Assessing the joint impact of DNAPL source-zone behavior and degradation products on the probabilistic characterization of human health risk

The release of industrial contaminants into the subsurface has led to a rapid degradation of groundwater resources. Contamination caused by Dense Non-Aqueous Phase Liquids (DNAPLs) is particularly severe owing to their limited solubility, slow dissolution and in many cases high toxicity. A greater insight into how the DNAPL source zone behavior and the contaminant release towards the aquifer impact human health risk is crucial for an appropriate risk management. Risk analysis is further complicated by the uncertainty in aquifer properties and contaminant conditions. This study focuses on the impact of the DNAPL release mode on the human health risk propagation along the aquifer under uncertain conditions. Contaminant concentrations released from the source zone are described using a screening approach with a set of parameters representing several scenarios of DNAPL architecture. The uncertainty in the hydraulic properties is systematically accounted for by high-resolution Monte Carlo simulations. We simulate the release and the transport of the chlorinated solvent perchloroethylene and its carcinogenic degradation products in randomly heterogeneous porous media. The human health risk posed by the chemical mixture of these contaminants is characterized by the low-order statistics and the probability density function of common risk metrics. We show that the zone of high risk (hot spot) is independent of the DNAPL mass release mode, and that the risk amplitude is mostly controlled by heterogeneities and by the source zone architecture. The risk is lower and less uncertain when the source zone is formed mostly by ganglia than by pools. We also illustrate how the source zone efficiency (intensity of the water flux crossing the source zone) affects the risk posed by an exposure to the chemical mixture. Results display that high source zone efficiencies are counter-intuitively beneficial, decreasing the risk because of a reduction in the time available for the production of the highly toxic subspecies.Peer ReviewedPostprint (author's final draft

The responsibilities of the board of directors in promoting the principles of corporate governance

Includes bibliographical references.Corporate governance is an important aspect of the way in which companies do business. It regulates the conduct of those in control of the corporation. An important aspect of corporate governance is the establishment of structures and processes that enable directors to discharge their legal responsibilities. The global financial crisis raised many corporate governance issues and this led to many reviews and changes to corporate governance systems worldwide. This is the main reason for this research. This dissertation will focus on these reviews and changes in the corporate governance systems of South Africa and the United Kingdom. The recent collapses of high profile companies, for example, Enron and WorldCom in America, Parmalat in Italy and Masterbond, Saambou and Fidentia in South Africa have resulted in the actions, skill and diligence of directors to again come under strict inspection. Companies and especially directors have to realise that they do not act independently but that their actions and decisions impact on the societies and environment in which they operate. This is the main problem in this dissertation. The new Companies Act 71 of 2008 incorporates into statute for the first time issues of corporate governance. Company law sets the framework in which the company operates and the recommended practices set out in the King Report on Governance for South Africa 2009 (‘the King III Report’) and the King Code of Governance for South Africa 2009 (‘the Code’) provide guidance for directors as to how they should direct the business of the company and make decisions on behalf of the company. In this sense, the Companies Act of 2008 and the King III Report and the Code complement each other. The King III Report and the Code deal broadly with the responsibilities of the board of directors. The Report confirms that it is for the board of directors to act as the focal point and custodian of corporate governance. This dissertation is in essence an examination of the responsibilities of the board of directors to promote the principles of corporate governance and recommendations in terms of the King III Report and the Code

Toward efficiency in heterogeneous multispecies reactive transport modeling: A particle-tracking solution for first-order network reactions

Modeling multispecies reactive transport in natural systems with strong heterogeneities and complex biochemical reactions is a major challenge for assessing groundwater polluted sites with organic and inorganic contaminants. A large variety of these contaminants react according to serial-parallel reaction networks commonly simplified by a combination of first-order kinetic reactions. In this context, a random-walk particle tracking method is presented. This method is capable of efficiently simulating the motion of particles affected by first-order network reactions in three-dimensional systems, which are represented by spatially variable physical and biochemical coefficients described at high resolution. The approach is based on the development of transition probabilities that describe the likelihood that particles belonging to a given species and location at a given time will be transformed into and moved to another species and location afterward. These probabilities are derived from the solution matrix of the spatial moments governing equations. The method is fully coupled with reactions, free of numerical dispersion and overcomes the inherent numerical problems stemming from the incorporation of heterogeneities to reactive transport codes. In doing this, we demonstrate that the motion of particles follows a standard random walk with time-dependent effective retardation and dispersion parameters that depend on the initial and final chemical state of the particle. The behavior of effective parameters develops as a result of differential retardation effects among species. Moreover, explicit analytic solutions of the transition probability matrix and related particle motions are provided for serial reactions. An example of the effect of heterogeneity on the dechlorination of organic solvents in a threedimensional random porous media shows that the power-law behavior typically observed in conservative tracers breakthrough curves can be largely compromised by the effect of biochemical reactions.Postprint (published version

Control of VEGF-A transcriptional programs by pausing and genomic compartmentalization.

Vascular endothelial growth factor A (VEGF-A) is a master regulator of angiogenesis, vascular development and function. In this study we investigated the transcriptional regulation of VEGF-A-responsive genes in primary human aortic endothelial cells (HAECs) and human umbilical vein endothelial cells (HUVECs) using genome-wide global run-on sequencing (GRO-Seq). We demonstrate that half of VEGF-A-regulated gene promoters are characterized by a transcriptionally competent paused RNA polymerase II (Pol II). We show that transition into productive elongation is a major mechanism of gene activation of virtually all VEGF-regulated genes, whereas only ∼40% of the genes are induced at the level of initiation. In addition, we report a comprehensive chromatin interaction map generated in HUVECs using tethered conformation capture (TCC) and characterize chromatin interactions in relation to transcriptional activity. We demonstrate that sites of active transcription are more likely to engage in chromatin looping and cell type-specific transcriptional activity reflects the boundaries of chromatin interactions. Furthermore, we identify large chromatin compartments with a tendency to be coordinately transcribed upon VEGF-A stimulation. We provide evidence that these compartments are enriched for clusters of regulatory regions such as super-enhancers and for disease-associated single nucleotide polymorphisms (SNPs). Collectively, these findings provide new insights into mechanisms behind VEGF-A-regulated transcriptional programs in endothelial cells

Risk managment of complex aquifers contaminated by chemical mixtures : numerical tools and human health risk assessment

Human impact on groundwater resources has led to a rapid growth of social concerns worldwide owing to an increasing presence of toxic chemicals released in the subsurface. Risk assessment provides the scientific tool needed to quantify the actual thread that these potential hazards pose to human health. Specifically, risk analysis enables decision makers to answer: What can happen? How likely is it to happen? What can be the consequences? Risk assessment is in this context essential. However, modeling efforts involve in risk analysis are still facing several problems. Among them, in some cases, degradation products can constitute new noxious chemical compounds not necessarily less toxic than their parent product. Thus, the original pollutants and their daughter products are susceptible to co-exist in the aquifer forming a hazardous chemical mixture composed of products of different toxicity. This renders the quantification and interpretation of human health risk a non-trivial and challenging task. Also, the lack of information in the hydraulic and biochemical properties renders transport predictions to be highly uncertain. Stochastic human health risk assessment incorporates hydrogeological uncertainty in human health predictions. This way, probabilistic risk models can be used to determine the likelihood of risk exceeding a given regulatory threshold value or the expected threat to the exposed population and its uncertainty. Unfortunately, these approaches are very computationally demanding. Moreover, the diverse mineralogical composition of a real soil and the complex spatial variability of aquifer properties can produce a mixture of rates of mass transfer between regions of mobile and immobile contaminants. Finally, risk predictions are typically challenged by the complexity of the source zone condition. Existing reactive transport models based on Eulerian methods still undergo computational burden and numerical problems when modeling strong hydro-biochemical heterogeneities with complex reactions in multi-porosity systems. In this context, Particle Tracking Methods constitute a feasible alternative but these methods are limited in the range of applicability. The work presented in this thesis proposes an efficient particle tracking solution capable to simulate serial-parallel degradation reactions in multiple porosity systems with rate-limited mass transfer and strong heterogeneities. The method is then used to characterize the human health risk posed by chemical mixtures in highly heterogeneous porous media under complex source zone conditions. In particular, we investigate the interaction between aquifer heterogeneity, connectivity, contaminant injection mode and chemical toxicity in the probabilistic characterization of health risk. We illustrate how chemical-specific travel times control the regime of the expected risk and its corresponding uncertainties. Results indicate conditions where preferential flow paths can favor the reduction of the overall risk of the chemical mixture. The overall human risk response to aquifer connectivity is shown to be non-trivial for multi-species transport. This non-triviality is a result of the interaction between aquifer heterogeneity and chemical toxicity. To quantify the joint effect of connectivity and toxicity in health risk, we propose a toxicity-based Damköhler number. Results also show that the degradation capacity of immobile water regions and the mass depletion model can play a significant role on the spatiotemporal behavior of the contaminant mixture. Our work furthermore highlights the potential impact of the water flux passing through the source zone on the effective increased lifetime cancer risk due to a reactive chemical mixture. Counter-intuitively, the source zone efficiency is shown to have a beneficial effect on the risk. The total risk tends indeed to decrease for high source zone efficiency due to the consequential decrease in travel times near the source zone.El impacto humano sobre los recursos hídricos que forman los acuíferos es actualmente una de las grandes preocupaciones sociales en crecimiento debido a la presencia antrópica cada vez mayor de productos químicos tóxicos liberados en el subsuelo. El análisis de riesgo proporciona la herramienta científica necesaria para cuantificar el peligro real que estos contaminantes suponen para la salud humana. En concreto, el análisis de riesgo permite tomar decisiones que respondan a las siguientes preguntas: Qué puede pasar?. Qué tan probable es que suceda? Cuál pueden ser las consecuencias?. El análisis de riesgo es una herramienta clave en este sentido. Sin embargo, los esfuerzos de modelación necesarios para llevar a cabo el análisis de riesgo se enfrentan con varios problemas. Entre ellos, algunos productos tóxicos de degradación pueden constituir nuevos compuestos químicos nocivos no necesariamente menos tóxico que su producto padre. Por lo tanto, los contaminantes originales y sus productos hijos son susceptibles de coexistir en los acuíferos formando una mezcla de compuestos químicos de diferente toxicidad. Esto hace que la cuantificación e interpretación del riesgo para la salud humana sea una tarea no trivial y desafiante. Por otra parte, la falta de informaci´on en las propiedades hidráulicas y bioquímicos hace que las predicciones sobre el comportamiento de dichos contaminantes en el subsuelo sean altamente inciertas. El análisis de riesgo estocástico incorpora de forma natural la incertidumbre hidrogeológica que existe en las predicciones de riesgo para la salud humana. De esta manera, estos modelos pueden ser utilizados para determinar la probabilidad de que el riesgo supere un valor umbral o el valor esperado del riesgo y su incertidumbre. Desafortunadamente, estos enfoques son muy exigentes en tiempo de cálculo. Además de estas dos problemáticas, también se tiene que tener en cuenta que la composición mineralógica de un suelo real es diversa y variable en el espacio. Muchas veces esto implica la transferencia de masa entre zonas de contaminantes móviles e inmóviles. Esto último exige modelos sofisticados de transporte que, por ejemplo, conceptualicen el medio poroso como un sistema multi-porosidad. Finalmente, la complejidad que existe en el comportamiento del foco de contaminación hace complicado un análisis de riesgo. Los líquidos tóxicos densos y no acuosos ilustran perfectamente esta complejidad. Una vez en el subsuelo, estos líquidos liberación lentamente los contaminantes dentro del acuífero de acuerdo con una tasa de agotamiento que depende fuertemente de la arquitectura errática del foco de contaminación. Los modelos de transporte reactivo eulerianos tienen problemas numéricos cuando se simulan fuertes heterogeneidades hidro-bioquímicos en el terreno al mismo tiempo que reacciones químicas complejas en sistemas multi-porosidad. En este contexto, los métodos de trayectorias de partículas constituyen una alternativa viable. Sin embargo, estos métodos pueden tener en cuenta un rango pequeño de reacciones químicas. El trabajo presentado en esta tesis propone una solución a estos problemas mediante un método de trayectoria de partículas. El método es eficiente y capaz de simular cadenas y redes de degradación en sistemas heterogéneos con porosidad múltiples. El método se basa en el desarrollo de probabilidades de transición que describen las probabilidades de que las partículas que pertenecen a un estado determinado (producto químico y región móvil / inmóvil) en un momento dado se transformará en otro estado en un momento posterior. El método se utiliza para caracterizar el riesgo para la salud humana que representan las mezclas de degradación en medios porosos altamente heterogéneos derivados de focos de contaminación complejos. En particular, se investiga la interacción entre la heterogeneidad, la conectividad, el modo de inyección de los contaminantes y su toxicidad química con respecto a la caracterización probabilística del riesgo para la salud humana. Los resultados indican las condiciones mediante las cuales las vías de flujo preferencial pueden favorecer la reducción del riesgo para la salud humana. La dependencia de la conectividad con el riego se demuestra que no es nada trivial cuando se trata de mezclas de compuestos químicos. Esta no trivialidad es el resultado de la interacción entre la heterogeneidad del acuífero y la toxicidad de los compuestos químicos. Para cuantificar el efecto conjunto de la conectividad y la toxicidad en el riesgo para la salud, se propone un número de Damköhler nuevo que tiene en cuenta la toxicidad. Además, el riesgo también se caracteriza en términos estadísticos mediante momentos de bajo orden y funciones de densidad de probabilidad. Los resultados también muestran que tanto la capacidad de degradación de zonas inmóviles como los modelos existentes de agotamiento del foco pueden desempeñar un papel muy significativo en el análisis espacio-temporal del riesgo. Este trabajo también muestra que la eficiencia del foco de contaminación para concentrar el flujo puede tener un efecto beneficioso sobre el riesgo. El riesgo total de hecho tiende a disminuir para eficiencias grandes debido a la disminución consecuente en tiempos de viaje cerca del foco de contaminación, limitando la producción de productos de degradación más tóxicos

Elimination of the reaction rate 'scale effect': application of the Lagrangian reactive particle-tracking method to simulate mixing-limited, field-scale biodegradation at the Schoolcraft (MI, USA) site

This is the peer reviewed version of the following article: [Ding, D., Benson, D. A., Fernàndez‐Garcia, D., Henri, C. V., Hyndman, D. W., Phanikumar, M. S., & Bolster, D. (2017). Elimination of the reaction rate “scale effect”: Application of the Lagrangian reactive particle‐tracking method to simulate mixing‐limited, field‐scale biodegradation at the Schoolcraft (MI, USA) site. Water Resources Research, 53, 10,411–10,432. https://doi.org/10.1002/2017WR021103], which has been published in final form at https://doi.org/10.1002/2017WR021103. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.Measured (or empirically fitted) reaction rates at groundwater remediation sites are typically much lower than those found in the same material at the batch or laboratory scale. The reduced rates are commonly attributed to poorer mixing at the larger scales. A variety of methods have been proposed to account for this scaling effect in reactive transport. In this study, we use the Lagrangian particle-tracking and reaction (PTR) method to simulate a field bioremediation experiment at the Schoolcraft, MI site. A denitrifying bacterium, Pseudomonas Stutzeri strain KC (KC), was injected to the aquifer, along with sufficient substrate, to degrade the contaminant, carbon tetrachloride (CT), under anaerobic conditions. The PTR method simulates chemical reactions through probabilistic rules of particle collisions, interactions, and transformations to address the scale effect (lower apparent reaction rates for each level of upscaling, from batch to column to field scale). In contrast to a prior Eulerian reaction model, the PTR method is able to match the field-scale experiment using the rate coefficients obtained from batch experiments.Peer ReviewedPostprint (author's final draft

Spontaneous emission enhancement at a photonic wire miniband edge

International audienceIn a multimode photonic-crystal waveguide, we observe strong enhancement of the photoluminescence of embedded quantum dots at the edges of the so-called mini-stopband that were opened by Bragg diffraction between two guided modes. Taking into account light collection, we relate this observation to the singular photon density of states that is characteristic of a one-dimensional photon system. Furthermore, we quantify by how much the radiation losses smooth the divergence. For the first time to our knowledge, a clear account of the control of spontaneous emission in a one-dimensional system is thus demonstrated

Synthèse de polyuréthanes par organo-catalyse dans le dioxyde de carbone supercritique

La synthèse de particules polyuréthane par organo-catalyse dans le dioxyde de carbone supercritique a été étudiée. Des réactions modèles ont été préalablement conduites et suivies par spectroscopie infrarouge in situ dans le CO2 supercritique afin d'identifier les catalyseurs organiques de réaction d'uréthanisation les plus efficients. Une série de polymères siliconés CO2-phile, porteurs de la fonction organo-catalytique en bout de chaîne (organo-catasurfs), a ensuite été préparée et testée dans le CO2 supercritique pour la polymérisation en dispersion de polyuréthane.The organocatalysed synthesis of polyurethane particles in supercritical carbon dioxide has been studied. Model reactions were first carried out in supercritical CO2 and monitored by in situ infrared spectroscopy in order to indentify the most efficient catalysts for the urethanisation reaction. A series of CO2-philic silicone polymers, end-functionalised with the organocatalytic group (organocatasurfs), were then synthesised and tested in supercritical CO2 for the dispersion polymerisation of polyurethane.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF