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See detailLarge-scale stochastic optimization using non-stationary geostatistics for uncertainty assessment of groundwater flow and solute transport, in the framework of a near surface radioactive waste disposal
Rogiers, Bart; Laloy, E.; Gedeon, Matej et al

Poster (2014, July 09)

Uncertainty quantification is very much needed to support decision making related to e.g. environmental impact assessment for waste disposal sites. A probabilistic result provides a much stronger basis ... [more ▼]

Uncertainty quantification is very much needed to support decision making related to e.g. environmental impact assessment for waste disposal sites. A probabilistic result provides a much stronger basis for decision making compared to a single deterministic outcome. Accurate posterior exploration of high-dimensional and CPU-intensive models, which are often used for environmental impact assessment, is however a challenging task. To quantify the uncertainty associated with solute transport in the framework of a near surface radioactive waste disposal in Mol/Dessel, Belgium, we investigate combining the adaptive Metropolis (AM) McMC algorithm for updating the global model parameters, and adaptive spatial resampling (ASR) for updating of the spatially distributed model parameters, by block sampling. The forward model used is a groundwater flow model conditioned on borehole and direct push data, that accounts for non-stationary heterogeneity in hydraulic conductivity. The obtained flow solutions are used for solute transport simulations, and the results are compared with a different groundwater flow model parameterization, that makes use of homogeneous hydrogeological layers. Moreover, a number of simulations is performed to assess the effect of realistic dispersivity, which is derived from outcrop investigations. The obtained results indicate that the combination of AM and ASR using block sampling seems not to be very efficient for McMC sampling with the forward model used in this study. However, using the algorithm in optimization mode seems to work fine, and provides an alternate way for exploring the parameter space and the prediction uncertainty. [less ▲]

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See detailMulti-scale aquifer characterization and groundwater flow model parameterization using direct push technologies
Rogiers, Bart; Vienken, Thomas; Gedeon, Matej et al

in Environmental Earth Sciences (2014)

Direct push (DP) technologies are typically used for cost-effective geotechnical characterization of unconsolidated soils and sediments. In more recent developments, DP technologies have been used for ... [more ▼]

Direct push (DP) technologies are typically used for cost-effective geotechnical characterization of unconsolidated soils and sediments. In more recent developments, DP technologies have been used for efficient hydraulic conductivity (K) characterization along vertical profiles with sampling resolutions of up to a few centimetres. Until date, however, only a limited number of studies document high-resolution in situ DP data for three-dimensional conceptual hydrogeological model development and groundwater flow model parameterization. This study demonstrates how DP technologies improve building of a conceptual hydrogeological model. We further evaluate the degree to which the DP-derived hydrogeological parameter K, measured across different spatial scales, improves performance of a regional groundwater flow model. The study area covers an area of ~60 km² with two overlying, mainly unconsolidated sand, aquifers separated by a 5-7 m thick highly heterogeneous clay layer (in north-eastern Belgium). The hydrostratigraphy was obtained from an analysis of cored boreholes and about 265 cone penetration tests (CPTs). The hydrogeological parameter K was derived from a combined analysis of core and CPT data and also from hydraulic direct push tests. A total of 50 three-dimensional realizations of K were generated using a non-stationary multivariate geostatistical approach. To preserve the measured K values in the stochastic realizations, the groundwater model Krealizations were conditioned on the borehole and direct push data. Optimization was performed to select the best performing model parameterization out of the 50 realizations. This model outperformed a previously developed reference model with homogeneous K fields for all hydrogeological layers. Comparison of particle tracking simulations, based either on the optimal heterogeneous or reference homogeneous groundwater model flow fields, demonstrate the impact DP-derived subsurface heterogeneity in K can have on groundwater flow and solute transport. We demonstrated that DP technologies, especially when calibrated with site-specific data, provide high-resolution 3D subsurface data for building more reliable conceptual models and increasing groundwater flow model performance. [less ▲]

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See detailHigh resolution saturated hydraulic conductivity logging of friable to poorly indurated borehole cores using air permeability measurements
Rogiers, Bart; Winters, P.; Huysmans, Marijke et al

in Hydrogeology Journal (2014)

Saturated hydraulic conductivity (Ks) is one of the most important parameters determining groundwater flow and contaminant transport in both unsaturated and saturated porous media. This paper investigates ... [more ▼]

Saturated hydraulic conductivity (Ks) is one of the most important parameters determining groundwater flow and contaminant transport in both unsaturated and saturated porous media. This paper investigates the hand-held air permeameter technique for high resolution hydraulic conductivity determination on borehole cores using a spatial resolution of ~0.05 m. We test the suitability of such air permeameter measurements on friable to poorly indurated sediments to improve the spatial prediction of classical laboratory based Ks measurements obtained at a much lower spatial resolution (~2 m). About 368 Ks measurements were made on ~350 m of borehole cores originating from the Campine basin, Northern Belgium, while ~5230 air permeability measurements were performed on the same cores. The heterogeneity in sediments, ranging from sand to clayey sand with distinct clay lenses, resulted in a Ks range of seven orders of magnitude. Cross-validation demonstrated that using air permeameter data as secondary variable and laboratory based Ks measurements as primary variable increased performance from R2 = 0.35 for ordinary kriging (laboratory Ks only) to R2 = 0.61 for co-kriging. Due to the large degree of small-scale variability detected by the air permeameter, the spatial granularity in the predicted laboratory Ks also increases drastically. The separate treatment of Kh and Kv revealed considerable anisotropy in certain lithostratigraphical units, while others where clearly isotropic at the sample scale. Air permeameter measurements on borehole cores provide a cost-effective way to improve spatial predictions of traditional laboratory based Ks. [less ▲]

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See detailThe usefulness of outcrop-analogue air-permeameter measurements for analysing aquifer heterogeneity: testing outcrop hydrogeological parameters with independent borehole data
Rogiers, Bart; Beerten, Koen; Smeekens, Tom et al

in Hydrology & Earth System Sciences (2013), 17

Outcropping sediments can be used as easily accessible analogues for studying subsurface sediments, especially to determine the small-scale spatial variability of hydrogeological parameters. The use of ... [more ▼]

Outcropping sediments can be used as easily accessible analogues for studying subsurface sediments, especially to determine the small-scale spatial variability of hydrogeological parameters. The use of cost-effective in situvmeasurement techniques potentially makes the study of outcrop sediments even more attractive. We investigate to what degree air-permeameter measurements on outcrops of unconsolidated sediments can be a proxy for aquifer saturated hydraulic conductivity (K) heterogeneity. The Neogene aquifer in northern Belgium, known as a major groundwater resource, is used as the case study. K and grain-size data obtained from different outcropping sediments are compared with K and grain-size data from aquifer sediments obtained either via laboratory analyses on undisturbed borehole cores (K and grain size) or via large-scale pumping tests (K only). This comparison shows a pronounced and systematic difference between outcrop and aquifer sediments. Part of this difference is attributed to grain-size variations and earth surface processes specific to outcrop environments, including root growth, bioturbation, and weathering. Moreover, palaeoenvironmental conditions such as freezing–drying cycles and differential compaction histories will further alter the initial hydrogeological properties of the outcrop sediments. A linear correction is developed for rescaling the outcrop data to the subsurface data. The spatial structure pertaining to outcrops complements that obtained from the borehole cores in several cases. The higher spatial resolution of the outcrop measurements identifies small-scale spatial structures that remain undetected in the lower resolution borehole data. Insights in stratigraphic and K heterogeneity obtained from outcrop sediments improve developing conceptual models of groundwater flow and transport. [less ▲]

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See detailThe usefulness of outcrop analogue air permeameter measurements for analyzing aquifer heterogeneity: quantifying outcrop hydraulic conductivity and its spatial variability
Rogiers, Bart; Beerten, Koen; Smeekens, Tuur et al

in Hydrological Processes (2013)

Saturated hydraulic conductivity (K) is one of the most important parameters determining groundwater flow and contaminant transport in both unsaturated and saturated porous media. Although several well ... [more ▼]

Saturated hydraulic conductivity (K) is one of the most important parameters determining groundwater flow and contaminant transport in both unsaturated and saturated porous media. Although several well-established laboratory methods exist for determining K, in situ measurements of this parameter remain very complex and scale dependent. Often, the limited accessibility of subsurface sediments for sampling means an additional impediment to our ability to quantify subsurface K heterogeneity. One potential solution is the use of outcrops as analogues for subsurface sediments. This paper investigates the use of air permeameter measurements on outcrops of unconsolidated sediments to quantify K and its spatial heterogeneity on a broad range of sediment types. The Neogene aquifer in northern Belgium is used as a case study for this purpose. To characterize the variability in K, 511 small-scale air permeability measurements were performed on outcrop sediments representative over five of the aquifer’s lithostratigraphic units. From these measurements, outcrop-scale equivalent K tensors were calculated using numerical upscaling techniques. Validation of the air permeameter-based K values by comparison with laboratory constant head K measurements reveals a correlation of 0.93. Overall, the results indicate that hand-held air permeameters are very efficient and accurate tools to characterize saturated K, as well as its small-scale variability and anisotropy on a broad range of unconsolidated sediments. The studied outcrops further provided a qualitative understanding of aquifer hydrostratigraphy and quantitative estimates about K variability at the centimetre-scale to metre-scale. [less ▲]

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See detailMulti-scale aquifer characterization and groundwater flow model parameterization using direct push technologies
Rogiers, Bart; Vienken, T; Batelaan, Okke et al

in Novel Methods for Subsurface Characterization and Monitoring: From Theory to Practice (2013, May)

Groundwater flow and contaminant transport models are used to support decision making regarding waste disposal options, sites contaminated by surface or subsurface sources, or to develop and test cost ... [more ▼]

Groundwater flow and contaminant transport models are used to support decision making regarding waste disposal options, sites contaminated by surface or subsurface sources, or to develop and test cost-effective groundwater remediation schemes. Such models are influenced by different sources of uncertainty, including those due to spatial variability in aquifer and aquitard properties including hydraulic conductivity (K). However, quantifying spatial variability in K remains challenging. Classical drilling techniques for shallow heterogeneous unconsolidated sedimentary deposits involving continuous coring are expensive and time-consuming, especially when the area of interest exceeds several tens of km². Alternative techniques such as direct push technologies use hydraulic rams, supplemented with vehicle weight, or high-frequency hammering, to advance small-diameter tools into the subsurface. These tools are typically used for cost-effective geotechnical characterization of unconsolidated deposits; recent developments also allow for hydraulic characterization. The depth of investigation is up to ~40 m, depending on the tools used (i.e applied load) and sediment properties (friction). Up to now, only a limited number of studies document using this type of data to parameterize regional groundwater flow models. To fill this gap, this study aims at parameterizing a regional groundwater flow model using data from various types of direct push technologies. We discuss the characterization of an area (~60 km²) near the nuclear zone of Mol/Dessel (Belgium), using various direct push technologies. Most of the measurements are concentrated in an area of 200×400 m². The data include 265 cone penetration tests (CPTs), 113 pore pressure dissipation tests (PPDTs), 17 direct push injection logs (DPIL), 6 hydraulic profiling tool (HPT) logs and 19 direct push slug tests (DPST). Resulting K values, either calculated or estimated, and the corresponding spatial variability are compared with that of borehole and outcrop studies. The benefit of using standard CPT data for the parameterization of an aquitard at the study site has previously been shown. The approach is now applied to the aquifer units and incorporates new direct push data for the entire upper ~40 m of the hydrogeological domain. The effect of the 3D heterogeneous hydraulic conductivity field on the performance of the groundwater flow model is discussed; the value of the different direct push technologies is equally addressed. [less ▲]

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See detailDerivation of flow and transport parameters from outcropping sediments of the Neogene aquifer, Belgium
Rogiers, Bart; Beerten, K.; Smeekens, T. et al

in Geologica Belgica (2013), 16(3), 129-147

Centimetre-scale saturated hydraulic conductivities (K) are derived from air permeability measurements on a selection of outcrops of the Neogene aquifer in the Campine area, Belgium. Outcrop sediments are ... [more ▼]

Centimetre-scale saturated hydraulic conductivities (K) are derived from air permeability measurements on a selection of outcrops of the Neogene aquifer in the Campine area, Belgium. Outcrop sediments are of Miocene to Quaternary age and have a marine to continental origin. Grain size analyses for the same outcrops and corresponding K predictions using previously developed models are also presented. We discuss outcrop hydrogeological properties and quantify the heterogeneity within the outcrops in detail using geostatistical variography. Moreover, outcrop-scale K values, their anisotropy and dispersivities are numerically calculated as a means to upscale such small-scale measurements to a larger scale commensurate with the scale of flow and transport modelling. By studying the small-scale variability as observed in outcrops, we gain crucial understanding of the larger-scale behaviour of the corresponding hydrogeological units within the Neogene aquifer, the most important groundwater reservoir of Flanders. The results of this study will equally improve conceptual hydrogeological model building and parameterization. [less ▲]

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See detailEstimation of hydraulic conductivity and its uncertainty from grain-size data using GLUE and artificial neural networks
Rogiers, Bart; Mallants, Dirk; Batelaan, Okke et al

in Mathematical Geosciences (2012), 44(6), 739-763

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 ... [more ▼]

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, i.e.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 generalized 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 literature demonstrates the importance of site specific calibration. The dataset 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 optimized models is presented for a borehole lacking Ks data. [less ▲]

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See detailSite-specific soil classification from cone penetration tests and borehole data: a multivariate statistical analysis
Rogiers, Bart; Mallants, Dirk; Batelaan, Okke et al

in NovCare 2011 edition:2 (2011, May 11)

Groundwater flow and contaminant transport models are influenced by different kinds of uncertainty, including spatial variability in aquifer and aquitard properties. Appropriate models are developed to ... [more ▼]

Groundwater flow and contaminant transport models are influenced by different kinds of uncertainty, including spatial variability in aquifer and aquitard properties. Appropriate models are developed to support decision making related to environmental impact assessment for waste disposal sites or to develop cost-effective groundwater remediation. These modelling tools are required to incorporate spatial variability as observed at different scales. Several studies have investigated correlations between geotechnical data, e.g. cone penetration tests (CPT) results, and hydrogeological parameters such as hydraulic conductivity (K). However, very few studies have used geotechnical data in groundwater flow models, although gathering of such information is usually much easier and cheaper compared to expensive drilling and pumping test campaigns. It is however generally known that CPT soil behaviour type (SBT) classifications are only indicative, and parameters attributed to these SBTs should therefore be treated with caution. Moreover, since most parameters of interest in groundwater flow modeling do not share a one-to-one relationship with the standard CPT parameters, the assessment of the uncertainty related to this relationship is of great importance in stochastic modeling. Since a unique data set is available for the nuclear zone of Mol/Dessel (Belgium) different approaches to describe the spatial variability in flow and transport parameters can be tested. A detailed hydrogeological characterization reaching depths of 40 to 50 m (including Quaternary and Neogene formations) has been carried out in 2008-2009 coordinated by ONDRAF/NIRAS (Belgian Agency for Radioactive Waste and Enriched Fissile Materials) in the frame of a surface disposal project for low and intermediate short-lived radioactive waste. A large amount of quantitative and semi-quantitative information has been collected in an area of 60 km², including borehole logs, more than 200 CPTs, and roughly 340 K measurements on undisturbed cores. This study uses exploratory cluster analysis to classify this multivariate dataset into different groups. This is achieved with k-means clustering, minimizing the within-group variance. The resulting classes are then interpreted with respect to the site-specific lithotypes using factor analysis. Next, a simplified classification is derived, as it is not possible to distinguish between some groups solely based on CPT data. The resulting classification is compared with literature SBT classifications in relation to the mean and variance of groundwater flow and physical/chemical parameters within each class. Both classifications are also compared in their ability to identify the local litho- and hydro-stratigraphy. Hydraulic conductivity values derived from pore pressure dissipation tests are compared with estimations obtained from SBT classifications. A final discussion is devoted to the integral scales of the corresponding indicator fields of soil classes and how these differ between the different classification schemes. [less ▲]

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See detailCaractérisation de l'hétérogénéité de la conductivité hydraulique à saturation au moyen d'essais de pénétration au cône
Rogiers, Bart; Mallants, Dirk; Batelaan, Okke et al

in Bulletin du GFHN n°56 (2010, November)

Dans le cadre du stockage des déchets radioactifs à activité faible et intermédiaire dans une installation de proche surface à Dessel, une caractérisation du site et de ses alentours a été réalisée ces ... [more ▼]

Dans le cadre du stockage des déchets radioactifs à activité faible et intermédiaire dans une installation de proche surface à Dessel, une caractérisation du site et de ses alentours a été réalisée ces dernières années. Les données recueillies contiennent à ce jour 388 mesures de conductivité hydraulique à saturation collectées sur 8 sites de sondage. En outre, des informations secondaires comme la granulométrie, la résistivité électrique, et la porosité ont été recueillies. Pour extrapoler ces informations à toute la région (70 km²), et bâtir un modèle hydrogéologique, plusieurs campagnes géotechniques ont été réalisées avec un total d'environ 260 essais de pénétration au cône. Avec cet ensemble de données, une approche probabiliste à petite échelle peut être utilisée 1) pour valider les modèles déterministes basée sur des paramètres moyens à grande échelle, et 2) pour aider à développer un réseau de surveillance. Parce que les essais de pénétration au cône délivrent une large information sur la variabilité spatiale du sous-sol, ils sont étudiés en détail. Une approche empirique est utilisée pour estimer la conductivité hydraulique à saturation à partir des paramètres des essais de pénétration au cône. En raison de la complexité de la relation entre la conductivité hydraulique à saturation et les variables mesurées sur le terrain, les modèles de régression ne sont pas satisfaisants pour le site investigué. De plus, le nombre de données est insuffisant pour appliquer des méthodes plus complexes comme par exemple les réseaux de neurones artificiels. Par analogie avec les méthodes classique de la classification du sous-sol en fonction des essais de pénétration au cône, une interpolation des données est faite dans le plan de la résistance du cône et le ratio de ce dernier avec la résistance au frottement latéral. Ainsi, une table de conversion est construite pour coupler les paramètres géotechniques à la conductivité hydraulique à saturation. Pour valider la méthode utilisée, les résultats sont comparés avec d'autres informations du sous-sol comme les carottes des sondages et avec des estimations de la conductivité hydraulique à saturation déduites d’analyses granulométriques. Finalement, une réalisation conditionnelle géostatistique de la conductivité hydraulique saturée d'un volume à trois dimensions de 400x175x40 m est proposée, et permet de se rendre compte de l’existence de 2 unités hydrogéologiques contrastées avec de différents modèles structurels. [less ▲]

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See detailGroundwater model parameter identification using a combination of cone-penetration tests and borehole data
Rogiers, Bart; Schiltz, Marco; Beerten, Koen et al

in International Groundwater Symposium 2010, IAHR (2010, September)

In the framework of the disposal of short-lived low- and intermediate-level radioactive waste in a near-surface disposal facility in Dessel, Belgium, additional extensive site characterisation has been ... [more ▼]

In the framework of the disposal of short-lived low- and intermediate-level radioactive waste in a near-surface disposal facility in Dessel, Belgium, additional extensive site characterisation has been performed in 2008. The gathered data now include 388 hydraulic conductivity measurements on samples from 8 cored boreholes. Detailed characterisation of these cored boreholes, together with geophysical logging, enabled to identify various hydrostratigraphical units at 8 discrete locations in the research area. Various analyses were performed on the cores, yielding information on grain size, mineralogy, density and total porosity. Geophysical logging parameters were derived from gamma-ray and resistivity measurements. Subsequently, an extensive geotechnical logging campaign was performed in order to establish a 3D-model of the hydrostratigraphical units, based on a dense network of investigation points. About 180 cone penetration tests (CPTs) were executed and lithology was deduced in detail based on existing soil classi cation charts. As such, a description of the regional subsurface up to depths of nearly 50 m was established, and this information was integrated with the borehole data. Most importantly, the lateral extent, depth and thickness of a hydrogeologically important aquitard was identi fied. Based on the 2008 site characterisation results and their interpretation, an update of a ground- water fl ow model used in safety assessments was made. The CPT-based stratigraphic model and the hydraulic conductivity data determined at different scales were combined into a new 3D hydrostratigraphical model. The small-scale measurements (on 100 cm³ core samples) are compared with hydraulic conductivity values obtained from pumping tests and the large-scale parameters derived by inverse modelling. The performance of the original and the updated flow model are compared. The presented approach was succesfull in substantially decreasing the conceptual model and parameter uncertainty and resulted in an improved calibration of the groundwater flow model. [less ▲]

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See detailGeostatistical analysis of primary and secondary data in a sandy aquifer at Mol/Dessel (Belgium)
Rogiers, Bart; Mallants, Dirk; Batelaan, Okke et al

in Cokx, L.; Van Meirvenne, M.; Bogaert, P. (Eds.) et al 8th International Conference on Geostatistics for Environmental Applications (GeoENV2010) (2010, September)

In the framework of the disposal of short-lived low- and intermediate-level radioactive waste in a near-surface disposal facility in Dessel, Belgium, additional extensive site characterization has been ... [more ▼]

In the framework of the disposal of short-lived low- and intermediate-level radioactive waste in a near-surface disposal facility in Dessel, Belgium, additional extensive site characterization has been performed in 2008. The gathered data now enclose 388 hydraulic conductivity measurements on samples of 8 cored boreholes. Secondary information as grain size analysis, porosity, and borehole geophysical parameters was also gathered. In addition, the geology of the study area has also been thoroughly characterized by a set of 178 cone penetration tests (CPTs) to approximate 50 m depth. This dataset allowed to refine the hydrostratigraphical model of the region. The existing groundwater model, based on large-scale effective hydraulic properties, was updated accordingly. The next step is a small-scale probabilistic approach 1) to validate the current existing deterministic groundwater models and 2) to support design for a monitoring network. In preparation for stochastic realizations of the subsurface, a geostatistical analysis of the available primary and secondary data is performed. [less ▲]

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See detailExploratory and structural data analysis of a sandy aquifer at Mol/dessel, Belgium
Rogiers, Bart; Mallants, Dirk; Batelaan, Okke et al

in Geophysical Research Abstracts, Vol. 12 (2010, May 03)

In the framework of the disposal of short-lived low- and intermediate-level radioactive waste in a near-surface disposal facility in Dessel, Belgium, additional extensive site characterization has been ... [more ▼]

In the framework of the disposal of short-lived low- and intermediate-level radioactive waste in a near-surface disposal facility in Dessel, Belgium, additional extensive site characterization has been performed in 2008. The gathered data now enclose 388 hydraulic conductivity measurements on samples of 8 cored boreholes. Additionally, secondary information as grain size analysis, resistivity, porosity, etc. was gathered. With this dataset, a small-scale probabilistic approach can be used 1) to validate the current existing deterministic groundwater models based on large-scale average hydraulic conductivity values, and 2) to support design for a monitoring network. In preparation for stochastic realizations of the subsurface, an exploratory and structural data analysis was performed and is presented here. The exploratory data analysis allows definition of hydrogeological units, independent from the existing lithostratigraphy. It also shows the presence of three conductivity classes within a strongly heterogeneous unit. Currently, only vertical structural data analysis is performed based on vertical data profiles. Semi-variograms and cross-variograms are calculated to characterize the spatial variance of the different hydrogeological units. Additional sampling and measurements will be performed in the future to provide information of the horizontal spatial variance. Together with the current results, these will serve as the basis for conditional stochastic simulation of groundwater flow and contaminant transport. [less ▲]

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