The effect of heterogeneity of diffusion parameters on chloride transport in low-permeability argillites

in Environmental Earth Sciences (2013), 68(7), 1835-1848

Understanding flow and transport in low-permeability media is very important in the context of nuclear waste disposal, oil and gas reservoirs and long term evolution of groundwater systems. In low ... [more ▼]

Understanding flow and transport in low-permeability media is very important in the context of nuclear waste disposal, oil and gas reservoirs and long term evolution of groundwater systems. In low-permeability media, transport by diffusion is often the most important mass transport process. This study investigates the effect of the heterogeneity of diffusion parameters on mass transport in low-permeability media. A geostatistical approach for integrating heterogeneity of diffusion parameters in groundwater flow and transport models is proposed and applied to the Toarcian argillites in France which are studied in the framework of feasibility of storing radioactive waste in deep clayey massifs. Stochastic fields of the diffusion parameters of the Toarcian argillites (France) are generated based on 64 measured values of diffusion coefficient and diffusion accessible porosity and used as input for a 3D local-scale groundwater flow and transport model. The chloride concentrations computed by these heterogeneous models are compared to the measured chloride concentrations and to concentrations calculated with a model in which the Toarcian argillites are subdivided into several homogeneous zones. The heterogeneous simulations result in a slightly better correspondence between measured and calculated values and have the additional advantage that the measured diffusion coefficient values in the Toarcian are perfectly honored in the model. This study shows that small-scale variability of diffusion parameters has a significant effect on solute concentrations and omitting this heterogeneity may be a problem in transport calculations in low-permeability media, depending on the specific setting and objectives of the study. [less ▲]

Modeling the effect of clay drapes on pumping test response in a cross-bedded aquifer using multiple-point geostatistics

in Journal of Hydrology (2012), 450-451

This study investigates whether fine-scale clay drapes can cause an anisotropic pumping test response at a much larger scale. A pumping test was performed in a sandbar deposit consisting of cross-bedded ... [more ▼]

This study investigates whether fine-scale clay drapes can cause an anisotropic pumping test response at a much larger scale. A pumping test was performed in a sandbar deposit consisting of cross-bedded units composed of materials with different grain sizes and hydraulic conductivities. The measured drawdown values in the different observation wells reveal an anisotropic or elliptically-shaped pumping cone. The major axis of the pumping ellipse is parallel with the strike of cm to m-scale clay drapes that are observed in several outcrops. To determine (1) whether this large-scale anisotropy can be the result of fine-scale clay drapes and (2) whether application of multiple-point geostatistics can improve interpretation of pumping tests, this pumping test is analyzed with a local 3D groundwater model in which fine-scale sedimentary heterogeneity is modelled using multiple-point geostatistics. To reduce CPU and RAM demand of the multiple-point geostatistical simulation step, edge properties indicating the presence of irregularly-shaped surfaces are directly simulated. Results show that the anisotropic pumping cone can be attributed to the presence of the clay drapes. Incorporating fine-scale clay drapes results in a better fit between observed and calculated drawdowns. These results thus show that fine-scale clay drapes can cause an anisotropic pumping test response at a much larger scale and that the combined approach of multiple-point geostatistics and cell edge properties is an efficient method for integrating fine-scale features in larger scale models. [less ▲]

Relationship between sedimentary features and permeability at different scales in the Brussels Sands

in Geologica Belgica (2012), 15(3), 156-164

The Brussels Sands display a complex three-dimensional subsurface architecture. This sedimentological heterogeneity induces a highly heterogeneous spatial distribution of hydrogeological parameters at ... [more ▼]

The Brussels Sands display a complex three-dimensional subsurface architecture. This sedimentological heterogeneity induces a highly heterogeneous spatial distribution of hydrogeological parameters at different scales and may consequently influence subsurface fluid flow and solute migration. This study aims at characterizing spatial variability of permeability at different scales in the Brussels Sands. Firstly, a literature review on the permeability distribution of the Brussels Sands was performed. Secondly, a field campaign was carried out consisting of field observations of the small-scale sedimentary structures and in situ measurements of air permeability. A total of 6550 cm-scale air permeability measurements were carried out in situ in three Brussels Sands quarries in the central part of Belgium: Bierbeek, Mont Saint Guibert and Chaumont Gistoux. On the large basin scale, substantial differences in permeability are observed. A literature data analysis shows that there is no clear correlation between hydraulic conductivity and sedimentary facies. At the small scale, results show that permeability heterogeneity and anisotropy are strongly influenced by sedimentary heterogeneity in all three quarries. Clay-rich sedimentary features such as bottomsets and distinct mud drapes exhibit a different statistical and geostatistical permeability distribution compared to the cross-bedded lithofacies, where the permeability anisotropy is dominated by the foreset lamination orientation. [less ▲]

Groundwater model parameter identification using a combination of cone-penetration tests and borehole data

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 ▲]

Geostatistical analysis of primary and secondary data in a sandy aquifer at Mol/Dessel (Belgium)

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 ▲]

Caractérisation de l'hétérogénéité de la conductivité hydraulique à saturation au moyen d'essais de pénétration au cône

in 35èmes Journées du GFHN (Groupe Francophone Humidimétrie et traNsferts en milieux poreux):Transferts en milieux poreux: Hétérogénéité des processus et des propriétés (2010)

Small-scale sedimentary structures and permeability in a cross-bedded aquifer

in Journal of Hydrology (2008), (361), 41-51

The objective of this study is to investigate the relation between small-scale sedimentary structures and permeability in the Brussels Sands formation, an early Middle-Eocene shallow marine sand deposit ... [more ▼]

The objective of this study is to investigate the relation between small-scale sedimentary structures and permeability in the Brussels Sands formation, an early Middle-Eocene shallow marine sand deposit in Central Belgium that constitutes a major groundwater source in the region. A field campaign was carried out consisting of field observations of the sedimentary structures and in situ measurements of air permeability. The sedimentary structures were interpreted, sketched, digitally photographed and measured in a representative outcrop. Additionally, a total of 2750 cm-scale air permeability measurements were carried out in situ. Analysis of the spatial distribution of sedimentary structures and permeability shows that clay-rich sedimentary features such as bottomsets and distinct mud drapes exhibit a different statistical and geostatistical permeability distribution compared to the other lithofacies in the cross-bedded sands. Spatial analysis of the air permeability data shows that permeability anisotropy in the cross-bedded lithofacies is dominated by the foreset lamination orientation. These results show that smallscale sedimentary heterogeneity strongly influences the local spatial distribution of the hydraulic properties and results in permeability heterogeneity and stratification that would produce anisotropy in upscaled permeability values. [less ▲]

Hydrogeological modeling of radionuclide transport in low permeability media: a comparison between Boom Clay and Ypresian Clay

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 ▲]

Stochastic analysis of the effect of spatial variability of diffusion parameters on radionuclide transport in low permeability clay layer

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 ▲]

Effect of excavation induced fractures on radionuclide migration through the Boom Clay (Belgium)

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 ▲]