input parameter values. The relation between the different Peclet numbers and the difference between the calculated concentration considering diffusion only and the calculated concentration considering both diffusion and advection is studied. These calculations show that some Peclet number definitions are not well suited to decide whether advection may be neglected in low permeability media. [less ▲]

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See detailCarte hydrogéologique, Seraing - Chênée 42/5-6, 1/25.000 : [notice explicative]
Ruthy, Ingrid ULg; Dassargues, Alain ULg

Book published by Ministère de la Région wallonne, Direction générale des ressources naturelles et de l'environnement - Edition provisoire : août 2005 (2005)

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See detailCarte hydrogéologique, Huy - Nandrin 48/3-4, 1/25.000 : [notice explicative]
Ruthy, Ingrid ULg; Dassargues, Alain ULg

Book published by Ministère de la Région wallonne, Direction générale des ressources naturelles et de l'environnement - Edition provisoire : août 2005 (2005)

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See detailCarte hydrogéologique de Wallonie, Huy - Nandrin 48/3-4
Ruthy, Ingrid ULg; Dassargues, Alain ULg

Cartographic material (2005)

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See detailCarte hydrogéologique de Wallonie, Seraing - Chênée 42/5-6
Ruthy, Ingrid ULg; Dassargues, Alain ULg

Cartographic material (2005)

Detailed reference viewed: 163 (14 ULg)
See detailCarte hydrogéologique de Wallonie, Hamoir - Ferrières 49/5-6, 1/25.000 : [notice explicative]
Ruthy, Ingrid ULg; Dassargues, Alain ULg

Book published by Ministère de la Région wallonne, Direction générale des ressources naturelles et de l'environnement - Edition provisoire : juin 2005 (2005)

Detailed reference viewed: 122 (25 ULg)