The contribution of phytoplankton productivity to higher trophic levels in salt marshes is not well understood. My study furthers our understanding of possible mechanisms controlling phytoplankton productivity, abundance, and community composition in salt marshes. Across three consecutive springs (2001 to 2003), I sampled the upper Nueces Delta in south Texas, a shallow, turbid, salt marsh system stressed by low freshwater inflow and wide ranging salinity (<15 to >300 ppt). Water column productivity and respiration were estimated using a light-dark bottle technique, and phytoplankton biovolume and community composition were determined using inverted light microscopy. To determine their effect on the phytoplankton community, zooplankton and bacterioplankton abundance and several physical parameters were also assessed. Meaningful relationships among the numerous variables evaluated in this study were identified using principal component analysis (PCA). Despite high turbidity, phytoplankton productivity and biovolume were substantial. Resuspension appeared to play a major role in phytoplankton dynamics, as indicated by a positive relationship between ash weight and biovolume that explained up to 46% of the variation in the PCA. Negative relationships between zooplankton grazers and pennate diatoms of optimal sizes for these grazers suggested a functional grazing food chain in this system. Salinity also may have been important in phytoplankton dynamics, whereas nutrients appeared to play a minor role. Salinity increases may have been responsible for a decoupling observed between phytoplankton and grazers during late spring. Findings suggest hypotheses for future studies focused on the role of phytoplankton in salt marshes, particularly those stressed by reduced freshwater inflow and high salinities.

The contribution of phytoplankton productivity to higher trophic levels in salt marshes is not well understood. My study furthers our understanding of possible mechanisms controlling phytoplankton productivity, abundance, and community composition in salt marshes. Across three consecutive springs (2001 to 2003), I sampled the upper Nueces Delta in south Texas, a shallow, turbid, salt marsh system stressed by low freshwater inflow and wide ranging salinity (<15 to >300 ppt). Water column productivity and respiration were estimated using a light-dark bottle technique, and phytoplankton biovolume and community composition were determined using inverted light microscopy. To determine their effect on the phytoplankton community, zooplankton and bacterioplankton abundance and several physical parameters were also assessed. Meaningful relationships among the numerous variables evaluated in this study were identified using principal component analysis (PCA). Despite high turbidity, phytoplankton productivity and biovolume were substantial. Resuspension appeared to play a major role in phytoplankton dynamics, as indicated by a positive relationship between ash weight and biovolume that explained up to 46% of the variation in the PCA. Negative relationships between zooplankton grazers and pennate diatoms of optimal sizes for these grazers suggested a functional grazing food chain in this system. Salinity also may have been important in phytoplankton dynamics, whereas nutrients appeared to play a minor role. Salinity increases may have been responsible for a decoupling observed between phytoplankton and grazers during late spring. Findings suggest hypotheses for future studies focused on the role of phytoplankton in salt marshes, particularly those stressed by reduced freshwater inflow and high salinities.

Die CFD-Modellierung des Fasertransports wurde auf der Basis des Euler/Euler-Ansatzes untersucht. Die Nachbildung des Transportverhaltens der Partikel erfordert die korrekte Beschreibung des Impulsaustausches. Hierfür liefert die Auswertung der Sinkgeschwindigkeiten aus den Experimenten am Versuchsstand "Säule" die notwendigen Informationen. Über die Vorgabe von Partikeldichte und Partikeldurchmesser kann die Sinkgeschwindigkeit nachgebildet und der Partikeltransport adäquat modelliert werden. Beispielsimulationen einer partikelbeladenen Strömung im Versuchsstand "Ringkanal" lieferten plausible Resultate. Das Anlagerungs- bzw. Abtragungsverhalten der Partikel an Hindernissen (z. B. Wehren) wurde über die Modellierung einer vom Feststoffanteil abhängigen Viskosität simuliert. Dieser Zusammenhang wurde an die Ergebnisse der Experimente im Ringkanal angepasst. Erste Demonstrationsrechnungen zur Beschreibung des Partikeltransportes an Hindernissen ergaben plausible Resultate. In anderen Beispielen zeigte die Parametervariation einer feststoffabhängigen Viskosität jedoch einen zu geringen Einfluss auf die berechneten Ablagerungsmengen, um die experimentellen Befunde zu reproduzieren. Es wurde ein Modell des Differenzdruckaufbaus aus den Kompaktierungseigenschaften der auf dem Sieb abgelagerten Fasern entwickelt und an Experimenten justiert. Damit steht neben den in der Literatur veröffentlichten Korrelationen ein weiteres Verfahren zur Berechnung des Differenzdrucks zur Verfügung. In einer Machbarkeitsstudie wurde die Wirkung eines Siebes auf das Strömungsfeld unter Nutzung des Modellansatzes des porösen Körpers in CFX-4.4 implementiert. Das implementierte Modell liefert qualitativ plausible Resultate.

Contaminated sediments are periodically subjected to resuspension processes during either storm events or due to dredging. In sediments, metals are often contained in insoluble low bioavailability forms. Upon resuspension, however, biogeochemical processes associated with the exposure to more oxic conditions may lead to transformation and release of the metals, giving rise to exposure and risk in the water column. Batch experiments suggested that oxidation of reduced species and corresponding pH decrease were the most importance processes controlling metals release upon sediment resuspension. A mathematical model was implemented to better understand the complex underlying biogeochemical reactions that affect metals release. The model described the metals dynamics and other inter-related important biogeochemical factors well and was successful at predicting the metals release from different sediment reported in the literature. Tidal and other cyclic variations in oxygen, pH and other relevant parameters in the overlying water may also lead to cyclic transformations and release of metals from surficial sediments. In simulated estuarine microcosm experiments, cyclic variations in pH and salinity due to freshwater/saltwater exchange were shown to lead to cyclic variations in metal release. Both pH and salinity were important factors controlling interstitial dissolved metals concentrations, however, in terms of freely dissolved metals concentrations, which have been considered to be more related with toxicity and bioavailability, pH was the single most important parameter. The mathematical model was extended to the conditions of the cyclic behavior in an estuary and successfully described metals release under such conditions. It is believed that the model can be used to predict the metals behavior in other sediments and conditions by model calibration with a similar experimental approach to that used in this study. / text

L’objectif de cette étude est de bien décrire les mécanismes impliqués dans la remise en suspension des microparticules en conduite ventilée. Une méthodologie expérimentale est utilisée, et cela en tenant compte de la période d’accélération de l’écoulement qui précède l’atteinte du régime permanent. Une méthode optique a été choisi pour étudier le mouvement initial des particules, et leur cinétique de remise en suspension. Parallèlement, nous avons recueilli des données locales de l’écoulement en période d’accélération et au régime permanent.Le démarrage de la remise en suspension a été analysé relativement à l’évolution temporelle de trois paramètres pouvant influencer le démarrage de la remise en suspension, le premier est la vitesse instantanée en proche paroi, le second est l’intensité turbulente, et enfin le troisième est l’énergie cinétique turbulente. Le paramètre prépondérant sur le démarrage de la remise en suspension semble être l’énergie cinétique turbulente. / The objective of this study is to well describe the mechanisms involved in the resuspension of particles in ventilated duct by using an experimental methodology and taking into account the acceleration of the air flow which always precedes steady state. For that purpose, we chose an optical method in order to investigate the initial movement of particles, and to quantify the resuspension kinetics. In parallel we collected local data of the flow during acceleration and steady state.We observed that the resuspension kinetics starts during the acceleration period and extends to steady state. We highlighted the relevant velocity characteristics (critical velocity at the center duct and close to the wall, critical kinetic energy range) to explain this phenomenon. The resuspension start seems to be linked with a critical kinetic energy range.

Surface-bound particulates containing radionuclides in the environment can become airborne through the process of resuspension. Once airborne, these radionuclides can be inhaled or ingested to deliver an internal dose of ionizing radiation. To that end, the resuspension factor method is a powerful tool for predicting a person's exposure to airborne particles from surface contaminations, and therefore is used to determine protective and intervening measures. The resuspension factor is calculated as the ratio measured airborne to surface mass concentration and has been found to generally decrease exponentially with time. Current models of the resuspension factor are empirical and have failed to predict recent measurement, motivating a stronger basis and physical model for the system. Additionally, federal guidances conservatively suggest an unphysical model of particulate radioactivity impact wherein the entirety of the radiation is absorped. For this dissertation, two- and three-compartment catenary models were derived which build on measured resuspension rate constants under various influences. These models were fit to a set of historic observations of resuspension factors using an instrumental uncertainty-weighting to resolve the large variances early in time which otherwise inflate calculations. When compared to previous resuspension models, our physical models better fit the data achieving reduced-chi-squared closer to 1. An experiment was undertaken to validate our basic environment resuspension models in an urban environment without wind. A resuspension chamber is constructed by placing an acrylic tube atop a poured concrete surface and lowering a low-volume air sampler head from above. Europium oxide powder was dispersed upon the surface or from above the air sampling height to emulate ideal compartmentalized release scenarios, and air is sampled on an hourly, daily, or weekly basis. Sampler filters then were evaluated for Europium content using neutron activation and gamma spectroscopy. Hourly measurements following airborne release are within an order of magnitude of early-timeframe historic resuspension factors (~10^−6 m^−1), whereas daily and weekly measurements from surface release demonstrate a gradual decrease in resuspension factor (∼10^−8 m^−1). These results support a need to critically assess the resuspension factor definition and its relationship to "initial suspension" and the indoor background, non-anthropogenic resuspension. Finally, a simulated model was generated to demonstrate loss of alpha radiation from relevant transuranic radioparticles. This was accomplished using the Geant4 Monte Carlo particle transport code. This basic model demonstrated a clear loss of average intensity and energy of exiting particles which are both directly related to the absorped dose. The data shows a loss from 10 to 90% of intensity to occur at particle sizes approaching the range of alphas within them, and a loss of roughly half the initial alpha energy at around the same particle sizes. The results establish a first-order baseline for a particulate self-absorption model which complement existing dosimetry models for inhaled radionuclides.

Die CFD-Modellierung des Fasertransports wurde auf der Basis des Euler/Euler-Ansatzes untersucht. Die Nachbildung des Transportverhaltens der Partikel erfordert die korrekte Beschreibung des Impulsaustausches. Hierfür liefert die Auswertung der Sinkgeschwindigkeiten aus den Experimenten am Versuchsstand "Säule" die notwendigen Informationen. Über die Vorgabe von Partikeldichte und Partikeldurchmesser kann die Sinkgeschwindigkeit nachgebildet und der Partikeltransport adäquat modelliert werden. Beispielsimulationen einer partikelbeladenen Strömung im Versuchsstand "Ringkanal" lieferten plausible Resultate. Das Anlagerungs- bzw. Abtragungsverhalten der Partikel an Hindernissen (z. B. Wehren) wurde über die Modellierung einer vom Feststoffanteil abhängigen Viskosität simuliert. Dieser Zusammenhang wurde an die Ergebnisse der Experimente im Ringkanal angepasst. Erste Demonstrationsrechnungen zur Beschreibung des Partikeltransportes an Hindernissen ergaben plausible Resultate. In anderen Beispielen zeigte die Parametervariation einer feststoffabhängigen Viskosität jedoch einen zu geringen Einfluss auf die berechneten Ablagerungsmengen, um die experimentellen Befunde zu reproduzieren. Es wurde ein Modell des Differenzdruckaufbaus aus den Kompaktierungseigenschaften der auf dem Sieb abgelagerten Fasern entwickelt und an Experimenten justiert. Damit steht neben den in der Literatur veröffentlichten Korrelationen ein weiteres Verfahren zur Berechnung des Differenzdrucks zur Verfügung. In einer Machbarkeitsstudie wurde die Wirkung eines Siebes auf das Strömungsfeld unter Nutzung des Modellansatzes des porösen Körpers in CFX-4.4 implementiert. Das implementierte Modell liefert qualitativ plausible Resultate.

The 2011–2012 Cordón Caulle eruption emitted about 1 km3 of rhyodacitic tephra. Dominant westerly winds in the region caused most of the primary tephra to deposit in neighboring Argentina. In addition to the impact of widespread dispersal and fallout of primary tephra during the eruption, Argentina was also significantly affected by remobilization of the primary ash even several years after the climactic phase of the eruption. In this mixed methods study, we combine aspects of natural and social sciences to characterize the ash resuspension events associated with the 2011–2012 Cordón Caulle deposits and assess the impacts on the Argentinian farming community of Ingeniero Jacobacci in the Patagonian Steppe. Our findings show the primary importance of wind, rainfall and ash availability in controlling the occurrence and persistence of ash resuspension events. The role played by these variables was also reflected in the seasonal distribution of events observed. Regarding the impacts, our results complement those of earlier studies and demonstrate that ash resuspension events can exacerbate the negative impact of primary tephra fallout events from the time of deposition to many years after the eruption. Only after five years has the environment and the farming community begun to show signs of recovery. Our findings also highlight the importance of assessing ash resuspension events in multi-hazard scenarios involving volcanic and hydrometeorologic hazards.

No description available.

No description available.