• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 6552
  • 449
  • 281
  • 259
  • 167
  • 90
  • 49
  • 17
  • 12
  • 5
  • 5
  • 5
  • 5
  • 5
  • 5
  • Tagged with
  • 9399
  • 9399
  • 5878
  • 868
  • 703
  • 559
  • 552
  • 415
  • 383
  • 365
  • 342
  • 328
  • 323
  • 322
  • 309
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

An Integrated Systems Approach to Performance Assessment of Near Surface Disposal Facilities for Low Level Radioactive Waste Management

Rustick, Joseph Henry 14 April 2016 (has links)
An integrated systems approach framework was developed that defines the performance of a near surface low-level radioactive waste disposal facility as a system of three components (or subsystems): the engineered component (cover systems and bottom liners); the properties of the waste (composition, waste form and waste package); and the site-specific environmental features (climate, geology, hydrology). US government radioactive waste disposal facility design and management were examined and compared using this approach. The waste component, historically not considered when calculating waste movement within the facility, was evaluated in greater detail by looking at corrosion of carbon steel boxes filled with waste and buried in a humid environment. The time to hydraulic failure from initial burial to development of corroded holes was calculated for four corrosion scenarios under a constant and a slowing corrosion case. Corrosion rates were estimated from several historical studies and related to the corrosivity and aeration profile of the soil. The scenarios were chosen to represent a range of possible conditions at current and future U.S. Department of Energy disposal facilities. A leachate model was then created that could show the amount of liquid leachate present in each waste package at the time of failure. This model was applied to three different infiltration situations based off of past, current, and proposed future installations of operational and interim cover systems over recently buried waste packages. It was found that for past practices of a 25-year operational period, the estimated amount of leachate within the waste zone over current practices was greater than 300 percent. In order to reduce leachate for future planned disposal facilities, it would be useful to install interim cover systems immediately after waste burial, and fill the waste packages with grout before disposal. For all disposal facilities, leachate movement from the waste zone into the vadose zone would be a good target for performance monitoring.

An Evaluation of Safety and Health Data, with an Emphasis on Developing Performance Measures for Nuclear Chemical Facilities, Using Quantitative and Semi-quantitative Methods

Fyffe, Lyndsey Morgan 09 April 2016 (has links)
Nuclear chemical facilities are facilities that contain the operating hazards of a complex chemical facility with the additional hazards of radioactive materials. The approach to safety management at nuclear chemical facilities is rooted in nuclear hazard analysis techniques, and could benefit from lessons learned from the chemical industry. A content analysis of chemical industry accident reports, as well as occurrences from select nuclear chemical facilities was undertaken to determine common themes of process safety accidents. Grounded theory was applied to these common causes and themes to develop a set of theories about safe operations at nuclear chemical facilities; these theories were, in turn, used to postulate a set of leading performance indicators to monitor safety. The performance measures were reviewed for practicality and effectiveness by subject matter experts, and impact on facility safety using an existing probabilistic risk assessment. Guidelines are provided for facility implementation of the performance measures.

The Attachment of Colloidal Particles to Environmentally Relevant Surfaces: Effect of Ionic Strength, Particle Shape, and Physicochemical Properties

McNew, Coy Phillip 04 December 2015 (has links)
The sharp increase in production of engineered nanomaterials (ENMs) combined with their high potential for aquatic toxicity, mean that understanding the transport of these materials throughout the environment is of utmost importance. In the presence of environmentally relevant surfaces, the relationship between particle attachment and relevant variables was quantitatively investigated and reported. Increasing temperature greatly altered the attachment onto two different humic substances by altering the hydration, and therefore confirmation, of the natural organic matter (NOM) matrix. By increasing the hydration of the NOM layer, the matrix swelled, allowing for more surface area for particle attachment and an increase in possible sorption sites. Similarly, high ionic strengths caused the NOM layer to condense, reducing surface area and sorption sites for particle attachment and effectively lowering particle attachment efficiency. The shape of the particle itself also played a role in attachment. A humic acid layer showed preference to smaller, more spherical particles due to the size of the voids within the layer, raising attachment efficiency for the smaller, spherical particles only, while a smoother, more condensed layer did not. As ionic strength increased, however, the layer condensed and the preference vanished. Finally, a predictive model for attachment efficiency was developed using a machine learning approach and trained on a database containing all the data gathered in this work combined with all currently available, relevant attachment efficiency literature. The model employed 13 training features, each of which was a physicochemical characteristic of the particle, surface, or solution system, to predict attachment efficiency with relatively high performance. The most important features for predicting attachment efficiency were also identified. The results presented in this work improve the understanding of particle attachment efficiency by identifying important variables, explaining why these variables have an effect on attachment efficiency, and also providing an empirical predictive model for attachment efficiency. By applying this approach to other areas of particle transport, we can close the gap between experimental and modeling efforts, advancing transport knowledge as quickly and efficiently as possible. Only by closing this gap can we expect to understand particle transport in a system as complex as the natural environment.

Response of the Brahmaputra River to Tectonic Deformation and Paleohydrological Events in the Foreland Bengal Basin

Pickering, Jennifer Lynne 01 August 2016 (has links)
The Brahmaputra River has constructed an upland fluvial fan delta with an estimated volume of >5 million km^3; of riverine sediments. Much of this stratigraphy was generated during the Quaternary, a period of time characterized globally by cyclic glacioeustatic fluctuations and regionally by ongoing deformation of the Bengal basin associated with Himalayan convergence. This research examines the extent to which these external climatic and tectonic conditions have influenced the paleogeography of the river and the evolution of the basin. Stratigraphic analysis of sediments constructed by the Brahmaputra reveals that the course of the river has evolved in direct response to uplift of the Shillong Plateau, a regional crust block associated with a forward jump of the Himalayan arc. Specifically, the river has been episodically deflected by uplifting terrain and subsequently attracted to topographic subsidence below the overriding thrust, resulting in the anfractuous course that the river follows today. Overprinted upon this tectonic steering of the river, episodic paleohydrological events in the form of ice-dam floods were instrumental in scouring a wide valley paved by the deposition of cobble to boulder-sized gravel during the late glacial to interglacial transition. Presently, the river braidbelt is constrained within this flood-generated paleovalley, reflecting the long-term influence of paleohydrological events that occurred more than 10,000 years ago on the modern course of the river. Importantly, these findings suggest that allogenic influences played a major role in the morphostratigraphic evolution of one of the worldâs largest fluvial systems.

Heat and mass transfer through disturbed soil| Multiscale experimental and modeling investigation

Wallen, Benjamin M. 15 June 2016 (has links)
<p> Landmines are one of the most prolific, human-made environmental hazards impacting the world. Although there are numerous technologies used to detect buried landmines, none enable a perfect find rate, in part, due to the heterogeneous nature of the environment in which they are buried. Variations in environmental conditions such as soil moisture and climate (e.g., temperature, diurnal fluctuations) impact detection performance. Improved understanding of the environmental conditions associated with minefield emplacement is needed to enable improvement in the algorithms used by detection technologies (e.g., infrared, ground penetrating radar), thus increasing their performance and probability of detection rates. However, there is a lack of understanding of the effect of the mine placement on the heat and mass transfer dynamics in the vicinity of the mine. More specifically, very little is known about how soil disturbance, a process that changes the soil thermal and hydraulic properties of the soil surrounding the mine, due to the placement and burial of the mine effects the soil moisture and temperature conditions in the vicinity of the mine. This is important because understanding these impacts enables increased ability to compare progressively complex models to measured aspects of interest specific to landmine emplacement conditions. The purpose of this research is to better understand the effect of soil disturbance (i.e., loosening the soil) and mixing (i.e., combining different soil types) on heat and mass transfer behavior in the vicinity of buried landmines. The aim is that this knowledge can help future research efforts to improve algorithms associated with various detection technologies. This research integrates a field experiment and numerous laboratory experiments with analytical modeling. In the first task, the thermal conductivity of mixed sands are evaluated at the small scale, providing critical knowledge of the unique behavior. Results indicate that for the test sands studied, knowledge of soil density enables identification of both saturated and dry thermal conductivity which enhances modeling of the thermal conductivity-saturation relationships. Experimental data were used to test thermal conductivity-saturation models. The analytical models varied in their ability to capture the thermal behavior, demonstrating the need for a physically based thermal conductivity-saturation model. The second task compares several approaches used to determine evaporation with several laboratory evaporation and evapotranspiration experiments in an effort to determine an appropriate method that can be applied to studies of landmine detection, specifically, disturbed soil conditions. Results demonstrate that the methods vary in their ability to capture atmospheric versus diffusion dominated evaporative stages for the test soils and boundary conditions studied. Although no one method is applicable for all boundary and initial conditions, the sensible heat balance and heat pulse method enabled the highest level of agreement between measured and modeled evaporation from bare soil experiments. Additionally, the ability of this method to isolate evaporation under evapotranspiration conditions has the potential to isolate transpiration which is significant for many agricultural applications as well as modeling efforts. The third task investigates the impact of soil disturbance and mixing on heat and mass transfer behavior under varying climate conditions at the laboratory scale. Using the methods established in Task 2, I could quantitatively understand the evaporation rates from soils under different conditions (e.g. disturbed or loose conditions compared to undisturbed or tight conditions) using both in-situ and remotely sensed temperature and soil moisture data. Results demonstrate that the disturbance and mixing cause a significant increase in evaporation compared to undisturbed soil conditions. Under disturbed conditions without mixing, the increase evaporation occurred in part to due capillary pumping from the loose soil into the tight soil. Additionally, higher evaporation rates were observed from the upstream tight region compared to the downstream tight region. Finally, the fourth task is a field scale proof of concept demonstration. The purpose of this task is to obtain a data set that includes aspects of tasks 1-3, thus testing our understanding of soil disturbance at the field scale. Experimental results demonstrate distinct behaviors in soil moisture and temperature distributions above and around buried objects that change with climate forcings (i.e., temperature and rain events).</p>

Diet and water source of Pleistocene Lamini camelids based on stable isotopes of tooth enamel: Implications for North American vegetation and paleoclimate

Yann, Lindsey Theresa 23 June 2014 (has links)
<p>Arid adapted taxa have evolved to live in some of the harshest environments on Earth, yet the adaptations that allowed them to transition from mesic to arid landscapes is poorly understood. Members of Camelidae (camels, vicunas, guanacos) provide a unique opportunity to study past climates as their ancestors are ubiquitous in the fossil record and all extant taxa live in arid environments. This dissertation examines Pleistocene Lamini camelids (<em>Camelops</em>, <em>Hemiauchenia</em>, <em>Palaeolama</em>); to better understand the paleoecology of the ancestors of modern South American camelids (<em>Vicugna</em>, <em>Lama</em>). To reconstruct the diet and ecology of these camelids, stable carbon (δ<sup>13</sup>C) and oxygen (δ<sup>18</sup>O) isotope values were used as a proxy for the vegetation and water consumed by herbivores. An aridity index was further developed and identified camelids as sensitive to changes in aridity. Examination of community sites in Florida suggests that warmer and drier sites had more heterogeneous environments during the Pleistocene, and likely provided the vegetation needed to support closely related taxa. In response to warmer temperatures and more heterogeneous environments, <em>Hemiauchenia</em>, <em>Platygonus</em>, and <em>Mylohyus </em>modified their dietary niches, but δ<sup>13</sup>C values suggest that <em>Equus</em>, <em>Mammut</em>, <em>Palaeolama</em>, and <em>Tapirus</em> were dietary specialist. The integration of δ<sup>13</sup>C and δ<sup>18</sup>O values indicates that <em>Palaeolama</em> was a specialized forest browser that did not modify its dietary niche in response to environmental changes or changes in the faunal composition of past ecosystems. This interpretation is further supported by its frequent co-occurrence with forest browsing <em>Tapirus</em> and <em>Odocoileus</em>.  <em>Hemiauchenia</em> was a true dietary generalist that could modify its diet in response to environmental changes and/or the presence of either <em>Palaeolama </em>or <em>Camelops</em>. Potential consumption of C<sub>4</sub> saltbush suggests <em>Camelops</em> was an opportunistic browser that may have taken advantage of its large body size and varied browsing diet to co-occur with <em>Palaeolama</em> or <em>Hemiauchenia</em>. Co-occurrence with a broader range of taxa further suggests that <em>Hemiauchenia</em> and <em>Camelops</em> lived in a range of habitats. This work further clarifies the isotopic ecology of three Pleistocene Lamini camelids, and suggests the opportunistic behavior of <em>Hemiauchenia</em> and its descendants potentially allowed for the adaptation to harsh, arid environments. </p>

Pharmaceuticals and personal care products in an effluent-dominated stream| Seasonal variability and downstream fate

Buswell, Bradley R. 25 April 2017 (has links)
<p>Wastewater treatment plant (WWTP) effluents are major sources of pharmaceuticals and personal care products (PPCPs) in the environment and effluent-dominated streams (EDSs) represent worst-case scenarios for PPCP exposures to aquatic organisms. The concentrations of PPCPs downstream from a WWTP can be altered by dilution and fate processes such as biodegradation, photodegradation and sorption. The relative importance of these processes depends on the individual PPCPs and environmental variables that vary seasonally. The primary objective of this study was to determine the concentrations of selected PPCPs in an EDS as a function of season and distance from a WWTP with the hypothesis being that the downstream attenuation of the PPCPs would vary based on their corresponding physicochemical properties. A secondary objective was to evaluate the ability of the constructed wetlands located between the plant and creek to reduce PPCP concentrations. Samples were collected seasonally from above and below the East Canyon Water Reclamation Facility (ECWRF) and within the constructed wetlands for selected PPCPs. Except for caffeine, downstream PPCP concentrations were higher than upstream, indicating that the ECWRF effluent is the major source of PPCPs in East Canyon Creek. Generally, the highest PPCP concentrations in the stream were observed in July and the lowest in May corresponding to the times of lowest and highest ratio of stream to effluent flows, respectively. Dilution was the major factor associated with the declining PPCP concentrations downstream of the ECWRF but the extent of decline varied between compounds suggesting other fate mechanisms also play a role. Sorption of PPCPs to wetland sediments was greater than stream sediments but overall the retention time within the wetlands was too short to significantly reduce the amount of PPCPs moving into the stream. The observed concentrations of individual PPCPs in East Canyon Creek were lower than those expected to negatively impact the health of aquatic organisms but mixture effects are still a potential concern.

Experimental Analysis of Bed Load Sediment Motions Using High-Speed Imagery in Support of Statistical Mechanics Theory

Fathel, Siobhan 11 November 2016 (has links)
Bed load sediment particles move as complex motions over the surface of a stream bed, accelerating and decelerating in response to the near-bed turbulence and due to particle-bed interactions. Detailed measurements of individual sand grains moving on a streambed allow us to obtain a deeper understanding of the characteristics of particle motions and evaluate spatial and temporal properties of particle diffusion, entrainment and disentrainment. I track bed load particle motions, measured start-to-stop, from high-speed (250 Hz) imaging of uniform, coarse-grained sand from two flume experiments, which have different mean fluid velocities near the bed. This work utilizes rich data sets to provide foundational support for a statistical mechanics approach to bed load transport. First, using these data, we characterize the underlying ensemble distributions of key measures of particle motions (particle velocities, accelerations, hop distances, and travel times). These distributions best represent the probabilistically expected behavior of sediment motions consistent with the macroscopic sediment and flow conditions, and thus provide a clear target for further analyses of transport, including statistical mechanics theory and numerical simulations. We then investigate the diffusive contribution to the sediment flux to demonstrate that conventional measures of particle spreading reveal different attributes of bed load particle behavior depending on details of the calculation. Our results indicate that while there are similarities between bed load transport and Brownian systems, care is needed in suggesting anomalous behavior when appealing to conventional measures of diffusion formulated for ideal particle systems. Finally, we analyze patterns and controls on sediment entrainment and disentrainment. Here we focus on entrainment and disentrainment in relation to near-bed fluid velocity measurements. This suggests that the connection between the fluid and particle entrainment is complex and will require a clearer understanding of the factors that jointly influence entrainment at the particle scale. Furthermore, this highlights and reveals important aspects of the mechanical behavior of particles during entrainment and disentrainment, and ultimately shows that these processes involve more than 'starting' and 'stopping'.

The Effects of Intermittent Drinking Water Supply in Arraijan, Panama

Erickson, John Joseph 02 February 2017 (has links)
<p>Over three hundred million people throughout the world receive supply from piped drinking water distribution networks that operate intermittently. This dissertation evaluates the effects of intermittent supply on water quality, pipe damage and service reliability in four study zones (one continuous and three intermittent) in a peri-urban drinking water distribution network in Arraijan, Panama. Normal water quality in all zones was good, with 97% of routine water quality grab samples from the distribution system and household taps having turbidity < 1 NTU, total coliform and E. coli bacteria concentrations < 1 MPN / 100 mL, and ? 0.3 mg/L free chlorine residual. However, negative pressures that represent a risk for contaminant intrusion and backflow were detected in three of the four study zones, and water quality during the first flush when supply resumed after an outage was sometimes degraded. High and transient pressures that could cause pipe damage were detected in study zones with intermittent pumping, but filling and emptying of distribution pipes due to intermittent supply was not associated with transient or extreme pressures. Operational challenges, including frequent infrastructure failures, difficulty monitoring the network, and a lack of system information, resulted in unreliable supply in the intermittent zones. Continuous pressure and flow monitoring methods used in this research could be helpful tools for operators of intermittent distribution networks to provide more reliable service and identify hydraulic conditions that could lead to contaminant intrusion or pipe breaks.

Role of Hydrological Process Representation on Erosion, Deposition, and Sediment Yield Estimate

Zi, Tan January 2016 (has links)
<p>Soil erosion by water is a major driven force causing land degradation. Laboratory experiments, on-site field study, and suspended sediments measurements were major fundamental approaches to study the mechanisms of soil water erosion and to quantify the erosive losses during rain events. The experimental research faces the challenge to extent the result to a wider spatial scale. Soil water erosion modeling provides possible solutions for scaling problems in erosion research, and is of principal importance to better understanding the governing processes of water erosion. However, soil water erosion models were considered to have limited value in practice. Uncertainties in hydrological simulations are among the reasons that hindering the development of water erosion model. Hydrological models gained substantial improvement recently and several water erosion models took advantages of the improvement of hydrological models. It is crucial to know the impact of changes in hydrological processes modeling on soil erosion simulation. </p><p>This dissertation work first created an erosion modeling tool (GEOtopSed) that takes advantage of the comprehensive hydrological model (GEOtop). The newly created tool was then tested and evaluated at an experimental watershed. The GEOtopSed model showed its ability to estimate multi-year soil erosion rate with varied hydrological conditions. To investigate the impact of different hydrological representations on soil erosion simulation, a 11-year simulation experiment was conducted for six models with varied configurations. The results were compared at varied temporal and spatial scales to highlight the roles of hydrological feedbacks on erosion. Models with simplified hydrological representations showed agreement with GEOtopSed model on long temporal scale (longer than annual). This result led to an investigation for erosion simulation at different rainfall regimes to check whether models with different hydrological representations have agreement on the soil water erosion responses to the changing climate. Multi-year ensemble simulations with different extreme precipitation scenarios were conducted at seven climate regions. The differences in erosion simulation results showed the influences of hydrological feedbacks which cannot be seen by purely rainfall erosivity method.</p> / Dissertation

Page generated in 0.1079 seconds