| 1. |
- Baresel, Christian, 1975-
(författare)
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Environmental management of water systems under uncertainty
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Hydrological drainage/river basins constitute highly heterogeneous systems of coupled natural and anthropogenic water and pollutant flows across political, national and international boundaries. These flows need to be appropriately understood, quantified and communicated to stakeholders, in order to appropriately guide environmental water system management. In this thesis, various uncertainties about water and pollutant flows in drainage/river basins and their implications for effective and efficient water pollution abatement are investigated, in particular for mine-related heavy metal loadings in the Swedish Dalälven River basin and for nitrogen loadings in the Swedish Norrström drainage basin. Economic cost-minimization modeling is used to investigate the implications of pollutant load uncertainties for the cost-efficiency of catchment-scale abatement of water pollution. Results indicate that effective and efficient pollution abatement requires explicit consideration of uncertainties about pollution sources, diffuse contributions of the subsurface water system to downstream pollutant observations in surface waters, and downstream effects of different possible measures to reduce water pollution. In many cases, downstream load abatement measures must be used, in addition to source abatement, in order to reduce not only expected, but also uncertainties around expected pollutant loads. Effective and efficient environmental management of water systems must generally also consider the entire catchments of these systems, rather than focusing only on discrete pollutant sources. The thesis presents some relatively simple, catchment-scale pollutant flow analysis tools that may be used to decrease uncertainties about unmonitored water and pollutant flows and subsurface pollutant accumulation-depletion and diffuse loading to downstream waters.
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| 2. |
- Bergström, Anders, et al.
(författare)
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Origins and genetic legacy of prehistoric dogs
- 2020
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 370:6516, s. 557-563
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Tidskriftsartikel (refereegranskat)abstract
- Dogs were the first domestic animal, but little is known about their population history and to what extent it was linked to humans. We sequenced 27 ancient dog genomes and found that all dogs share a common ancestry distinct from present-day wolves, with limited gene flow from wolves since domestication but substantial dog-to-wolf gene flow. By 11,000 years ago, at least five major ancestry lineages had diversified, demonstrating a deep genetic history of dogs during the Paleolithic. Coanalysis with human genomes reveals aspects of dog population history that mirror humans, including Levant-related ancestry in Africa and early agricultural Europe. Other aspects differ, including the impacts of steppe pastoralist expansions in West and East Eurasia and a near-complete turnover of Neolithic European dog ancestry.
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| 3. |
- Cai, Zipan, et al.
(författare)
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How Does ICT Expansion Drive "Smart" Urban Growth? : A Case Study of Nanjing, China
- 2020
-
Ingår i: Urban Planning. - : COGITATIO PRESS. - 2183-7635. ; 5:1, s. 129-139
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Tidskriftsartikel (refereegranskat)abstract
- In the context of accelerated urbanization, socioeconomic development, and population growth, as well as the rapid advancement of information and communication technology (ICT), urban land is rapidly expanding worldwide. Unplanned urban growth has led to the low utilization efficiency of land resources. Also, ecological and agricultural lands are continuously sacrificed for urban construction, which in the long-term may severely impact the health of citizens in cities. A thorough understanding of the mechanisms and driving forces of a city's urban land use changes, including the influence of ICT development, is therefore crucial to the formation of optimal and feasible urban planning in the new era. Taking Nanjing as a study case, this article attempts to explore the measurable "smart" driving indicators of urban land use change and analyze the tapestry of the relationship between these and urban land use change. Different from the traditional linear regression analysis method of driving force of urban land use change, this study focuses on the interaction relationship and the underlying causal relationship among various "smart" driving factors, so it adopts a fuzzy statistical method, namely the grey relational analysis (GRA). Through the integration of literature research and known effective data, five categories of "smart" indicators have been taken as the primary driving factors: industry and economy, transportation, humanities and science, ICT systems, and environmental management. The results show that these indicators have different impacts on driving urban built-up land growth. Accordingly, optimization possibilities and recommendations for development strategies are proposed to realize a "smarter" development direction in Nanjing. This article confirms the effectiveness of GRA for studies on the driving mechanisms of urban land use change and provides a theoretical basis for the development goals of a smart city.
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| 4. |
- Cai, Zipan, et al.
(författare)
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Spatial dynamic modelling for urban scenario planning : A case study of Nanjing, China
- 2020
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Ingår i: Environment and Planning B: Urban Analytics and City Science. - : SAGE Publications. - 2399-8083 .- 2399-8091. ; 47:8, s. 1380-1396
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Tidskriftsartikel (refereegranskat)abstract
- Although urban scenario planning is widely applied for exploring various directions of urban development, it often has high requirements on the medium of quantitative information analysis and transformation. Thus, this study establishes a method of combining scenario planning with a spatial dynamic planning support system to predict urban growth. Specifically, a scenario-based spatial dynamic modelling method is integrated with the information module of planning policy for better decision support. The integrated modelling method is applied for an actual urban land use planning case of Nanjing, an evolving city in China. The spatial forms of future urban land use are simulated under four different pre-set policy scenarios. The differences in simulated results under multi-criteria restrictions reveal the effectiveness and practical value of the integration approach. The findings of this study provide policymakers with a process-based approach to test and evaluate ‘what-if’ consequences and help stakeholders reach consensus.
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| 5. |
- Cai, Zipan, 1992-
(författare)
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Towards a Scenario-based Spatial Dynamic Modeling for Predicting Urban Land Use Change : Planning Tools and Comparative Analysis
- 2023
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- As global urbanization progresses, cities worldwide are growing in size, which leads to many economic, environmental, and management challenges. Recent advancements in spatial data analysis and algorithmic geography have also led to the development of various urban model-based planning support systems (PSS) for urban planning. These PSSs aim to assist urban decision-makers in understanding urban information and collaborating on planning to address urban development challenges. Advanced urban planning concepts are, however, always multidisciplinary, multi-situational, and continuously evolving. In addition to the development of more advanced urban information and communications technology (ICT) and management, planning concepts that promote urban health and sustainable development are needed to meet residents’ physical, spiritual, and social needs, and promote more sustainable lifestyles. These factors create the need for a more rigorous methodological and theoretical foundation to apply PSS to urban planning at the microscale. A scenario-based spatial dynamic modeling approach is proposed in this thesis to address this research gap, allowing for a more precise matching of local policy scenarios and desired development patterns for practical planning support purposes. Several urban development scenarios and their potential impacts are explored by analyzing future urban land use changes. The establishment of this planning support approach effectively integrates spatial analysis, simulation model, policy revision, and participative planning. First, the thesis examines the rules and correlations underlying land unit transformations resulting from human-land interactions in spatial dynamic models by investigating mechanisms driving changes in urban land use. Second, a series of possible urban development simulations are generated through several case studies that employ a variety of representative cities with different urban contexts as model test sites including Nanjing in China, Stockholm in Sweden, and Chicago in the USA to evaluate their validity and practicality. Socioeconomics, ecological systems, and urban amenities are among the research themes that provide a more realistic and practical view of urban development. Last, visualization of the simulation results and quantitative information analyses and transformation is utilized to arrive at recommendations for revising planning policies and promoting sustainable development strategies.The challenge of adapting successful experiences of urban transformation from one city to another is considerable and cannot be achieved by merely replicating single projects or developments. An important goal was to address this challenge by developing general methods for model-assisted planning and then exploring their applicability and scalability across different contexts and geographical regions. Obtained results confirm that prioritizing industrial and transportation sectors in urban development is the most significant factor contributing to the rapid expansion of cities. This allocation of resources leads to the development of supporting infrastructure and employment opportunities, thereby attracting more people and industries to urban areas. Limiting the expansion of built-up areas and preserving green spaces is a desired measure to protect natural assets and the composition of cities and mitigate the negative environmental consequences of urbanization. Moreover, it is found that there are significant differences in the spatial and temporal needs and dependencies of residents in different areas with respect to natural and social amenities, providing a basis for future land development in residential and commercial areas of a city. Based on these findings, policymakers can more readily test and evaluate “what-if” scenarios using a process-based approach to avoid uncontrolled urban growth. In spite of limitations and uncertainties, the tools presented in this thesis are relevant for urban policymakers to enhance stakeholder interaction and consensus building in the decision-making process. This work has demonstrated the methodological steps for the implementation of these tools, as well as the general potential benefits of dynamic modeling for sustainable city planning and development.
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| 6. |
- Cai, Zipan, et al.
(författare)
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Urban Ecosystem Vulnerability Assessment of Support Climate‐ResilientCity Development
- 2021
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Ingår i: Urban Planning. - : Cogitatio. - 2183-7635. ; :3, s. 227-239
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Tidskriftsartikel (refereegranskat)abstract
- Climate change poses a threat to cities. Geospatial information and communication technology (Geo-ICT) assisted planning is increasingly being utilised to foster urban sustainability and adaptability to climate change. To fill the theoretical and practical gaps of urban adaptive planning and Geo-ICT implementation, this article presents an urban ecosystem vulnerability assessment approach using integrated socio-ecological modelling. The application of the Geo-ICT method is demonstrated in a specific case study of climate-resilient city development in Nanjing (China), aiming at helping city decision-makers understand the general geographic data processing and policy revision processes in response to hypothetical future disruptions and pressures on urban social, economic, and environmental systems. Ideally, the conceptual framework of the climate-resilient city transition proposed in this study effectively integrates the geographic data analysis, policy modification, and participatory planning. In the process of model building, we put forward the index system of urban ecosystem vulnerability assessment and use the assessment result as input data for the socio-ecological model. As a result, the model reveals the interaction processes of local land use, economy, and environment, further generating an evolving state of future land use in the studied city. The findings of this study demonstrate that socio-ecological modelling can provide guidance in adjusting the human-land interaction and climate-resilient city development from the perspective of macro policy. The decision support using urban ecosystem vulnerability assessment and quantitative system modelling can be useful for urban development under a variety of environmental change scenarios.
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| 7. |
- Cai, Zipan, 1992-, et al.
(författare)
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Urban spatial dynamic modeling based on urban amenity data to inform smart city planning
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- An ideal form of smart city planning would focus on the availability of urban amenities that can meet the basic needs of a residents material life, civil connections, and humanistic spirit. Previous studies have concentrated on analyzing the spatial distribution of urban services, with less attention on their contribution as local urban amenities. In this study, we propose a spatial dynamic modeling approach based on urban amenities using social media data from Google Place API to provide locational information on potential resident interactions. We use a representative region in Europe (Stockholm County, SE) to simulate and project urban development in the region, until 2050. Our circular conceptual framework of spatial information and feedback supports decision-makers in testing possible urban planning scenarios that align with the vision of a smart city. Simulation results reveal the interplay between human-land interactions on a specific spatial-temporal scale, and we analyze scenario outcomes in relation to commercial and residential land uses. Overall, our study provides a new perspective on human-social behavior-driven urban development, through a smart, spatial dynamic model as a planning support system that can enhance accuracy and realism, and ultimately help realize planned development objectives in the region.
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| 8. |
- Cai, Zipan, et al.
(författare)
-
Urban spatial dynamic modeling based on urban amenity data to inform smart city planning
- 2023
-
Ingår i: Anthropocene. - : Elsevier BV. - 2213-3054. ; 42
-
Tidskriftsartikel (refereegranskat)abstract
- An ideal form of smart city planning would focus on the availability of urban amenities that can meet the basic needs of a resident's material life, civil connections, and humanistic spirit. Previous studies have concentrated on analyzing the spatial distribution of urban services, with less attention on their contribution as local urban amenities. In this study, we propose a spatial dynamic modeling approach based on urban amenities using social media data from Google Place API to provide locational information on potential resident interactions. We use a representative region in Europe (Stockholm County, SE) to simulate and project urban development in the region until 2050. Our circular conceptual framework of spatial information and feedback supports decision-makers in testing possible urban planning scenarios that align with the vision of a smart city. Simulation results reveal the interplay between human-land interactions on a specific spatial-temporal scale, and we analyze scenario outcomes in relation to commercial and residential land uses. Overall, our study provides a new perspective on human-social behavior-driven urban development, through a smart, spatial dynamic model as a planning support system that can enhance realism, and ultimately help realize planned development objectives in the region.
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| 9. |
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| 10. |
- Carstens, Christoffer, 1981-
(författare)
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In the Pipe or End of Pipe? : Transport and Dispersion of Water-borne Pollutants and Feasibility of Abatement Measures
- 2012
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Eutrophication is one of the key environmental problems of today, both in terms of complexity and magnitude. For the Baltic Sea (BS), eutrophication is an acute problem, leading to hypoxic conditions at the bottom; a situation that is sustained and amplified, when phosphorus is released from hypoxic sediments. Reducing nutrient loading is a top political priority but the present situation is believed to require active measures within the catchments and recipients to reduce both loading and adverse effects. Implementation of effective and cost-efficient abatement methods requires understanding of natural processes in watersheds, streams and recipients as well as technological expertise in order to compare the effects of measures of different kinds and locations. This thesis tries to combine process understanding of catchment transport behaviour, especially in coastal zones, and feasibility of certain technologies for reducing nutrient loading and effects of eutrophication in-situ. The over-arching theme is the fate of the individual contaminant, from injection to removal. Transport and dispersion in catchments are investigated, combining physically-based, distributed, numerical groundwater models with Lagrangian stochastic advective reactive solute (LaSAR) transport modelling. The approach is powerful in the sense that it incorporates catchment structural, geomorphological dispersion in the numerical model with hydrodynamic and sub-scale dispersion as well as uncertainty in the LaSAR framework. The study exemplifies the complex nature of transport time distributions in catchments in general and when varying source size and location, importance of dispersion parameters and retention due to molecular diffusion. It is shown that geomorphological control on dispersion is present even for relatively heterogeneous systems and that neither the mean residence time nor a statistical distribution may provide accurate representations of hydrological systems. To combat internal loading of P from sediments in-situ, large-scale aeration of deep waters, halocline ventilation, has been suggested. This study further investigates the feasibility of wave-powered devices to meet the energy demands for such an operation. It is shown that the required amount of oxygen needed to keep the sediments at oxic conditions could be provided, cheaply and efficiently, through the use of wave power.
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| 11. |
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| 12. |
- Chasset, Coralie, et al.
(författare)
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Scenario simulations of CO(2) injection feasibility, plume migration and storage in a saline aquifer, Scania, Sweden
- 2011
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Ingår i: International Journal of Greenhouse Gas Control. - : Elsevier BV. - 1750-5836 .- 1878-0148. ; 5:5, s. 1303-1318
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Tidskriftsartikel (refereegranskat)abstract
- Deep saline aquifers have large capacity for geological CO(2) storage, but are generally not as well characterized as petroleum reservoirs. We here aim at quantifying effects of uncertain hydraulic parameters and uncertain stratigraphy on CO(2) injectivity and migration, and provide a first feasibility study of pilot-scale CO(2) injection into a multilayered saline aquifer system in southwest Scania, Sweden. Four main scenarios are developed, corresponding to different possible interpretations of available site data. Simulation results show that, on the one hand, stratigraphic uncertainty (presence/absence of a thin mudstone/claystone layer above the target storage formation) leads to large differences in predicted CO(2) storage in the target formation at the end of the test (ranging between 11% and 98% of injected CO(2) remaining), whereas other parameter uncertainty (in formation and cap rock permeabilities) has small impact. On the other hand, the latter has large impact on predicted injectivity, on which stratigraphic uncertainty has small impact. Salt precipitation at the border of the target storage formation affects CO(2) injectivity for all considered scenarios and injection rates. At low injection rates, salt is deposited also within the formation, considerably reducing its availability for CO(2) storage.
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| 13. |
- Chen, Yuanying, et al.
(författare)
-
Dominant Hydro-Climatic Drivers of Water Temperature, Salinity, and Flow Variability for the Large-Scale System of the Baltic Coastal Wetlands
- 2019
-
Ingår i: Water. - : MDPI. - 2073-4441. ; 11:3
-
Tidskriftsartikel (refereegranskat)abstract
- For the large-scale coastal wetland system of the Baltic Sea, this study develops a methodology for investigating if and to what degree the variability and changes in certain hydro-climatic drivers control key coastal-marine physical conditions. The studied physical conditions include: (a) water temperature, (b) water salinity, and (c) flow structures (magnitudes and directions of flows between marine basins and the associated coastal zones and wetlands). We use numerical simulations of three hydro-climatically distinct cases to investigate the variations in hydro-climatic drivers and the resulting physical conditions (a-c) among the cases. The studied hydro-climatic forcing variables are: net surface heat flux, wind conditions, saltwater influx from the North Sea, and freshwater runoff from land. For these variables, the available observation-based data show that the total runoff from land is significantly and positively correlated with precipitation on the sea itself, and negatively correlated with saltwater influx from the North Sea to the Baltic Sea. Overall, the physical condition (a-c) variability in the Baltic Sea and its coastal zones is found to be pairwise well-explained by simulation case differences as follows: (a) Net heat flux is a main control of sea water temperature. (b) Runoff from land, along with the correlated salt water influx from the North Sea, controls average sea salinity; with the variability of local river discharges shifting some coastal zones to deviate from the average sea condition. (c) Wind variability and change control the Baltic Sea flow structure, primarily in terms of flow magnitude and less so in terms of flow direction. For specific coastal wetland zones, considerable salinity differences from average Baltic Sea conditions (due to variability in local river discharges) are found for the coasts of Finland and Estonia, while the coastal wetland zones of south-eastern Sweden, and of Estonia and Latvia, emerge as particularly sensitive to wind shifts.
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| 14. |
- Chen, Yuanying
(författare)
-
Land-Sea Interactions in the Coastal-Marine System of the Baltic Sea under Hydro-Climatic Variability
- 2020
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis investigates a few important component processes for understanding and quantifying eutrophication in the Baltic Sea, that include characterization of nutrient loadings from land, water flow in the sea under changing climate conditions and transport of solutes originating from different locations along the coast. Furthermore, this study aims to improve our understanding on how processes from land (the nutrient loading conditions) and the sea (transport dynamics and water quality) couple to determine the fate of nutrients in the sea and the water quality in a selected localized coastal area, the Himmerfjärden Bay.Comprehensive data are compiled as a basis for numerical simulations. An open source tool for oceanographic studies FVCOM is used to simulate flow and transport processes in the Baltic Sea. Hydrodynamic simulations are verified in terms of temperature, salinity and water level for the year 2005. Results show that most of the investigated Swedish watersheds along the coastline are dominated by subsurface legacy sources, the loads of which are positively and linearly correlated with river discharges. Moreover, subsurface legacy sources are less likely to decrease over time compared with the current surface sources. The Baltic Sea has a stable flow structure considering flux directions between basins, while the flux magnitudes between basins are mainly determined by different wind conditions. The spreading patterns in the sea with solute released from different coastal areas are similar when the released amounts are comparable, even though different cases have different source input and water flow conditions. The overall spreading patterns in the sea are generally dominated by the total mass of released solute. Local transport dynamics and patterns around the coast differ greatly for different cases and are determined by the local flow conditions. Different water quality indicators are influenced by different land-based or sea-based measures for water quality improvement. The dry-cold hydro-climatic condition is the most favorable for improving the water quality and elevating the ecological status in the Himmerfjärden Bay.Based on this investigation, varying hydro-climatic factors impose important influence on the different component processes of nutrient loading from land to the sea. For example, the change of river discharges from land in the future would influence the total load into the sea from subsurface legacy sources, and finally influence the general spreading patterns of nutrients in the sea. The change of wind conditions would affect the flow and transport dynamics at local scale and flow fluxes magnitudes between marine basins at the sea scale. Change towards a dry-cold condition would be beneficial for the water quality and lead to improvement of coastal water quality, while the change towards a wet-warm condition will be generally unfavorable for improving the water quality. Clearly more comprehensive studies are needed based on the component processes considered in this thesis, for mapping water quality and eutrophication long-term trends in the Baltic Sea with confidence that is sufficient for effective mitigation measures and policies.
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| 15. |
- Chen, Yuanying, et al.
(författare)
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Scenarios of Nutrient-Related Solute Loading and Transport Fate from Different Land Catchments and Coasts into the Baltic Sea
- 2019
-
Ingår i: Water. - : MDPI. - 2073-4441. ; 11:7
-
Tidskriftsartikel (refereegranskat)abstract
- This study uses controlled numerical experimentation to comparatively simulate and investigate solute transport and concentration responses and patterns in the Baltic Sea for various solute releases from the land through two different coastal cases. These cases are the Swedish Kalmar County coast and the Polish coast of the Vistula River outlet. For equivalent solute releases, the coastal flow conditions and their interactions with main marine currents determine the local coastal solute spreading, while the overall spreading over the Baltic Sea is similar for the two coastal cases, despite their large local differences. For nutrient-proportional solute release scenarios, the highly-populated Vistula catchment yields much greater total, but smaller per-capita nutrient impacts, in the Baltic Sea than the Kalmar County catchment. To be as low as from the Vistula catchment, the per-capita nutrient contribution from Kalmar County would have to be reduced much more than required on average per Swedish inhabitant by the Baltic Sea Action Plan. This highlights an unfairness issue in the per-capita distribution of nutrient load allowance among the Baltic countries, which needs to be considered and handled in further research and international efforts aimed to combat the Baltic Sea eutrophication.
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| 16. |
- Cheng, Hua, et al.
(författare)
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Evaluation of Colloid Transport Experiments in a Quarried Block
- 2009
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Ingår i: SCIENTIFIC BASIS FOR NUCLEAR WASTE MANAGEMENT XXXII. - 9781605110967 ; , s. 519-524
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Konferensbidrag (refereegranskat)abstract
- Colloid tracer experiments were performed in a single, heterogeneous fracture contained in a quarried block (QB) under the configuration of dipole tracer tests. The experiments were first performed using bentonite and 100 nm latex colloids, as well as conservative tracer iodide and bromide, under conditions of different flow rates in order to identify the flow rates that favour colloid transport. The tracer experiments were later expanded to include experiments with different colloid sizes and longer transport distances. The aims of the present study are to identify the processes that affect colloid transport in the QB fracture and to estimate the retention parameters for the different sized colloids. We model the measured breakthrough curves (BTCs) using an advection-retention approach. The key feature of the advection-retention model is that advective transport and retention processes are related in a dynamic manner through the flow equation. Two Lagrangian random variables, tau and beta, that depend solely on flow conditions, control the retention processes. Here tau is the nonreactive travel time and beta is related to tau but also depends on the local aperture value. We assume the water residence time distribution g(tau) to be inverse-gaussian. The first two moments of g(tau) were obtained by calibrating the measured BTCs of conservative tracers. We then model the colloid BTCs using g(tau) and take into account the retention processes. The modelling results indicate that dominating retention processes include first-order linear kinetic attachment/detachment on the fracture surface, and mass loss (removal) by filtration/sedimentation. Diffusion into the rock matrix is of a much lesser importance.
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| 17. |
- Cheng, Hua, 1962-
(författare)
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Hydrodynamic control of retention in heterogeneous aquifers and fractured rock
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, fluid flow and solute transport in heterogeneous aquifers and particularly in frac-tured rock have been investigated using Lagrangian Stochastic Advective-Reaction (LaSAR) framework. The heterogeneity of the aquifer structure or fracture configuration, as well as the various reaction/retention processes have been considered in the modelling approach. Advection and retention processes are considered to be the dominant transport processes. Monte-Carlo simulation results for transport of nonreactive tracers in 2D generic heterogeneous aquifers indicate that the travel time τ can be well approximated by a lognormal distribution up to a relative high degree of heterogeneity of the aquifers. Comparison between the Monte-Carlo simulation results and the results of first-order approximation reveals that the analytical solutions of the statistical moments of τ are valid only when the variability of the aquifer properties is small. For reactive tracers, Monte-Carlo simulations have been conducted by accounting for spatial variability of both hydraulic conductivity and one sorption parameter simultaneously. The simulation results indicate that the reaction flow path μ is a nonlinear function of distance for shorter distance, linear function for longer distance, and also that μ and τ are well correlated over the considered parameter range. The parameter β, which is purely determined by the flow condi-tions, quantifies the hydrodynamic control of retention processes for transport of tracers in frac-tures. Numerical simulations have been performed to study the statistical properties of the pa-rameter β, travel time τ and flow rate Q in a single heterogeneous fracture and in a sequence of fractures. The results of Monte-Carlo simulations indicate that the parameter β and τ are corre-lated with a power-law relationship β ∼ τm. The correlation between β and the flow rate Q have also been studied and an inverse power-law relationship β ∼ Q-m is proposed. The establishment of these relationships provides a link between the parameter β and measurable parameters τ (or Q). The LaSAR approach has been applied for prediction, evaluation and interpretation of the results of a number of tracer tests (TRUE-1, TRUE Block Scale and TRUE Block Scale Continuation) conducted by SKB at the Äspö site for tracer transport in fractures. The breakthrough curves may be predicted reasonably well, provided that the retention parameters, boundary conditions and hydraulic properties of the domain are given. The evaluation of TRUE tests indicates that the retention occurs mainly in the rim zone on site characterization time scales, while on the per-formance assessment time scale, diffusion and sorption in the unaltered rock matrix are likely to become dominant retention mechanisms.
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| 18. |
- Cheng, Hua, et al.
(författare)
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Hydrodynamic control of tracer retention in heterogeneous rock fractures
- 2003
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Ingår i: Water resources research. - 0043-1397 .- 1944-7973. ; 39:5, s. 1130-1139
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the statistical properties of a Lagrangian random variable beta[T/L], which has been shown to quantify hydrodynamic impact on retention [Cvetkovic et al., 1999], using Monte Carlo simulations of flow and transport in a single fracture. The "local cubic law'' of water flow is generalized to a power law Qsimilar tob(n), where Q is the flow rate, b is the half aperture, and nless than or equal to3. Simulations of flow and particle transport are carried out assuming "local cubic law'' (n=3) and "local quadratic law'' (n=2), and for two typical flow configurations: uniform flow and radially converging flow. We find that beta is related to tau as betasimilar totau(m), where m is dependent on the power n and the configuration of flow and transport. Simulation results for uniform flow indicate that betasimilar totau(n/n-1) for a small source section; as the source section increases, we have the convergence to betasimilar totau. For radially converging flow, we find betasimilar totau for a small source section and a convergence to beta=const for an increasing source section. Simulation results for both flow configurations are consistent with the results for a homogeneous fracture. The results for a homogeneous fracture provide reasonable bounds for simulated beta. The correlation between beta and Q is relatively weak for all cases studied.
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| 19. |
- Cheng, Hua, et al.
(författare)
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Impact of temperature increase on nuclide transport in crystalline rock on the near field scale
- 2004. - 2
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Ingår i: Coupled Thermo-Hydro-Mechanical-Chemical Processes in Geo-Systems. - : Elsevier. ; , s. 413-418
-
Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- The TRUE (“Tracer Retention Understanding Experiments”) programme at the Äspö Hard Rock Laboratory (Winberg et al., 2000, 2002) has since 1995 generated a unique database for quantifying retention of radionuclides in crystalline rock on the 5-30m scales. Temperature conditions in the TRUE analysis were about 15°C. In this study, we consider the effect of temperature increase to ca 60°C consistent with conditions after say 1000 years in the rock volume surrounding a KBS-3 type of repository, anticipated to persist over a relatively long time. Temperature elevation will decrease the mean aperture of a conducting fracture by approximately 30%, which in turn will enhance retention. Furthermore, diffusion in the rock matrix will increase at elevated temperatures by approximately factor 4, further enhancing retention. Sorption coefficients are assumed to be unchanged since there is still considerable uncertainty regarding sorption and its change with increasing temperature. We show that elevated temperature on the near field scale enhances nuclide retention, thereby providing an additional safety margin.
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| 20. |
- Cvetkovic, Vladimir
(författare)
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A general memory function for modeling mass transfer in groundwater transport
- 2012
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Ingår i: Water resources research. - 0043-1397 .- 1944-7973. ; 48, s. W04528-
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Tidskriftsartikel (refereegranskat)abstract
- A power-law extension of the gamma distribution is proposed as a general memory function for capturing rate limitations of retention in groundwater transport. Using moments, we show how the new memory function can be reduced to most other forms available in the literature, exactly or approximately. The proposed formulation is suitable for field scale or laboratory scale transport modeling. Rate limitation effects are illustrated for solute transport by considering the fractional mass release over a given transport scale. The equilibrium and no-retention cases set bounds for contaminant attenuation, between which the impact of rate limitations is clearly exposed.
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| 21. |
- Cvetkovic, Vladimir, et al.
(författare)
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Analysis of nonlinear effects on tracer migration in heterogeneous aquifers using Lagrangian travel time approach
- 1996
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Ingår i: Water resources research. - 0043-1397 .- 1944-7973. ; 32:6, s. 1671-1680
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Tidskriftsartikel (refereegranskat)abstract
- Advective transport in a heterogeneous two-dimensional aquifer is simulated and quantified by the statistical moments of tracer travel time and transverse displacement. These moments depend on the statistics of the Lagrangian velocity given as a function of space rather than time. A comparison is made with first-order results that appear to be robust for log transmissivity variance at least up to 1. Lagrangian statistics for travel time clearly expose a few essential features of nonlinear transport: nonstationary distribution and the increasing integral scale of the Lagrangian velocity. These reflect the tendency of streamlines to concentrate into relatively few flow paths ('preferential flow') for increasing log transmissivity variance. Two simple empirical relationships are identified that in combination with first-order results for the flow capture the main features of nonlinear transport. The proposed methodology can readily be generalized for analyzing advective transport under more complex flow conditions and for establishing relatively simple analytical models for estimating solute mass flux in heterogeneous aquifers.
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| 22. |
- Cvetkovic, Vladimir
(författare)
-
Colloid-facilitated tracer transport by steady random ground-water flow
- 2000
-
Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 12:9, s. 2279-2294
-
Tidskriftsartikel (refereegranskat)abstract
- We study the transport of reactive solute in a three-phase system (water-solid matrix-colloids) in natural porous media. Semianalytical (integral) solutions are derived for the first time, which can be used for computing expected concentration, mass flux, or discharge for the dissolved as well as for colloid-bounded tracer. The results are based on a few simplifying assumptions: advection-dominated transport, linear mass transfer reactions, and steady-state colloidal concentration. Derived semianalytical expressions capture the main features of colloid-facilitated transport (the reversible-equilibrium and irreversible-kinetic sorption of tracers on colloids), and are applicable for the general class of linear sorption processes on the porous matrix. Derived solutions account for spatial variability of flow and sorption parameters, which is relevant for field-scale applications. We apply the theoretical results to the transport of neptunium and plutonium, using flow and transport data from the alluvial aquifer near Yucca Mountain, Nevada. Based on the zeroth and first temporal moment, dimensionless indicators are proposed for assessing the potential impact of colloid-facilitated tracer transport in aquifers. Generic sensitivity curves show the importance of tracer-colloid kinetic rates. Even very low irreversible rates (which will generally be difficult to determine in the laboratory) may yield observable effects for sufficiently long transport times. The obtained results can be used for assessing the significance of colloid-facilitated tracer transport under field conditions, as well as for setting further constraints on relevant parameters which need to be estimated in the field.
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| 23. |
- Cvetkovic, Vladimir, et al.
(författare)
-
Comparative measures of radionuclide containment in the crystalline geosphere
- 2002
-
Ingår i: Nuclear science and engineering. - : Informa UK Limited. - 0029-5639 .- 1943-748X. ; 142:3, s. 292-304
-
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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| 24. |
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| 25. |
- Cvetkovic, Vladimir D.
(författare)
-
On the significance of hydrodynamic control for radionuclide retention in fractured porous media
- 2004
-
Ingår i: Coupled Thermo-Hydro-Mechanical-Chemical Processes in Geo-Systems — Fundamentals, Modelling, Experiments and Applications. - : Elsevier. ; , s. 507-511
-
Bokkapitel (refereegranskat)abstract
- It is demonstrated that the approximate means of quantifying hydrodynamic control of retention is reasonably accurate for low values of the transport resistance on the 100m and 1000m scales; for high values, the approximate expression may significantly underestimate retention. Our results emphasize the need for further development of practical methodologies for quantifying statistical distributions of transport resistance by effectively combining field measurements, numerical simulations and theoretical/analytical considerations.
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| 26. |
- Cvetkovic, Vladimir
(författare)
-
Diffusion-controlled tracer retention in crystalline rock on the field scale
- 2010
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 37, s. L13401-
-
Tidskriftsartikel (refereegranskat)abstract
- Tracer retention is a key process for the barrier function of crystalline rock to any contaminant. Here we investigate the nature of retention mechanisms and their field-scale parametrization using results of a comprehensive tracer transport experiment in crystalline rock on the field scale (Aspo Hard Rock Laboratory, Sweden). A method for identifying dominant retention mechanisms and inferring key parameters on the site scale is presented. Taking advantage of multiple tracer tests with a wide range of sorption affinities, retention is shown to be diffusion-controlled. For the considered site, robust features of tracer migration can be reasonably well predicted within a rock volume on at least 200 m scale, by combining independent information with a simple model.
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| 27. |
- Cvetkovic, Vladimir, et al.
(författare)
-
Evaluation of single-well injection-withdrawal tests in Swedish crystalline rock using the Lagrangian travel time approach
- 2011
-
Ingår i: Water resources research. - 0043-1397 .- 1944-7973. ; 47, s. W02527-
-
Tidskriftsartikel (refereegranskat)abstract
- A series of 10 single-well injection-withdrawal (SWIW) tests are evaluated with two tracers each: uranine and cesium (Cs). An evaluation tool for SWIW tests in crystalline rock is presented on the basis of the Lagrangian travel time approach, whereby probabilities of tracer particle residence times are computed for key stages of the test cycle. Calibration results for three transport parameters and each breakthrough curve are presented. We show that estimates of the controlling retention parameter group psi [1/root T] are robust for Cs but highly uncertain for uranine. The estimated retention for Cs is larger for the Laxemar-Simpevarp site compared to the Forsmark site. Deviations from the -3/2 asymptotic breakthrough curve slope observed in a few of the tests at Forsmark are possibly due to a thin fracture coating that has been identified in mineralogical studies at some locations of the site.
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| 28. |
- Cvetkovic, Vladimir, et al.
(författare)
-
Flow-dependence of matrix diffusion in highly heterogeneous rock fractures
- 2013
-
Ingår i: Water resources research. - 0043-1397 .- 1944-7973. ; 49:11, s. 7587-7597
-
Tidskriftsartikel (refereegranskat)abstract
- Diffusive mass transfer in rock fractures is strongly affected by fluid flow in addition to material properties. The flow-dependence of matrix diffusion is quantified by a random variable ("transport resistance'') denoted as beta [T/L] and computed from the flow field by following advection trajectories. The numerical methodology for simulating fluid flow is mesh-free, using Fup basis functions. A generic statistical model is used for the transmissivity field, featuring three correlation structures: (i) highly connected non-multiGaussian; (ii) poorly connected (or disconnected) non-multi-Gaussian; and (iii) multi-Gaussian. The moments of beta are shown to be linear with distance, irrespective of the structure, after approximately 10 integral scales of ln T. Percentiles of beta are found to be linear with the mean beta when considering all three structures. Taking advantage of this property, a potentially useful relationship is presented between beta percentiles and the fracture mean water residence time that integrates all structures with high variability; it can be used in discrete fracture network simulations where T statistical data on individual fractures are not available.
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| 29. |
- Cvetkovic, Vladimir, et al.
(författare)
-
Global sensitivity analysis of groundwater transport
- 2015
-
Ingår i: Journal of Hydrology. - : Elsevier. - 0022-1694 .- 1879-2707. ; 531
-
Tidskriftsartikel (refereegranskat)abstract
- In this work we address the model and parametric sensitivity of groundwater transport using the Lagrangian-Stochastic Advection-Reaction (LaSAR) methodology. The 'attenuation index' is used as a relevant and convenient measure of the coupled transport mechanisms. The coefficients of variation (CV) for seven uncertain parameters are assumed to be between 0.25 and 3.5, the highest value being for the lower bound of the mass transfer coefficient k0. In almost all cases, the uncertainties in the macro-dispersion (CV = 0.35) and in the mass transfer rate k0 (CV = 3.5) are most significant. The global sensitivity analysis using Sobol and derivative-based indices yield consistent rankings on the significance of different models and/or parameter ranges. The results presented here are generic however the proposed methodology can be easily adapted to specific conditions where uncertainty ranges in models and/or parameters can be estimated from field and/or laboratory measurements.
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| 30. |
- Cvetkovic, Vladimir
(författare)
-
How accurate is predictive modeling of groundwater transport? : A case study of advection, macrodispersion, and diffusive mass transfer at the Forsmark site (Sweden)
- 2013
-
Ingår i: Water resources research. - : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 49:9, s. 5317-5327
-
Tidskriftsartikel (refereegranskat)abstract
- We study predictive modeling of groundwater transport that accounts for three mechanisms: mean advection, macrodispersion, and mass transfer. A general methodology is presented and applied to a series of nonsorbing tracer tests along multiple pathways on scales ranging from ca. 70 to 300 m, in a highly heterogeneous aquifer at Forsmark (Sweden). The mean water residence time cannot be predicted well using a simple water balance model. Longitudinal macrodispersivity (L) (L) and a mass transfer parameter group (1/root T) are extrapolated from the control tracer experiments, to yield accurate predictions of tracer discharge, once the mean water residence time is constrained. A relatively simple modeling framework based on Fickian macrodispersion and diffusion seems to be adequate for reproducing the tracer discharge in this complex and highly heterogeneous porous media.
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| 31. |
- Cvetkovic, Vladimir, et al.
(författare)
-
Inference of Retention Time From Tracer Tests in Crystalline Rock
- 2020
-
Ingår i: Water resources research. - : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 56:2
-
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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| 32. |
- Cvetkovic, Vladimir, et al.
(författare)
-
On the upscaling of chemical transport in fractured rock
- 2014
-
Ingår i: Water resources research. - 0043-1397 .- 1944-7973. ; 50:7, s. 5797-5816
-
Tidskriftsartikel (refereegranskat)abstract
- The impact of flow heterogeneity on chemical transport from single to multiple fractures is investigated. The emphasis is on the dynamic nature of the specific surface area (SSA) due to heterogeneity of the flow, relative to a purely geometrical definition. The flow-dependent SSA is interpreted probabilistically, following inert tracer particles along individual fractures. Upscaling to a fracture network is proposed as a time domain random walk based on the statistics of SSA for single fractures. Statistics of SSA are investigated for three correlation structures of transmissivity: multi-Gaussian and two non-multi-Gaussian. The mean of SSA stabilizes after similar to 20 fractures at different values depending on whether the cubic or quadratic hydraulic law is assumed. The results are tested against comprehensive DFN simulations based on site-specific data but also against direct estimates from a wider range of tracer tests. The proposed time domain random walk methodology sets bounds for SSA in a 75% confidence interval as similar to 1800 1/m and 27,000 1/m, with a median of 14,000 1/m; these values capture reasonably well both the DFN simulation and tracer test SSA data. Presented results may be particularly relevant when quantifying uncertainty of reactive transport modeling in fractured rock.
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| 33. |
- Cvetkovic, Vladimir, et al.
(författare)
-
Parameter and model sensitivities for colloid-facilitated radionuclide transport on the field scale
- 2004
-
Ingår i: Water resources research. - : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 40:6
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate the potential effects of inorganic colloids on radionuclide transport in groundwater using generic sensitivity studies and an example based on the alluvial aquifer near Yucca Mountain, Nevada. Our emphasis is on kinetically controlled sorption of radionuclides on mobile and immobile colloids. Three kinetic sorption models are considered for the sensitivity analysis: bilinear, Langmuir, and linear. Plutonium is assumed to be injected into the Yucca Mountain alluvial aquifer at a constant rate and follows a random stream tube to a monitoring boundary. The linear sorption model provides a reasonable upper bound on colloid-facilitated plutonium transport for the site-specific conditions. In the absence of colloid filtration and retardation, colloids enhance the plutonium discharge by a large factor over the situation without colloids. Exchange of plutonium between solution and reversibly attached colloids makes colloid retardation relatively ineffective at reducing colloid-facilitated transport except when the retardation factor is large. Irreversible removal of colloids (filtration) is more effective than retardation at reducing colloid-facilitated transport. For fixed filtration rate the degree of attenuation depends sensitively and nonmonotonically on the rate of plutonium desorption from colloids. These results emphasize the need for accurate measurements of rates of desorption from colloids as well as in situ studies of filtration of naturally occurring colloids.
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| 34. |
- Cvetkovic, Vladimir
(författare)
-
Significance of fracture rim zone heterogeneity for tracer transport in crystalline rock
- 2010
-
Ingår i: Water resources research. - : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 46:3, s. W03504-
-
Tidskriftsartikel (refereegranskat)abstract
- Conducting fractures of crystalline rock are typically altered over long periods of time. The fracture rim zone, a result of these alterations, will as a rule have different physical and chemical properties from the unaltered ("fresh'') rock, depending on various microscopic and macroscopic factors of the alterations. In this paper, we study the impact of rim zone heterogeneity, exemplified by a decreasing porosity trend as inferred from the Aspo Hard Rock Laboratory site (Sweden), on short- and long-term tracer transport. Our main finding is that this particular rim zone structure will have a dominant effect on transport of moderately to strongly sorbing tracers on experimental time scales and a notable effect on application time scales. The findings of this work lend further support to the interpretation of the relatively strong retention reported by Cvetkovic et al. The fracture rim zone porosity structure may provide an additional safety margin for sorbing radionuclides in crystalline rock at sites where fracture alteration is prevalent.
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| 35. |
- Cvetkovic, Vladimir, et al.
(författare)
-
Solute transport and retention in three-dimensional fracture networks
- 2012
-
Ingår i: Water resources research. - 0043-1397 .- 1944-7973. ; 48, s. W02509-
-
Tidskriftsartikel (refereegranskat)abstract
- Resolving the hydrodynamic control of retention is an important step in predictive modeling of transport of sorbing tracers in fractured rock. The statistics of the transport resistance parameter beta [T/L] and the related effective active specific surface area s(f) [1/L] are studied in a crystalline rock volume on a 100 m scale. Groundwater flow and advective transport are based on generic boundary conditions and realistic discrete fracture networks inferred from the Laxemar site, southeast Sweden. The overall statistics of beta are consistent with statistics of the water residence time tau; the moments of beta vary linearly with distance, at least up to 100 m. The correlation between log tau and log beta is predominantly linear, however, there is significant dispersion; the parameter s(f) strongly depends on the assumed hydraulic law (theoretical cubic or empirical quadratic). Fast and slow trajectories/segments in the network determine the shape of the beta distribution that cannot be reproduced by infinitely divisible model over the entire range; the low value range and median can be reproduced reasonably well with the tempered one-sided stable density using the exponent in the range 0.35-0.7. The low percentiles of the beta distribution seems to converge to a Fickian type of behavior from a 50 to 100 m scale.
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| 36. |
- Cvetkovic, Vladimir, et al.
(författare)
-
Solute transport in aquifers of arbitrary variability : A time-domain random walk formulation
- 2014
-
Ingår i: Water resources research. - 0043-1397 .- 1944-7973. ; 50:7, s. 5759-5773
-
Tidskriftsartikel (refereegranskat)abstract
- Solute transport in three-dimensional aquifers, with spatially varying hydraulic conductivity of arbitrary point distribution is investigated. The basis of our study is a multiindicator model (MIM) representation of the heterogeneity, combined with a self-consistent approximation for groundwater flow and particle transport. A time-domain random walk (TDRW) approach is presented for computing the expected mass arrival along the longitudinal transport direction that is simple and honors the hydrodynamics of flow for any variability. Using hydraulic conductivity measurements at the MADE site and the MIM, it is shown that the travel time distribution for large variability, cannot be well reproduced by the common distributions used for modeling hydrological transport, such as the log-normal distribution, or the inverse-Gaussian distribution. The proposed TDRW approach directly relates to the Lagrangian trajectory formulation and is appropriate for applications where occurrence of negative flow velocities is small. These results open new possibilities for modeling solute transport in aquifers of arbitrary variability by the time-domain random walk that can readily account for a wide range of mass transfer reactions.
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| 37. |
- Cvetkovic, Vladimir, et al.
(författare)
-
Sorbing tracer experiments in a crystalline rock fracture at Aspo (Sweden) : 2. Transport model and effective parameter estimation
- 2007
-
Ingår i: Water resources research. - : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 43:11
-
Tidskriftsartikel (refereegranskat)abstract
- [1] Transport and retention of sorbing tracers in a single, altered crystalline rock fracture on a 5 m scale is investigated. We evaluate the results of a comprehensive field study ( referred to as Tracer Retention Understanding Experiments, first phase ( TRUE- 1)), at a 400 m depth of the Aspo Hard Rock Laboratory ( Sweden). A total of 16 breakthrough curves are analyzed, from three test configurations using six radioactive tracers with a broad range of sorption properties. A transport- retention model is proposed, and its applicability is assessed based on available data. We find that the conventional model with an asymptotic power law slope of - 3/ 2 ( one- dimensional diffusion into an unlimited rock matrix) is a reasonable approximation for the conditions of the TRUE- 1 tests. Retention in the altered rock of the rim zone appears to be significantly stronger than implied by retention properties inferred from generic ( unaltered) rock samples. The effective physical parameters which control retention ( matrix porosity and retention aperture) are comparable for all three test configurations. The most plausible in situ ( rim zone) porosity is in the range 1% - 2%, which constrains the effective retention aperture to the range 0.2 - 0.7 mm. For all sorbing tracers the estimated in situ sorption coefficient appears to be larger by at least a factor of 10, compared to the value inferred from through- diffusion tests using unaltered rock samples.
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| 38. |
- Cvetkovic, Vladimir, et al.
(författare)
-
Sorbing tracer experiments in a crystalline rock fracture at Aspo (Sweden) : 3. Effect of microscale heterogeneity
- 2008
-
Ingår i: Water resources research. - : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 44:12
-
Tidskriftsartikel (refereegranskat)abstract
- We study the impact of microscale (10(-3) -10(-2) m) heterogeneity in material and structural properties on sorbing tracer transport in a single crystalline fracture of the TRUE-1 tests (mesoscale, 5 m) at Aspo (Sweden). The analysis is based on the microscale characterization results as presented in part 1 of this series. Our main objective in this last part of the series is to provide an independent interpretation (or "prediction'') of the effective parameters as estimated from calibration in part 2 by combining the data presented in part 1 with analytical and numerical transport modeling. We show here that the independent information from microscopic characterization can be used for "predicting'' the effective diffusion time t(d) reasonably well; a discrepancy is to be expected given the uncertainties of microscale retention properties, in particular of the sorption coefficient.
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| 39. |
- Cvetkovic, Vladimir
(författare)
-
Statistical Formulation of Generalized Tracer Retention in Fractured Rock
- 2017
-
Ingår i: Water resources research. - : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 53:11, s. 8736-8759
-
Tidskriftsartikel (refereegranskat)abstract
- We study tracer retention in fractured rock by combing Lagrangian and time domain random walk frameworks, as well as a statistical representation of the retention process. Mass transfer is quantified by the retention time distribution that follows from a Lagrangian coupling between advective transport and mass exchange processes, applicable for advection-dominated transport. A unifying parametrization is presented for generalized diffusion using two rates denoted by k(1) and k(2) where k(1) is a forward rate and k(2) a reverse rate, plus an exponent as an additional parameter. For the Fickian diffusion model, k(1) and k(2) are related to measurable retention properties of the fracture-matrix by the method of moments, whereas for the non-Fickian case dimensional analysis is used. The derived retention time distributions are exemplified for interpreting tracer tests as well as for predictive modeling of expected tracer breakthrough. We show that non-Fickian effects can be notable when transport is upscaled based on a non-Fickian interpretation of a tracer test for which deviations from Fickianity are relatively small. The statistical representation of retention clearly shows the significance of the forward rate k(1) which depends on the active specific surface area and is the most difficult parameter to characterize in the field.
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| 40. |
- Cvetkovic, Vladimir, et al.
(författare)
-
Stochastic simulation of radionuclide migration in discretely fractured rock near the Aspo Hard Rock Laboratory
- 2004
-
Ingår i: Water resources research. - : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 40:2, s. W02404-
-
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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| 41. |
- Cvetkovic, Vladimir
(författare)
-
The tempered one-sided stable density : a universal model for hydrological transport?
- 2011
-
Ingår i: Environmental Research Letters. - : IOP Publishing. - 1748-9326. ; 6:3, s. 034008-
-
Tidskriftsartikel (refereegranskat)abstract
- A generalized distribution for the water residence time in hydrological transport is proposed in the form of the tempered one-sided stable (TOSS) density. It is shown that limiting cases of the TOSS distribution recover virtually all distributions that have been considered in the literature for hydrological transport, from plug flow to flow reactor, the advection-dispersion model, and the gamma and Levy densities. The stable property of TOSS is particularly important, enabling a seamless transition between a time-domain random walk, and the Lagrangian (trajectory) approach along hydrological transport pathways.
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| 42. |
- Cvetkovic, Vladimir
(författare)
-
Tracer attenuation in groundwater
- 2011
-
Ingår i: Water resources research. - 0043-1397 .- 1944-7973. ; 47:12, s. W12541-
-
Tidskriftsartikel (refereegranskat)abstract
- The self-purifying capacity of aquifers strongly depends on the attenuation of waterborne contaminants, i.e.,irreversible loss of contaminant mass on a given scale as a result of coupled transport and transformation processes. A general formulation of tracer attenuation in groundwater is presented. Basic sensitivities of attenuation to macrodispersion and retention are illustrated for a few typical retention mechanisms. Tracer recovery is suggested as an experimental proxy for attenuation. Unique experimental data of tracer recovery in crystalline rock compare favorably with the theoretical model that is based on diffusion-controlled retention. Non-Fickian hydrodynamic transport has potentially a large impact on field-scale attenuation of dissolved contaminants.
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| 43. |
- Cvetkovic, Vladimir, et al.
(författare)
-
Tracer travel and residence time distributions in highly heterogeneous aquifers : Coupled effect of flow variability and mass transfer
- 2016
-
Ingår i: Journal of Hydrology. - : Elsevier. - 0022-1694 .- 1879-2707.
-
Tidskriftsartikel (refereegranskat)abstract
- The driving mechanism of tracer transport in aquifers is groundwater flow which is controlled by the heterogeneity of hydraulic properties. We show how hydrodynamics and mass transfer are coupled in a general analytical manner to derive a physically-based (or process-based) residence time distribution for a given integral scale of the hydraulic conductivity; the result can be applied for a broad class of linear mass transfer processes. The derived tracer residence time distribution is a transfer function with parameters to be inferred from combined field and laboratory measurements. It is scalable relative to the correlation length and applicable for an arbitrary statistical distribution of the hydraulic conductivity. Based on the derived residence time distribution, the coefficient of variation and skewness of residence time are illustrated assuming a log-normal hydraulic conductivity field and first-order mass transfer. We show that for a low Damkohler number the coefficient of variation is more strongly influenced by mass transfer than by heterogeneity, whereas skewness is more strongly influenced by heterogeneity.
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| 44. |
- Cvetkovic, Vladimir, et al.
(författare)
-
Transport and retention from single to multiple fractures in crystalline rock at Aspo (Sweden) : 1. Evaluation of tracer test results and sensitivity analysis
- 2010
-
Ingår i: Water resources research. - 0043-1397 .- 1944-7973. ; 46, s. W05505-
-
Tidskriftsartikel (refereegranskat)abstract
- We evaluate the breakthrough curves obtained within a comprehensive experimental program for investigating the retention properties of crystalline rock, referred to as Tracer Retention Understanding Experiments (TRUE). The tracer tests were conducted at the Aspo Hard Rock Laboratory (Sweden) in two phases jointly referred to as TRUE Block Scale (TBS); the TBS tests comprise a total of 17 breakthrough curves with nonsorbing and a range of sorbing tracers. The Euclidian length scales are between 10 and 30 m, compared to 5 m for the earlier tests TRUE-1. The unlimited diffusion model is consistent with measured breakthrough curves and is adopted here for evaluation. The model has four independent parameters, two of which are related to advection and dispersion, one which is related to diffusion-sorption, and one which is related to surface sorption; the individual retention parameters or properties cannot be inferred from breakthrough curves alone and require additional constraints. The mean water residence times for the TBS tests are in the range 15-250 h, whereas the coefficient of variation of the water residence times is in the range 0.4-0.6. A consistent trend is found in the calibrated retention parameters with the sorption affinities of the tracers involved. Using Bode sensitivity functions, it is shown that sensitivity increases for the retention parameter with increasing sorption affinity; for nonsorbing tracers, diffusion and hydrodynamic dispersion are shown to "compete," exhibiting similar effects; hence, their estimates are uncertain. The analysis presented here exposes a few fundamental limitations and sensitivities when evaluating diffusion-controlled retention in the subsurface; it is general and applicable to any site with comparable tracer test data. In part 2, it will be shown how discrete fracture network simulations based on the hydrostructural information available can be used for further constraining individual retention parameters, in particular, the active specific surface area (s(f)) and the rock matrix porosity (theta).
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| 45. |
- Cvetkovic, Vladimir, et al.
(författare)
-
Transport and retention from single to multiple fractures in crystalline rock at Aspo (Sweden) : 2. Fracture network simulations and generic retention model
- 2010
-
Ingår i: Water resources research. - : American Geophysical Union. - 0043-1397 .- 1944-7973. ; 46, s. W05506-
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Tidskriftsartikel (refereegranskat)abstract
- Hydrogeologic characterization of crystalline rock formations on the field scale is important for many applications but still presents a multitude of challenges. In this work we use comprehensive hydrostructural information and present a detailed simulation study of flow and advective transport in a discrete fracture network (DFN) that replicates the Tracer Retention Understanding Experiments (TRUE) Block Scale rock volume at the Aspo Hard Rock Laboratory (Sweden). Simulated water residence time tau and hydrodynamic retention parameter beta are used as independent constraints for estimating material retention properties as presented in paper 1 of this series, whereas simulated mean water residence times are compared with observed values. We find that the DFN simulations reproduce water residence times reasonably well, indicating that the characterization data are sufficient and that the DFN model does capture dominant features of the flow paths analyzed. The empirical quadratic law that relates aperture and transmissivity seems to better reproduce calibrated mean water residence times than the theoretical cubic law for the five flow paths. The active specific surface area (beta/tau) [1/L] as inferred from simulations is used for defining a generic retention model for the dominant rock type (Aspo diorite) that matches fairly well the entire range of calibrated retention parameters of the TRUE tests. The combination of paper 1 and this work provides a general, comprehensive methodology for evaluating tracer test results in crystalline rock where a comparable amount of information is available; critical to this methodology is that tracer tests are carried out using tracers with sufficiently different sorption affinities (of factor 10-100).
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| 46. |
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| 47. |
- Cvetkovic, Vladimir, et al.
(författare)
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Transport with multiple-rate exchange in disordered media
- 2002
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1063-651X .- 1095-3787. ; 65:5
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Tidskriftsartikel (refereegranskat)abstract
- We investigate transport of particles subject to exchange using the continuous-time random-walk framework. Transition is controlled by macroscale, and exchange by both macroscale and microscale disorder. A wide class of exchange mechanisms is represented using the multiple-rate exchange model. Particles are transported along random trajectories viewed as one-dimensional lattices. The solution of the transport problem is obtained in the form of the crossing-time density, h(t; L), at an exit surface L; h is dependent on two functions, g and f. g characterizes exchange controlled by microscale disorder. The joint density f is central for the solution as it relates the microscale and macroscale disorder along random trajectories. For the case of transition and exchange disorder, we show that power-law exponent eta (characterizing microscale disorder) and power-law exponents alpha(tau) and alpha(mu) (characterizing macroscale disorder), define two regions delimited by a line alpha(tau) = alpha(mu)(eta + 1): One in which the asymptotic transport is dominated by transition, and one in which it is dominated by the exchange. For the case of transition disorder with uniform exchange, both transition and exchange cart influence the late-time behavior of h(t). Microscale exchange processes will unconditionally influence the late-time behavior of h(t) only if eta<0. If η>0, exchange will dominate at late time provided that transition is asymptotically Gaussian.
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| 48. |
- 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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| 49. |
- Dagan, G., et al.
(författare)
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Tailing of the breakthrough curve in aquifer contaminant transport : The impact of permeability spatial variability
- 2008
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Konferensbidrag (refereegranskat)abstract
- A contaminant plume of mass Mo is inserted at time t = 0 at an injection plane at × = 0 in an aquifer of spatially variable conductivity K. The log-conductivity Y = InK is modelled as stationary and isotropic, of univariate distribution f(Y), and of finite integral scale I. The flow of water is uniform in the mean (natural gradient) and the plume is of large transverse extent relative to the integral scale. Advective transport and longitudinal spread are quantified by the solute mass arrival ("breakthrough curve", BTC) M(t,x) at a control plane at × > I. For a large plume (ergodic conditions) the relative mass flux μ(t,x) = (l/Mo)M/t is approximately equal to the probability density function of travel times of solute particles f(τx) and the latter is used to analyse transport. f(τx) is derived by adopting a structural model of the aquifer that contains spherical or cubic inclusions of uniform size and of independent Y that fill the space. Such a structure can represent any formation of given f(Y) and I. The flow and transport solutions are obtained by a simple semianalytical model and by accurate numerical simulations. The travel time distribution at few control planes is determined for a log-normal f(K) first. Under the assumption of weak heterogeneity, i.e. for small variance σy 2 and for x»I, the travel time distribution is symmetrical and Gaussian. Subsequently, by using the semi-analytical model and numerical simulations we derive f(τx) for a highly heterogeneous formation of σ y 2 = 2. The main finding is f(τx) is highly skewed due to the presence of a thin, but long tail, for large travel times. The tail is of significance to applications that deal with aquifer pollution and remediation. The tail is related to the large residence time of solute particles in blocks of low conductivity. A simple relationship is established between the tail of f(Y) for low K and that f(τx) for large τ. To further examine the impact of the log-conductivity distribution on BTC tailing, a non-Gaussian model, the subordinate model, is adopted for f(Y). This distribution depends on an additional parameter Is; travel time distribution tends to normal for Is→0, whereas the tails of the two distributions are different for Is > 0. This choice reflects the difficulty of identification of the tail of f(Y) based on field data. The relevance of results to applications is examined in terms of impact of conductivity spatial distribution, as well as influence of plume size (non-ergodic behaviour) and diffusion.
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| 50. |
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