| 1. |
- Berglund, Sten, et al.
(författare)
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Identification and Characterization of Potential Discharge Areas for Radionuclide Transport by Groundwater from a Nuclear Waste Repository in Sweden
- 2013
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Ingår i: Ambio. - : Springer Nature. - 0044-7447 .- 1654-7209. ; 42:4, s. 435-446
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Tidskriftsartikel (refereegranskat)abstract
- This paper describes solute transport modeling carried out as a part of an assessment of the long-term radiological safety of a planned deep rock repository for spent nuclear fuel in Forsmark, Sweden. Specifically, it presents transport modeling performed to locate and describe discharge areas for groundwater potentially carrying radionuclides from the repository to the surface where man and the environment could be affected by the contamination. The modeling results show that topography to large extent determines the discharge locations. Present and future lake and wetland objects are central for the radionuclide transport and dose calculations in the safety assessment. Results of detailed transport modeling focusing on the regolith and the upper part of the rock indicate that the identification of discharge areas and objects considered in the safety assessment is robust in the sense that it does not change when a more detailed model representation is used.
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| 2. |
- Bosson, Emma, et al.
(författare)
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Exchange and pathways of deep and shallow groundwater in different climate and permafrost conditions using the Forsmark site, Sweden, as an example catchment
- 2013
-
Ingår i: Hydrogeology Journal. - : Springer Nature. - 1431-2174 .- 1435-0157. ; 21:1, s. 225-237
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Tidskriftsartikel (refereegranskat)abstract
- This study simulates and quantifies the exchange and the pathways of deep and shallow groundwater flow and solute transport under different climate and permafrost conditions, considering the example field case of the coastal Forsmark catchment in Sweden. A number of simulation scenarios for different climate and permafrost condition combinations have been performed with the three-dimensional groundwater flow and transport model MIKE SHE. Results show generally decreasing vertical groundwater flow with depth, and smaller vertical flow under permafrost conditions than under unfrozen conditions. Also the overall pattern of both the vertical and the horizontal groundwater flow, and the water exchange between the deep and shallow groundwater systems, change dramatically in the presence of permafrost relative to unfrozen conditions. However, although the vertical groundwater flow decreases significantly in the presence of permafrost, there is still an exchange of water between the unfrozen groundwater system below the permafrost and the shallow groundwater in the active layer, via taliks. ‘Through taliks’ tend to prevail in areas that constitute groundwater discharge zones under unfrozen conditions, which then mostly shift to net recharge zones (through taliks with net downward flow) under permafrost conditions.
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| 3. |
- Cvetkovic, Vladimir, et al.
(författare)
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Comparative measures of radionuclide containment in the crystalline geosphere
- 2002
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Ingår i: Nuclear science and engineering. - : Informa UK Limited. - 0029-5639 .- 1943-748X. ; 142:3, s. 292-304
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Tidskriftsartikel (refereegranskat)abstract
- A probabilistic model for assessing the capacity of a fractured crystalline rock volume to contain radionuclides is developed The rock volume is viewed as a network of discrete fractures through which radionuclides are transported by flowing water. Diffusive mass transfer between the open fractures and the stagnant water in the pore space of the rock matrix allow radionuclides access to mineral grains where physical and chemical processes-collectively known as sorption-can retain radionuclides. A stochastic Lagrangian framework is adopted to compute the probability that a radionuclide particle will be retained by the rock, i.e., the probability that it will decay before being released from the rock volume. A dimensionless quantity referred to as the containment index is related to this probability and proposed as a suitable measure for comparing different rock volumes; such a comparative measure may be needed, for example, in a site selection program for geological radioactive waste disposal. The probabilistic solution of the transport problem is based on the statistics of two Lagrangian variables: T, the travel time of an imaginary tracer moving with the flowing water, and beta, a suitably normalized surface area available for retention. Statistics of tau and beta may be computed numerically using site-specific discrete fracture MP network simulations. Fracture data from the well-characterized Aspo Hard Rock Laboratory site in southern Sweden are used to illustrate the implementation of the proposed containment index for six radionuclides (Sn-126, I-129, Cs-135, Np-237, Pu-239, and Se-79). It is found that fractures of small aperture imply prolonged travel times and hence long tails in both beta and tau. This, in turn, enhances retention and is favorable from a safely assessment perspective.
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| 4. |
- Cvetkovic, Vladimir, et al.
(författare)
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Inference of Retention Time From Tracer Tests in Crystalline Rock
- 2020
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Ingår i: Water resources research. - : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 56:2
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Tidskriftsartikel (refereegranskat)abstract
- A statistical parametrization of transport combined with a new, general partition function for diffusive mass transfer (Cvetkovic, 2017, ) is here developed into a practical tool for evaluating tracer tests in crystalline rock. The research question of this study is how to separate the characteristic times of retention and advection, using tracer test information alone; this decoupling is critical for upscaling of transport. Three regimes are identified based on the unconditional mean number of trapping events. Analytical expressions are derived for inferring transport-retention parameters; these are first tested on a series of generic examples and then using two sets of tracer test data. Our results indicate that the key transport-retention parameters can be inferred separately with reasonable accuracy by a few simple steps, provided that the macrodispersion is not too large and retention not too strong. Of particular interest is inference of the retention time from the breakthrough curve peak that avoids costly asymptotic monitoring. Finally, we summarize the retention times as inferred from a series of nonsorbing tracer tests in the Swedish granite, demonstrating the uncertainties when estimating retention based on material and structural properties from samples. Possible strategies for reducing these uncertainties that combine improved understanding of crystalline rock evolution with numerical simulations are noted as topics for future research.
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| 5. |
- Cvetkovic, Vladimir, et al.
(författare)
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Stochastic simulation of radionuclide migration in discretely fractured rock near the Aspo Hard Rock Laboratory
- 2004
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Ingår i: Water resources research. - : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 40:2, s. W02404-
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Tidskriftsartikel (refereegranskat)abstract
- We study the migration of sorbing tracers through crystalline rock by combining relatively simple transport measures with particle tracking in a discrete fracture network. The rock volume is on a 100 m scale and is a replica of a thoroughly characterized site at the Aspo Hard Rock Laboratory, Sweden. Flow is driven by generic boundary conditions consistent with the natural gradient in the region. The emphasis is on the global effect of fracture-to-fracture hydraulic variability where individual fractures are assumed to be of uniform aperture. The transport measures are conditioned on two random variables: the water residence time (tau) and a parameter which quantifies the hydrodynamic control of retention (beta). Results are illustrated for two radionuclides: technetium (strongly sorbing) and strontium (weakly sorbing). It is found that the assumption of streamline routing or full mixing at fracture intersections has comparatively little impact on transport. The choice of the cubic or quadratic hydraulic law (i.e., relation between transmissivity and aperture) strongly affects water residence times but has little impact on average transport since it does not affect the statistics of beta. If the statistics of beta are known, then the distribution of water residence time (tau) is of little importance for transport. We assess the applicability of a linearized model beta = tau/b(ret) using two different approaches to estimate the effective retention'' aperture 2b(ret): from transmissivity data and from fracture density and flow porosity data. Under some conditions, these conventional estimates may provide acceptable representation of transport. The results stress the need for further studies on upscaling of tau, beta distributions as well as on estimating effective parameters for hydraulic control of retention.
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| 6. |
- Cvetkovic, Vladimir, et al.
(författare)
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Water and solute transport along hydrological pathways
- 2012
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Ingår i: Water resources research. - : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 48:6, s. W06537-
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Tidskriftsartikel (refereegranskat)abstract
- A Lagrangian framework for material transport along hydrological pathways is presented and consequences of statistically stationary space-time flow velocity variations on advective transport are investigated. The two specific questions addressed in this work are: How do temporal fluctuations affect forward and backward water travel time distributions when combined with spatial variability? and Can mass transfer processes be quantified using conditional probabilities in spatially and temporally variable flow? Space-time trajectories are studied for generic conditions of flow, with fully ergodic or only spatially ergodic velocity. It is shown that forward and backward distributions of advective water travel time coincide for statistically stationary space-time variations. Temporal variability alters the statistical structure of the Lagrangian velocity fluctuations. Once this is accounted for, integration of the memory function with the travel time distribution is applicable for quantifying retention. Further work is needed to better understand the statistical structure of space-time velocity variability in hydrological transport, as well as its impact on tracer retention and attenuation.
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| 7. |
- Davy, Philippe, et al.
(författare)
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Scaling of fractured rock flow. Proposition of indicators for selection of DFN based flow models
- 2023
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Ingår i: Comptes rendus Geoscience. - : Cellule MathDoc/CEDRAM. - 1631-0713 .- 1778-7025. ; 355
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Tidskriftsartikel (refereegranskat)abstract
- The objective of the paper is to better understand and quantify the flow structure in fractured rocks from flow logs, and to propose relevant indicators for validating, calibrating or even rejecting hydrogeological models. We first studied what the inflow distribution tells us about the permeability structure from a series of analyses: distribution of transmissivities as a function of depth, proportion of flowing sections as a function of section scale, and scaling of the arithmetically-averaged and geometrically-averaged permeability. We then define three indicators that describe few fundamental characteristics of the flow/permeability, whatever the scale: a percolation scale ls, the way permeability increases with scale above ls, and the variability of permeability. A 4th indicator on the representative elemental volume could in principle be defined but the data show that this volume/scale is beyond the 300 m investigated. We tested a series of numerical models built in three steps: the geo-DFN based on the observed fracture network, the open-DFN which is the part of the geo-DFN where fractures are open, and a transmissivity model applying on each fracture of the open-DFN (Discrete Fracture Network). The analysis of the models showed that the percolation scale is controlled by the open-DFN structure and that the percolation scale can be predicted from a scale analysis of the percolation parameter (basically, the third moment of the fracture size distribution that provides a measure of the network connectivity). The way permeability increases with scale above the percolation threshold is controlled by the transmissivity model and in particular by the dependence of the fracture transmissivity on either the orientation of the fractures via a stress-controlled transmissivity or their size or both. The comparison with data on the first two indicators shows that a model that matches the characteristics of the geo-DFN with an open fraction of 15% as measured adequately fits the data provided that the large fractures remain open and that the fracture transmissivity model is well selected. Most of the other models show unacceptable differences with data but other models or model combinations has still to be explored beforerejecting them. The third indicator on model variability is still problematic since the natural data show a higher variability than the models but the open fraction is also much more variable in the data than in the models.
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| 8. |
- Doolaeghe, D., et al.
(författare)
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Controls on fracture openness and reactivation in Forsmark, Sweden
- 2023
-
Ingår i: Scientific Reports. - : Springer Nature. - 2045-2322. ; 13:1
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Tidskriftsartikel (refereegranskat)abstract
- In crystalline bedrock, the open fraction of the fracture network constitutes the main pathways for fluids. Many observations point out that the state of stress influences the open fraction, likely indicating recent reactivation. But how this occurs is still unresolved. We analyse the conditions for fracture reactivation from fracture data collected in the uppermost 1 km of bedrock in Forsmark, Sweden. The open fraction is mainly correlated to the stress acting normally on the fracture but even away from critical failure, leading us to analyse the potential fluid pressure required for reactivation, P-c. We observe that 100% of the fractures are open when P-c is hydrostatic, and the ratio decreases exponentially to a plateau of similar to 17% when P-c is lithostatic and above. Exceptions are the oldest fractures, having a low open fraction independent of P-c. We suggest that these results reflect past pressure build-ups, potentially related to recent glaciations, and developing only if the preexisting open fraction is large enough.
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| 9. |
- Grenier, Christophe, et al.
(författare)
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Groundwater flow and heat transport for systems undergoing freeze-thaw : Intercomparison of numerical simulators for 2D test cases
- 2018
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Ingår i: Advances in Water Resources. - : Elsevier BV. - 0309-1708 .- 1872-9657. ; 114, s. 196-218
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Tidskriftsartikel (refereegranskat)abstract
- In high-elevation, boreal and arctic regions, hydrological processes and associated water bodies can be strongly influenced by the distribution of permafrost. Recent field and modeling studies indicate that a fully-coupled multidimensional thermo-hydraulic approach is required to accurately model the evolution of these permafrost-impacted landscapes and groundwater systems. However, the relatively new and complex numerical codes being developed for coupled non-linear freeze-thaw systems require verification.This issue is addressed by means of an intercomparison of thirteen numerical codes for two-dimensional test cases with several performance metrics (PMs). These codes comprise a wide range of numerical approaches, spatial and temporal discretization strategies, and computational efficiencies. Results suggest that the codes provide robust results for the test cases considered and that minor discrepancies are explained by computational precision. However, larger discrepancies are observed for some PMs resulting from differences in the governing equations, discretization issues, or in the freezing curve used by some codes.
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| 10. |
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| 11. |
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| 12. |
- Gylling, Björn, et al.
(författare)
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SKB Task Force GWFTS : Lessons Learned from Modeling Field Tracer Experiments in Finland and Sweden
- 2022
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Konferensbidrag (refereegranskat)abstract
- SKB and several other waste management organizations have established the international SKB Task Force on Modeling of Groundwater Flow and Transport of Solutes (TF GWFTS) to support and interpret field experiments. Objectives of the task force are to develop, test and improve tools for conceptual understanding and simulating groundwater flow and transport of solutes in fractured rocks. Work is organized in collaborative modeling tasks. Task 9 focuses on realistic modeling of coupled matrix diffusion and sorption in heterogeneous crystalline rock matrix at depth, e.g. by inverse and predictive modeling of in-situ transport experiments. Posiva’s REPRO (rock matrix REtention PROperties) experimental campaign has been performed at the ONKALO rock characterization facility in Finland. The two REPRO experiments considered were the Water Phase Diffusion Experiment (WPDE), addressing matrix diffusion in gneiss around a single borehole interval (modeled in Task 9A), and the Through Diffusion Experiment, which is performed between sections of three boreholes and addressed by modeling in Task 9C. The Long-Term Diffusion and Sorption Experiment (LTDE-SD) was an in-situ radionuclide tracer test performed at the Swedish Äspö Hard Rock Laboratory at a depth of about 410 m below sea level. The experimental results indicated a possible deeper penetration of sorbing tracers into the rock matrix than expected. The shape of these tracer penetration profiles was difficult to reproduce. This experiment was modeled and interpreted in Task 9B. Task 9D is addressing the possible benefits of detailed models of the in-situ experiments in safety assessment calculations. The task is performed by upscaling of the WPDE models to conditions applicable for nuclear waste repositories. As Task 9 is now in a finalization process, a number of lessons learned from the 4 sub-tasks have been identified. These include: • field tracer experiments can provide surprises even when well designed and executed, • interaction between the experimentalists and modelers is important and mutually beneficial when investigating anomalous results, • differences in conceptual models have the greatest impact on model outcomes, • it is not trivial to go from modeling of field experiments to safety assessment modeling without making substantial simplifications.
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| 13. |
- Iraola, Aitor, et al.
(författare)
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Microtomography-based Inter-Granular Network for the simulation of radionuclide diffusion and sorption in a granitic rock
- 2017
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Ingår i: Journal of Contaminant Hydrology. - : Elsevier BV. - 0169-7722 .- 1873-6009. ; 207, s. 8-16
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Tidskriftsartikel (refereegranskat)abstract
- Field investigation studies, conducted in the context of safety analyses of deep geological repositories for nuclear waste, have pointed out that in fractured crystalline rocks sorbing radionuclides can diffuse surprisingly long distances deep into the intact rock matrix; i.e. much longer distances than those predicted by reactive transport models based on a homogeneous description of the properties of the rock matrix. Here, we focus on cesium diffusion and use detailed micro characterisation data, based on micro computed tomography, along with a grain-scale Inter-Granular Network model, to offer a plausible explanation for the anomalously long cesium penetration profiles observed in these in-situ experiments. The sparse distribution of chemically reactive grains (i.e. grains belonging to sorbing mineral phases) is shown to have a strong control on the diffusive patterns of sorbing radionuclides. The computed penetration profiles of cesium agree well with an analytical model based on two parallel diffusive pathways. This agreement, along with visual inspection of the spatial distribution of cesium concentration, indicates that for sorbing radionuclides the medium indeed behaves as a composite system, with most of the mass being retained close to the injection boundary and a non-negligible part diffusing faster along preferential diffusive pathways.
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| 14. |
- Johansson, Emma, et al.
(författare)
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Data evaluation and numerical modeling of hydrological interactions between active layer, lake and talik in a permafrost catchment, Western Greenland
- 2015
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Ingår i: Journal of Hydrology. - : Elsevier BV. - 0022-1694 .- 1879-2707. ; 527, s. 688-703
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Tidskriftsartikel (refereegranskat)abstract
- This study investigates annual water balance conditions and their spatiotemporal variability under a wide variety of atmospheric driving conditions in the periglacial permafrost catchment of Two Boat Lake in Western Greenland. The study uses and combines a comprehensive hydrological multi-parameter dataset measured at the site with site conceptualization and numerical model development, application and testing. The model result reproduces measured lake and groundwater levels, as well as observations made by time-lapse cameras. The results highlights the importance of numerical modeling that takes into account and combines evapotranspiration with other surface and subsurface hydrological processes at various depths, in order to quantitatively understand and represent the dynamics and complexity of the interactions between meteorology, active layer hydrology, lakes, and unfrozen groundwater below permafrost in periglacial catchments. Regarding these interactions, the water flow between the studied lake and a through talik within and beneath it is found to be small compared to other water balance components. The modeling results show that recharge and discharge conditions in the talik can shift in time, while the lake and active layer conditions in the studied catchment are independent of catchment-external landscape features, such as the unfrozen groundwater system below the permafrost and the nearby continental-scale ice sheet.
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| 15. |
- Jutebring Sterte, Elin, et al.
(författare)
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Hydrological control of water quality – Modelling base cation weathering and dynamics across heterogeneous boreal catchments
- 2021
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Ingår i: Science of the Total Environment. - : Elsevier BV. - 0048-9697 .- 1879-1026. ; 799, s. 149101-149101
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Tidskriftsartikel (refereegranskat)abstract
- Linking biogeochemical processes to water flow paths and solute travel times is important for understanding internal catchment functioning and control of water quality. Base cation weathering is a process closely linked to key factors affecting catchment functioning, including water pathways, soil contact time, and catchment characteristics, particularly in silicate-dominated areas. However, common process-based weathering models are often calibrated and applied for individual soil profiles, which can cause problems when trying to extrapolate results to catchment scale and assess consequences for stream water and groundwater quality. Therefore, in this work, base cation export was instead modelled using a fully calibrated 3D hydrological model (Mike SHE) of a boreal catchment, which was expanded by adding a relatively simple but still reasonably flexible and versatile weathering module including the base cations Na, K, Mg, and Ca. The results were evaluated using a comprehensive dataset of water chemistry from groundwater and stream water in 14 nested sub-catchments, representing different catchment sizes and catchment characteristics. The strongest correlations with annual and seasonal observations were found for Ca (r = 0.89-0.93, p < 0.05), Mg (r = 0.90-0.95, p < 0.05), and Na (r = 0.80-0.89, p < 0.05). These strong correlations suggest that catchment hydrology and landscape properties primarily control weathering rates and stream dynamics of these solutes. Furthermore, catchment export of Mg, Ca, and K was strongly connected to travel times of discharging stream water (r = 0.78-0.83). Conversely, increasing Na export was linked to a reduced areal proportion of mires (r = -0.79). The results suggest that a significant part (~45%) of the catchment stream export came from deep-soil weathering sources (>2.5 m). These results have implications for terrestrial and aquatic water quality assessments. If deep soils are present, focusing mainly on the shallow soil could lead to misrepresentation of base cation availability and the acidification sensitivity of groundwater and water recipients such as streams and lakes.
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| 16. |
- Libby, Simon, et al.
(författare)
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Exploring the impact of fracture interaction on connectivity and flow channelling using grown fracture networks
- 2024
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Ingår i: Quarterly journal of engineering geology and hydrogeology. - : Geological Society of London. - 1470-9236 .- 2041-4803. ; 57:1
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Tidskriftsartikel (refereegranskat)abstract
- Quantitative assessment of the flow properties and mechanical stability of naturally fractured rock is frequently practised across the mining, petroleum, geothermal, geological disposal, construction and environmental remediation industries. These fluid and mechanical behaviours are strongly influenced by the connectivity of the fracture system and the size of the intact rock blocks. However, these are amongst the more difficult fracture system properties to characterize and honour in numerical simulations. Nonetheless, they are still the product of interactions between fractures that can be conceptualized as a series of deformation events following geomechanical principles. Generating numerical models of fracture networks by simulating this deformation with a coupled and evolving rock mass and stress field is a significant undertaking. Instead, large-scale fracture network models can be 'grown' dynamically according to rules that mimic the underlying mechanical processes and deformation history. This paper explores a computationally efficient rules-based method to generate fracture networks, demonstrates how different types of fracture patterns can be simulated, and illustrates how inclusion of fracture interactions can affect flow and mechanical properties. Relative to methods without fracture interaction and in contrast to some other rules-based approaches, the method described here regularizes and increases fracture connectivity and decreases flow channelling.
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| 17. |
- Liljedahl, Lillemor Claesson, et al.
(författare)
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Rapid and sensitive response of Greenland’s groundwater system to ice sheet change
- 2021
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Ingår i: Nature Geoscience. - : Springer Nature. - 1752-0894 .- 1752-0908. ; 14:10, s. 751-755
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Tidskriftsartikel (refereegranskat)abstract
- Greenland Ice Sheet mass loss is impacting connected terrestrial and marine hydrologic systems with global consequences. Groundwater is a key component of water cycling in the Arctic, underlying the 1.7e6 km2 ice sheet and forming offshore freshwater reserves. However, despite its vast extent, the response of Greenland’s groundwater to ongoing ice sheet change is unknown. Here we present in-situ observations of deep groundwater conditions under the Greenland Ice Sheet, obtained in a 651-metre-long proglacial bedrock borehole angled under the ice sheet margin. We find that Greenland’s groundwater system responds rapidly and sensitively to relatively minor ice sheet forcing. Hydraulic head clearly varies over multi-annual, seasonal and diurnal timescales, which we interpret as a response to fluid pressure forcing at the ice/bed interface associated with changes in overlying ice loading and ice sheet hydrology. We find a systematic decline in hydraulic head over the eight-year observational period is linked primarily to ice sheet mass loss. Ongoing and future ice thinning will probably reduce groundwater discharge rates, with potential impacts to submarine freshwater discharge, freshwater delivery to fjords and biogeochemical fluxes in the Arctic.
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| 18. |
- Molron, Justine, et al.
(författare)
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GPR-inferred fracture aperture widening in response to a high-pressure tracer injection test at the Äspö Hard Rock Laboratory, Sweden
- 2021
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Ingår i: Engineering Geology. - : Elsevier BV. - 0013-7952 .- 1872-6917. ; 292
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Tidskriftsartikel (refereegranskat)abstract
- We assess the performance of the Ground Penetrating Radar (GPR) method in fractured rock formations of very low transmissivity (e.g. T ≈ 10−9–10−10 m2/s for sub-mm apertures) and, more specifically, to image fracture widening induced by high-pressure injections. A field-scale experiment was conducted at the Äspö Hard Rock Laboratory (Sweden) in a tunnel situated at 410 m depth. The tracer test was performed within the most transmissive sections of two boreholes separated by 4.2 m. The electrically resistive tracer solution composed of deionized water and Uranine was expected to lead to decreasing GPR reflections with respect to the saline in situ formation water. The injection pressure was 5000 kPa leading to an injection rate of 8.6 mL/min (at steady state) that was maintained during 25 h, which resulted in a total injected volume of 13 L. To evaluate the fracture pathways between the boreholes, we conducted 3-D surface-based GPR surveys before and at the end of the tracer tests, using 160 MHz and 450 MHz antennas. Difference GPR data between the two acquisitions highlight an increasing fracture reflectivity in-between the boreholes at depths corresponding to the injection interval. GPR-based modeling suggests that the observed increasing reflectivity is not due to the tracer solution, but rather to a 50% widening of the fracture. Considering prevailing uncertainties in material properties, a hydromechanical analysis suggests that such a degree of widening is feasible. This research demonstrates that field-scale in situ GPR experiments may provide constraints on fracture widening by high-pressure injection and could help to constrain field-scale elastic parameters in fractured rock.
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| 19. |
- Molron, Justine, et al.
(författare)
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Which fractures are imaged with Ground Penetrating Radar? : Results from an experiment in the Äspö Hardrock Laboratory, Sweden
- 2020
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Ingår i: Engineering Geology. - : Elsevier BV. - 0013-7952. ; 273, s. 105674-
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Tidskriftsartikel (refereegranskat)abstract
- Identifying fractures in the subsurface is crucial for many geomechanical and hydrogeological applications. Here, we assess the ability of the Ground Penetrating Radar (GPR) method to image open fractures with sub-mm apertures in the context of future deep disposal of radioactive waste. GPR experiments were conducted in a tunnel located 410 m below sea level within the Äspö Hard Rock Laboratory (Sweden) using 3-D surface-based acquisitions (3.4 m × 19 m) with 160 MHz, 450 MHz and 750 MHz antennas. The nature of 17 identified GPR reflections was analyzed by means of three new boreholes (BH1-BH3; 9–9.5 m deep). Out of 21 injection and outflow tests in packed-off 1-m sections, only five provided responses above the detection threshold with the maximum transmissivity reaching 7.0 × 10−10 m2/s. Most GPR reflections are situated in these permeable regions and their characteristics agree well with core and Optical Televiewer data. A 3-D statistical fracture model deduced from fracture traces on neighboring tunnel walls show that the GPR data mainly identify fractures with dips between 0 and 25°. Since the GPR data are mostly sensitive to open fractures, we deduce that the surface GPR method can identify 80% of open sub-horizontal fractures. We also find that the scaling of GPR fractures in the range of 1–10 m2 agrees well with the statistical model distribution indicating that fracture lengths are preserved by the GPR imaging (no measurement bias). Our results suggests that surface-GPR carries the resolution needed to identify the most permeable sub-horizontal fractures even in very low-permeability formations, thereby, suggesting that surface-GPR could play an important role in geotechnical workflows, for instance, for industrial-scale siting of waste canisters below tunnel floors in nuclear waste repositories.
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| 20. |
- Painter, S., et al.
(författare)
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Power-law velocity distributions in fracture networks : Numerical evidence and implications for tracer transport
- 2002
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 29:14
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Tidskriftsartikel (refereegranskat)abstract
- Velocity distributions in two- and three-dimensional networks of discrete fractures are studied through numerical simulations. The distribution of 1/v, where v is the velocity along particle trajectories, is closely approximated by a power law (Pareto) distribution over a wide range of velocities. For the conditions studied, the power law exponents are in the range 1.1-1.8, and generally increase with increasing fracture density. The same is true for the quantity 1/bv, which is related to retention properties of the rock; b is the fracture half-aperture. Using a stochastic Lagrangian methodology and statistical limit theorems applicable to power-law variables, it is shown that the distributions of residence times for conservative and reacting tracers are related to one-sided stable distributions. These results are incompatible with the classical advection dispersion equation and underscore the need for alternative modeling approaches.
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| 21. |
- Sanglas, Jordi, et al.
(författare)
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Significance of Low-Velocity Zones on Solute Retention in Rough Fractures
- 2024
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Ingår i: Water resources research. - : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 60:4
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Tidskriftsartikel (refereegranskat)abstract
- Natural fractures are characterized by high internal heterogeneity. This internal variability is the cause of flow channeling, which in turn leads to contaminant transport taking place primarily along the high-velocity channels. Mass exchange between the high-velocity channels and the low-velocity zones has the potential to enhance contaminant retention, due to solute diffusion into the low-velocity zones and subsequent exposure to additional surface area for diffusion into the bordering rock matrix. Here, we derive a random walk particle tracking method for heterogeneous fractures, which includes an additional term to account for the aperture gradient. The method takes into account advection, diffusion in the fracture and matrix diffusion. The developed numerical framework is applied to assess the effect of low-velocity zones in rough self-affine fractures. The results show that diffusion into low-velocity zones has a visible but modest impact on contaminant retention. The magnitude of this impact does not change considerably, regardless of whether diffusion into the rock matrix is considered in the model, and increases for a decreasing average Peclet number of the fracture. Natural fractures are highly heterogeneous, comprising flowing channels and lower-velocity zones We study the effects that diffusion into low-velocity zones has in contaminant transport through rough fractures Accounting for diffusion in low-velocity zones has a relatively modest impact on contaminant retention
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| 22. |
- Selroos, Jan-Olof, et al.
(författare)
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Effect of transport-pathway simplifications on projected releases of radionuclides from a nuclear waste repository (Sweden)
- 2012
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Ingår i: Hydrogeology Journal. - : Springer Science and Business Media LLC. - 1431-2174 .- 1435-0157. ; 20:8, s. 1467-1481
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Tidskriftsartikel (refereegranskat)abstract
- The Swedish Nuclear Fuel and Waste Management Company has recently submitted an application for alicense to construct a final repository for spent nuclear fuel, at approximately 500m depth in crystalline bedrock. Migration pathways through the geosphere barrier are geometrically complex, with segments in fractured rock, deformation zones, backfilled tunnels, and near-surface soils. Several simplifications of these complex migration pathways were used in the assessments of repository performance that supported the license application. Specifically, in the geosphere transport calculations, radionuclide transport in soils and tunnels was neglected, and deformation zones were assumed to have transport characteristics of fractured rock. The effects of these simplifications on the projected performance of the geosphere barrier system are addressed. Geosphere performance is shown to be sensitive to how transport characteristics of deformation zones are conceptualized and incorporated into the model. Incorporation of advective groundwater travel time within backfilled tunnels reduces radiological dose from non-sorbing radionuclides such as I-129, while sorption in near-surface soils reduces radiological doses from sorbing radionuclides such as Ra-226. These results help quantify the degree to which geosphere performance was pessimistically assessed, and provide some guidance on how future studies to reduce uncertainty in geosphere performance may be focused.
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| 23. |
- Selroos, Jan-Olof, et al.
(författare)
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How Findings from a Multi-Annual International Modeling Initiative Are Implemented in a Nuclear Waste Management Organization
- 2023
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Ingår i: Energies. - : MDPI AG. - 1996-1073. ; 16:2
-
Tidskriftsartikel (refereegranskat)abstract
- In the present paper, we discuss various aspects of the SKB Task Force on Modeling of Groundwater Flow and Transport of Solutes (TFGWFTS). The TFGWTS is a multi-lateral forum for modeling of groundwater flow and solute transport, focusing on issues of relevance for disposal of nuclear waste. We discuss the objectives and set-up of the different tasks performed during the last 30 years, and specifically how the results of the modeling have informed performance and safety assessment applications within SKB (Swedish Nuclear Fuel and Waste Management Company, Solna, Sweden). We conclude that the TFGWFTS has been instrumental in developing modeling methodologies and tools, and in training and fostering modelers. While the early tasks were related to the construction of the Äspö Hard Rock Laboratory in Sweden and developed general modeling competence, the later tasks have served performance and safety assessment purposes in a more substantial manner.
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| 24. |
- Selroos, Jan-Olof, et al.
(författare)
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Influence of spatial and temporal flow variability on solute transport in catchments
- 2015
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Ingår i: Hydrological Processes. - : Wiley. - 0885-6087 .- 1099-1085. ; 29:16, s. 3592-3603
-
Tidskriftsartikel (refereegranskat)abstract
- This study quantifies the separate and combined effects of spatial and temporal variability for waterborne solute transport through catchments. The questions addressed regard whether, when and why the different types of variability may dominate catchment-scale transport. We utilize a versatile numerical solute transport code with a particle-based Monte Carlo time-domain random walk method to simulate waterborne transport through a generic catchment. Simulations are concretized and exemplified using data on spatiotemporal flow-transport variability from direct stream discharge observations and independently calculated advective solute travel time distributions for catchments within the water management district Northern Baltic Proper in Mid-Eastern Sweden. A main conclusion is that the projections of catchment mass loading based on spatial variability alone are robust estimates of long-term average solute transport development. This is especially true when annually aggregated mass load rather than finer temporal resolution of mass flux is considered. Temporal variability yields short-term fluctuations around the long-term average solute breakthrough development, and earlier or later arrival than the latter, depending on the timing and duration of solute input relative to the temporal flow variability. The exact temporal characteristics of future solute breakthrough are thus fundamentally uncertain, but their statistical expectation may be well quantified by accounting only for spatial variability.
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| 25. |
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| 26. |
- Selroos, Jan-Olof, et al.
(författare)
-
Overview of hydrogeological site-descriptive modeling conducted for the proposed high-level nuclear waste repository site at Forsmark, Sweden
- 2014
-
Ingår i: Hydrogeology Journal. - : Springer Nature. - 1435-0157 .- 1431-2174. ; 22:2, s. 295-298
-
Tidskriftsartikel (refereegranskat)abstract
- This study quantifies the separate and combined effects of spatial and temporal variability for waterborne solute transport through catchments. The questions addressed regard whether, when and why the different types of variability may dominate catchment-scale transport. We utilize a versatile numerical solute transport code with a particle-based Monte Carlo time-domain random walk method to simulate waterborne transport through a generic catchment. Simulations are concretized and exemplified using data on spatiotemporal flow-transport variability from direct stream discharge observations and independently calculated advective solute travel time distributions for catchments within the water management district Northern Baltic Proper in Mid-Eastern Sweden. A main conclusion is that the projections of catchment mass loading based on spatial variability alone are robust estimates of long-term average solute transport development. This is especially true when annually aggregated mass load rather than finer temporal resolution of mass flux is considered. Temporal variability yields short-term fluctuations around the long-term average solute breakthrough development, and earlier or later arrival than the latter, depending on the timing and duration of solute input relative to the temporal flow variability. The exact temporal characteristics of future solute breakthrough are thus fundamentally uncertain, but their statistical expectation may be well quantified by accounting only for spatial variability.
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| 27. |
- Selroos, Jan-Olof, et al.
(författare)
-
Permafrost Thaw with Thermokarst Wetland-Lake and Societal-Health Risks : Dependence on Local Soil Conditions under Large-Scale Warming
- 2019
-
Ingår i: Water. - : MDPI AG. - 2073-4441. ; 11:3, s. 574-
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Tidskriftsartikel (refereegranskat)abstract
- A key question for the evolution of thermokarst wetlands and lakes in Arctic and sub-Arctic permafrost regions is how large-scale warming interacts with local landscape conditions in driving permafrost thaw and its spatial variability. To answer this question, which also relates to risks for ecology, society, and health, we perform systematic model simulations of various soil-permafrost cases combined with different surface-warming trends. Results show that both the prevalence and the thaw of permafrost depended strongly on local soil conditions and varied greatly with these for the same temperature conditions at the surface. Greater ice contents and depth extents, but also greater subsurface volumes thawing at depth under warming, are found for peat soils than other studied soil/rock formations. As such, more thaw-driven regime shifts in wetland/lake ecosystems, and associated releases of previously frozen carbon and pathogens, may be expected under the same surface warming for peatlands than other soil conditions. Such risks may also increase in fast permafrost thaw in mineral soils, with only small thaw-protection effects indicated in the present simulations for possible desertification enhancement of mineral soil covers.
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| 28. |
- Selroos, Jan-Olof, et al.
(författare)
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Radionuclide transport during glacial cycles : Comparison of two approaches for representing flow transients
- 2013
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Ingår i: Physics and Chemistry of the Earth. - : Elsevier BV. - 1474-7065 .- 1873-5193. ; 64, s. 32-45
-
Tidskriftsartikel (refereegranskat)abstract
- The effect of future, transient ice sheet movement and permafrost development on transport of radionuclides from a proposed repository site is investigated using numerical groundwater flow and radionuclide transport modelling. Two different transport approaches are compared, both utilizing groundwater flow simulations of future climate conditions. The first transport approach uses steady-state particle trajectories representing temperate climate conditions, but modifies the transport velocity along the trajectories according to the changing climate. The second approach is pseudo-transient by performing particle tracking in each individual flow field representing a given time epoch.Two different climate sequences are analyzed. First, a simplified sequence is assessed in order to understand if the two different transport approaches yield significantly different breakthrough characteristics. Second, a sequence representing conditions relevant for real safety assessment applications is considered.Results indicate that the transport approach using fixed trajectories tends to significantly over predict breakthrough during permafrost conditions relative to the pseudo-transient approach. The major difference between the two approaches is related to discharge locations. The fixed trajectory approach yields discharge locations constant in time whereas the pseudo-transient approach is characterized by discharge centres moving in time according to the different climate conditions.
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| 29. |
- Soltani, Sofie Safeyeh, 1983-
(författare)
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Hydrological Transport in Shallow Catchments: : tracer discharge, travel time and water age
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This focuses on hydrological transport in shallow catchments with topography-driven flow paths. The thesis gives new insight to kinematic pathway models for estimation of tracer discharge at the catchment outlet. A semi-analytical methodology is presented for transient travel time and age distributions referred to as "kinematic pathway approach“(KPA) that accounts for dispersion at two levels of morphological and macro-dispersion. Macro-dispersion and morphological dispersion components are reflected in KPA by assuming an effective Péclet number and topographically driven pathway length distributions, respectively. The kinematic measure of the transport, defined as a characteristic velocity of water flow through the catchment is obtained from the overall water balance in the catchment. To include transformation process in its simplest form of linear decay/degradation a framework is presented that solves one-dimensional reactive transport with numerically simulated travel times as the independent variable. The proposed KPA and coupled transport framework for quantifying tracer discharge at the shallow catchment outlet are applied to two selected catchments in Sweden. KPA is applied to modeling of a 23-year long chloride data series for the Kringlan catchment whereas the implantation of the framework for quantifying natural attenuation is illustrated for the Forsmark catchment. Numerical simulations of Forsmark catchment advective travel times are obtained by means of particle tracking using the fully-integrated flow model MIKE SHE. The KPA is found to provide reasonable estimates of tracer discharge distribution when considering the transport controlled by hillslope processes associated with short topographically driven flow paths to adjacent discharge zones, e.g. rivers and lakes. Simulated natural attenuation for Forsmark is also estimated well provided that the pathway length distribution is skewed toward short pathway lengths. This fact is indicative of the controlling impact of topography on flow path length and travel time distributions in shallow catchments. Our work has shown that the pathway (Lagrangian) methodologies are promising as predictive tools for hydrological transport.
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| 30. |
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| 31. |
- Svensson, Urban, et al.
(författare)
-
Grains, grids and mineral surfaces : approaches to grain-scale matrix modeling based on X-ray micro-computed tomography data
- 2019
-
Ingår i: SN Applied Sciences. - : Springer Nature. - 2523-3963 .- 2523-3971. ; 1:10
-
Tidskriftsartikel (refereegranskat)abstract
- X-ray micro-computed tomography (X-μCT) generates 3D mineral distribution maps currently with a resolution of about 10 μm. For tight crystalline rocks, this implies that the mineral grains are well resolved, while micro-fractures, having apertures of less than 10 μm, are not resolved. In this study, we propose a method to analyze the properties (size, volume, surface area) of the mineral grains based on X-μCT data. The numerical approach uses a resolution similar to that of the X-μCT data and hence shares the same limitations. For example, it is clear that a large fraction of the mineral surface area is due to so-called roughness, with scales below 10 μm. In the second part of the study, methods to generate the diffusion-available pore space are discussed. The inter-granular space (distance between grains) is often smaller than 10 μm, and we need to design methods to be able to perform diffusion simulations in the matrix. Three methods, all based on X-μCT, are discussed, and it is demonstrated that models with realistic global properties (mean porosity and effective diffusion coefficient) can be developed based on the suggested techniques.
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| 32. |
- Svensson, Urban, et al.
(författare)
-
Modelling the diffusion-available pore space of an unaltered granitic rock matrix using a micro-DFN approach
- 2018
-
Ingår i: Journal of Hydrology. - : Elsevier BV. - 0022-1694 .- 1879-2707. ; 559, s. 182-191
-
Tidskriftsartikel (refereegranskat)abstract
- In sparsely fractured rock, the ubiquitous heterogeneity of the matrix, which has been observed in different laboratory and in situ experiments, has been shown to have a significant influence on retardation mechanisms that are of importance for the safety of deep geological repositories for nuclear waste. Here, we propose a conceptualisation of a typical heterogeneous granitic rock matrix based on micro-Discrete Fracture Networks (micro-DFN). Different sets of fractures are used to represent grain-boundary pores as well as micro fractures that transect different mineral grains. The micro-DFN model offers a great flexibility in the way inter- and intra-granular space is represented as the different parameters that characterise each fracture set can be fine tuned to represent samples of different characteristics. Here, the parameters of the model have been calibrated against experimental observations from granitic rock samples taken at Forsmark (Sweden) and different variant cases have been used to illustrate how the model can be tied to rock samples with different attributes. Numerical through-diffusion simulations have been carried out to infer the bulk properties of the model as well as to compare the computed mass flux with the experimental data from an analogous laboratory experiment. The general good agreement between the model results and the experimental observations shows that the model presented here is a reliable tool for the understanding of retardation mechanisms occurring at the mm-scale in the matrix.
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| 33. |
- Trinchero, P., et al.
(författare)
-
A Particle-Based Conditional Sampling Scheme for the Simulation of Transport in Fractured Rock With Diffusion Into Stagnant Water and Rock Matrix
- 2020
-
Ingår i: Water resources research. - : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 56:4
-
Tidskriftsartikel (refereegranskat)abstract
- In situ experiments and field-scale characterization studies have pointed out that, in fractured crystalline media, groundwater flow is highly channelized. This implies that, at the scale of a single fracture, only part of the fracture surface area is in contact with flowing water, while the rest of in-plane water is essentially stagnant and can be accessed by solutes via molecular diffusion. Despite their importance for contaminant retention, to date, there are no numerical or analytical approaches that could be used to assess the implication of stagnant water zones on solute transport in realistic large-scale Discrete Fracture Network-based models. Here, we present an efficient and flexible algorithm for the simulation of transport in fractured rock with diffusion into stagnant water and rock matrix. The algorithm is a generalization of a previously developed numerical framework for time domain particle tracking in sparsely fractured rock. The key of the generalization is that total time in fracture (τf) is first evaluated using a Monte Carlo sampling and then a second sampling is performed conditioned on τf. The algorithm has been successfully validated against existing independent solutions and the implication of diffusion into stagnant water and secondary diffusion into the matrix has been assessed for a realistic modeling scenario. The results show that, due to diffusion into stagnant water, contaminants are more strongly retarded. This increased retention is more significant for sorbing species, as a larger number of sorption sites is accessible. A high sensitivity to the flowing channel/stagnant water zone geometry has also been observed.
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| 34. |
- Trinchero, Paolo, et al.
(författare)
-
FASTREACT : An efficient numerical framework for the solution of reactive transport problems
- 2014
-
Ingår i: Applied Geochemistry. - : Elsevier BV. - 0883-2927 .- 1872-9134. ; 49, s. 159-167
-
Tidskriftsartikel (refereegranskat)abstract
- In the framework of safety assessment studies for geological disposal, large scale reactive transport models are powerful inter-disciplinary tools aiming at supporting regulatory decision making as well as providing input to repository engineering activities. Important aspects of these kinds of models are their often very large temporal and spatial modelling scales and the need to integrate different non-linear processes (e.g., mineral dissolution and precipitation, adsorption and desorption, microbial reactions and redox transformations). It turns out that these types of models may be computationally highly demanding. In this work, we present a Lagrangian-based framework, denoted as FASTREACT, that aims at solving multi-component-reactive transport problems with a computationally efficient approach allowing complex modelling problems to be solved in large spatial and temporal scales. The tool has been applied to simulate radionuclide migration in a synthetic heterogeneous transmissivity field and the results have been successfully compared with those obtained using a standard Eulerian approach. Finally, the same geochemical model has been coupled to an ensemble of realistic three-dimensional transport pathways to simulate the migration of a set of radionuclides from a hypothetical repository for spent nuclear fuel to the surface. The results of this modelling exercise, which includes key processes such as the exchange of mass between the conductive fractures and the matrix, show that FASTREACT can efficiently solve large-scale reactive transport models.
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| 35. |
- Trinchero, Paolo, et al.
(författare)
-
Modelling radionuclide transport in fractured media with a dynamic update of Kd values
- 2016
-
Ingår i: Computers & Geosciences. - : Elsevier BV. - 0098-3004 .- 1873-7803. ; 86, s. 55-63
-
Tidskriftsartikel (refereegranskat)abstract
- Radionuclide transport in fractured crystalline rocks is a process of interest in evaluating long term safety of potential disposal systems for radioactive wastes. Given their numerical efficiency and the absence of numerical dispersion, Lagrangian methods (e.g. particle tracking algorithms) are appealing approaches that are often used in safety assessment (SA) analyses. In these approaches, many complex geochemical retention processes are typically lumped into a single parameter: the distribution coefficient (Kd). Usually, the distribution coefficient is assumed to be constant over the time frame of interest. However, this assumption could be critical under long-term geochemical changes as it is demonstrated that the distribution coefficient depends on the background chemical conditions (e.g. pH, Eh, and major chemistry). In this work, we provide a computational framework that combines the efficiency of Lagrangian methods with a sound and explicit description of the geochemical changes of the site and their influence on the radionuclide retention properties.
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| 36. |
- Trinchero, Paolo, et al.
(författare)
-
Modelling the water phase diffusion experiment at Onkalo (Finland) : Insights into the effect of channeling on radionuclide transport and retention
- 2020
-
Ingår i: Journal of Hydrology. - : Elsevier BV. - 0022-1694 .- 1879-2707. ; 590
-
Tidskriftsartikel (refereegranskat)abstract
- In fractured crystalline rocks, contaminants are transported through open fractures by fluid flow and can access the connected pore space of the adjacent rock matrix by diffusion. The mass exchange term between the fracture and the matrix depends on in-plane groundwater flow patterns. Thus, channeling and preferential flow, due to heterogeneity in fracture aperture, have an impact on the overall rock retention capacity. Here, we have used experimental data from the recent Water Phase Diffusion Experiment, carried out at ONKALO (Finland), to assess the influence of channeling on contaminant transport and retention at increasingly larger scales. The upscaling is performed by extracting non-parametric Retention Time Distribution functions from the experimental data and using them to carry out transport simulations along a segmented pathway. The analysis shows that channeling leads to anomalous early breakthroughs at short scales, whereas at increasingly larger distances this effect is smeared out and the nuclide breakthrough curves become mostly controlled by mass exchange processes between the fracture and the matrix. This homogenisation occurs at shorter scales for sorbing nuclides. The influence of channelling on radionuclide retention is shown to be modest due to compensating effects between the shorter groundwater travel times and the larger specific fracture surface area made available through in-plane diffusion into stagnant water. Also, we have shown that the late-time behaviour of the experimental breakthrough curves can be used to infer equivalent parameters that, combined with existing homogeneous-based solutions, provide a good description of contaminant breakthrough curves at larger downstream distances.
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| 37. |
- Trinchero, Paolo, et al.
(författare)
-
Upscaling of radionuclide transport and retention in crystalline rocks exhibiting micro-scale heterogeneity of the rock matrix
- 2020
-
Ingår i: Advances in Water Resources. - : Elsevier BV. - 0309-1708 .- 1872-9657. ; 142
-
Tidskriftsartikel (refereegranskat)abstract
- In different in-situ diffusion experiments carried out in fractured crystalline rocks, sorbing radionuclides have shown a behaviour that strongly differs from what is predicted by homogeneous-based models. Their breakthrough curves are in fact often characterised by a fast first-arrival and these radionuclides can penetrate surprisingly long distances deep into the matrix. The heterogeneous structure of mineral distribution and porosity geometry had been offered as an explanation for these discrepancies. Here, we use reactive transport simulations to investigate the effect of the sparse distribution of sorption sites on the breakthrough curves of sorbing radionuclides. At small scale, the computed breakthrough curves significantly differ from those predicted using homogeneous models. For instance, the early part of these curves does not show any clear separation with the corresponding part of the curve of a non-sorbing tracer and a long transition zone is observed, with a very smooth slope of the tailing. Two different upscaling strategies, aimed at propagating the signal of heterogeneous retention over larger scales, are proposed and demonstrated against independent solutions computed at intermediate scales. The upscaling strategies are also used to show that at large scales (e.g. the scale of interest in a safety assessment study for a deep geological repository for nuclear waste) the signature of mineralogical heterogeneity is smoothed out and the heterogeneous breakthrough curve is well approximated by a homogeneous solution where the radionuclide distribution coefficient for the pure mineral phase is scaled by the mineral volume fraction. However, the spatial persistence of the heterogeneous signature is significant when the sorbing mineral is present in a low amount.
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| 38. |
- Vidstrand, Patrik, et al.
(författare)
-
Groundwater flow modeling of periods with periglacial and glacial climate conditions for the safety assessment of the proposed high-level nuclear waste repository site at Forsmark, Sweden
- 2014
-
Ingår i: Hydrogeology Journal. - : Springer Nature. - 1431-2174 .- 1435-0157. ; 22:6, s. 1251-1267
-
Tidskriftsartikel (refereegranskat)abstract
- The impact of periglacial and glacial climate conditions on groundwater flow in fractured crystalline rock is studied by means of groundwater flow modeling of the Forsmark site, which was recently proposed as a repository site for the disposal of spent high-level nuclear fuel in Sweden. The employed model uses a thermal-hydraulically coupled approach for permafrost modeling and discusses changes in groundwater flow implied by the climate conditions found over northern Europe at different times during the last glacial cycle (Weichselian glaciation). It is concluded that discharge of particles released at repository depth occurs very close to the ice-sheet margin in the absence of permafrost. If permafrost is included, the greater part discharges into taliks in the periglacial area. During a glacial cycle, hydraulic gradients at repository depth reach their maximum values when the ice-sheet margin passes over the site; at this time, also, the interface between fresh and saline waters is distorted the most. The combined effect of advances and retreats during several glaciations has not been studied in the present work; however, the results indicate that hydrochemical conditions at depth in the groundwater flow model are almost restored after a single event of ice-sheet advance and retreat.
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| 39. |
- Vidstrand, Patrik, et al.
(författare)
-
Modeling of groundwater flow at depth in crystalline rock beneath a moving ice-sheet margin, exemplified by the Fennoscandian Shield, Sweden
- 2012
-
Ingår i: Hydrogeology Journal. - : Springer Nature. - 1431-2174 .- 1435-0157. ; 21:1, s. 239-255
-
Tidskriftsartikel (refereegranskat)abstract
- On-going geological disposal programs for spent nuclear fuel have generated strong demands for investigation and characterization of deep-lying groundwater systems. Because of the long time scales for which radiological safety needs to be demonstrated in safety assessment applications, an analysis of the hydrogeological performance of the geosphere system during glacial climate conditions is needed. Groundwater flow at depth in crystalline rock during the passage of an ice-sheet margin is discussed based on performed groundwater-flow modeling of two bedrock sites, Forsmark and Laxemar, in the Fennoscandian Shield, Sweden. The modeled ice sheet mimics the Weichselian ice sheet during its last major advance and retreat over northern Europe. The paper elaborates and analyzes different choices of top boundary conditions at the ice sheet-subsurface interface (e. g. ice-sheet thickness and ice-margin velocity) and in the proglacial area (presence or lack of permafrost) and relates these choices to available groundwater-flow-model hydraulic output and prevailing conceptual hydrogeochemical models of the salinity evolution at the two sites. It is concluded that the choice of boundary conditions has a strong impact on results and that the studied sites behave differently for identical boundary conditions due to differences in their structural-hydraulic properties.
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| 40. |
- Voutilainen, Mikko, et al.
(författare)
-
Characterization of spatial porosity and mineral distribution of crystalline rock using X-ray micro computed tomography, C-14-PMMA autoradiography and scanning electron microscopy
- 2019
-
Ingår i: Applied Geochemistry. - : Elsevier BV. - 0883-2927 .- 1872-9134. ; 101, s. 50-61
-
Tidskriftsartikel (refereegranskat)abstract
- The spatial porosity and mineral distribution of geological materials strongly affects transport processes in them. X-ray micro computed tomography (X-μCT) has proven to be a powerful tool for characterizing the spatial mineral distribution of geological samples in 3-D. However, limitations in resolution prevent an accurate characterization of pore space especially for tight crystalline rock samples and 2-D methods such as C-14-polymethylmethacrylate (C-14-PMMA) autoradiography and scanning electron microscopy (SEM) are needed. The spatial porosity and mineral distributions of tight crystalline rock samples from Äspö Sweden, and Olkiluoto, Finland, were studied here. The X-μCT were used to characterize the spatial distribution of the main minerals in 3-D. Total porosities, fracture porosities, fracture densities and porosity distributions of the samples were determined using the C-14-PMMA autoradiography and characterization of mineral-specific porosities were assisted using chemical staining of rock surfaces. SEM and energy dispersive X-ray spectroscopy (EDS) were used to determine pore apertures and identify the minerals. It was shown that combination of the different imaging techniques creates a powerful tool for the structural characterization of crystalline rock samples. The combination of the results from different methods allowed the construction of spatial porosity, mineral and mineral grain distributions of the samples in 3-D. These spatial distributions enable reactive transport modeling using a more realistic representation of the heterogeneous structure of samples. Furthermore, the realism of the spatial distributions were increased by determinig the densities and porosities of fractures and by the virtual construction heterogeneous mineral distributions of minerals that cannot be separated by X-μCT.
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| 41. |
- Williams, Thomas, et al.
(författare)
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Recovering the Effects of Subgrid Heterogeneity in Simulations of Radionuclide Transport Through Fractured Media
- 2021
-
Ingår i: Frontiers in Earth Science. - : Frontiers Media SA. - 2296-6463. ; 8
-
Tidskriftsartikel (refereegranskat)abstract
- Groundwater flow and contaminant transport through fractured media can be simulated using Discrete Fracture Network (DFN) models which provide a natural description of structural heterogeneity. However, this approach is computationally expensive, with the large number of intersecting fractures necessitated by many real-world applications requiring modeling simplifications to be made for calculations to be tractable. Upscaling methods commonly used for this purpose can result in some loss of local-scale variability in the groundwater flow velocity field, resulting in underestimation of particle travel times, transport resistance and retention in transport calculations. In this paper, a transport downscaling algorithm to recover the transport effects of heterogeneity is tested on a synthetic Brittle Fault Zone model, motivated by the problem of large safety assessment calculations for geological repositories of spent nuclear fuel. We show that the variability in the local-scale velocity field which is lost by upscaling can be recovered by sampling from a library of DFN transport paths, accurately reproducing DFN transport statistic distributions and radionuclide breakthrough curves in an upscaled model.
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