| 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
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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. - : 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
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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)
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Colloid-facilitated tracer transport by steady random ground-water flow
- 2000
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 12:9, s. 2279-2294
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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)
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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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| 24. |
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| 25. |
- Cvetkovic, Vladimir D.
(författare)
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On the significance of hydrodynamic control for radionuclide retention in fractured porous media
- 2004
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Ingår i: Coupled Thermo-Hydro-Mechanical-Chemical Processes in Geo-Systems — Fundamentals, Modelling, Experiments and Applications. - : Elsevier. ; , s. 507-511
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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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