Dentz M.,CSIC - Institute of Environmental Assessment And Water Research
Advances in Water Resources | Year: 2012
We study concentration statistics for tracer transport in the flow through a heterogeneous porous medium. Spatial heterogeneity and temporal variability cause fluctuations in spatial characteristics of the tracer distribution such as the center of mass velocity v(t) and the spreading rate D e(t), which leads to fluctuations in the concentration. These fluctuations can be quantified statistically by the one-point probability density function (PDF) of concentration and by its simplest characteristics, the concentration mean and variance. We focus on the impact of variability in the center of mass velocity and treat D e(t) as deterministic. The fluctuating v(t) is modeled as a Gaussian stochastic process. Based on this stochastic description, we derive explicit expressions for the concentration variance and the full one-point concentration PDF. Their behavior is mainly controlled by the ratio of the spatial variance of the tracer distribution κ e(t) in single realizations and the variance κ m(t) of the plume centroids between realizations. We explore the behavior of concentration variance and PDF for two velocity correlation models, one that leads to a diffusive increase of centroid variance κ m(t) and one that yields an asymptotically constant κ m(t). The obtained results give insight into the dynamics of concentration statistics in heterogeneous media and the mechanisms by which they are controlled. © 2011 Elsevier Ltd.
Babin P.J.,University of Bordeaux 1 |
Goizet C.,University of Bordeaux 1 |
Goizet C.,Bordeaux University Hospital Center |
Raldua D.,CSIC - Institute of Environmental Assessment And Water Research
Progress in Neurobiology | Year: 2014
Motor neuron diseases (MNDs) are an etiologically heterogeneous group of disorders of neurodegenerative origin, which result in degeneration of lower (LMNs) and/or upper motor neurons (UMNs). Neurodegenerative MNDs include pure hereditary spastic paraplegia (HSP), which involves specific degeneration of UMNs, leading to progressive spasticity of the lower limbs. In contrast, spinal muscular atrophy (SMA) involves the specific degeneration of LMNs, with symmetrical muscle weakness and atrophy. Amyotrophic lateral sclerosis (ALS), the most common adult-onset MND, is characterized by the degeneration of both UMNs and LMNs, leading to progressive muscle weakness, atrophy, and spasticity. A review of the comparative neuroanatomy of the human and zebrafish motor systems showed that, while the zebrafish was a homologous model for LMN disorders, such as SMA, it was only partially relevant in the case of UMN disorders, due to the absence of corticospinal and rubrospinal tracts in its central nervous system. Even considering the limitation of this model to fully reproduce the human UMN disorders, zebrafish offer an excellent alternative vertebrate model for the molecular and genetic dissection of MND mechanisms. Its advantages include the conservation of genome and physiological processes and applicable in vivo tools, including easy imaging, loss or gain of function methods, behavioral tests to examine changes in motor activity, and the ease of simultaneous chemical/drug testing on large numbers of animals. This facilitates the assessment of the environmental origin of MNDs, alone or in combination with genetic traits and putative modifier genes. Positive hits obtained by phenotype-based small-molecule screening using zebrafish may potentially be effective drugs for treatment of human MNDs. © 2014 The Authors.
Postigo C.,CSIC - Institute of Environmental Assessment And Water Research |
Richardson S.D.,University of South Carolina
Journal of Hazardous Materials | Year: 2014
Pharmaceuticals are emerging contaminants of concern and are widespread in the environment. While the levels of these substances in finished drinking waters are generally considered too low for human health concern, there are now concerns about their disinfection by-products (DBPs) that can form during drinking water treatment, which in some cases have been proven to be more toxic than the parent compounds. The present manuscript reviews the transformation products of pharmaceuticals generated in water during different disinfection processes, i.e. chlorination, ozonation, chloramination, chlorine dioxide, UV, and UV/hydrogen peroxide, and the main reaction pathways taking place. Most of the findings considered for this review come from controlled laboratory studies involving reactions of pharmaceuticals with these oxidants used in drinking water treatment. © 2014 Elsevier B.V.
Izquierdo M.,British Geological Survey |
Querol X.,CSIC - Institute of Environmental Assessment And Water Research
International Journal of Coal Geology | Year: 2012
Coal-based power generation produces over 750. Mt of coal ash per year globally, but under 50% of world production is utilised. Large amounts of fly ash are either stored temporarily in stockpiles, disposed of in ash landfills or lagooned. Coal ash is viewed as a major potential source of release of many environmentally sensitive elements to the environment. This paper encompasses over 90 publications on coal fly ash and demonstrates that a large number of elements are tightly bound to fly ash and may not be easily released to the environment, regardless of the nature of the ash. This review provides an extensive look at the extent to which major and trace elements are leached from coal fly ash. It also gives an insight into the factors underlying the leachability of elements and addresses the causes of the mobility. The mode of occurrence of a given element in the parent coal was found to play an important role in the leaching behaviour of fly ash. The amount of calcium in fly ash exerts a dominant influence on the pH of the ash-water system. The mobility of most elements contained in ash is markedly pH sensitive. The alkalinity of fly ash attenuates the release of a large number of elements of concern such as Cd, Co, Cu, Hg, Ni, Pb, Sn or Zn among others, but at the same time, it enhances the release of oxyanionic species such as As, B, Cr, Mo, Sb, Se, V and W. The precipitation of secondary phases such as ettringite may capture and bind several pollutants such ash As, B, Cr, Sb, Se and V. © 2011 Elsevier B.V.
Galban-Malagon C.,CSIC - Institute of Environmental Assessment And Water Research
Nature communications | Year: 2012
Semivolatile persistent organic pollutants have the potential to reach remote environments, such as the Arctic Ocean, through atmospheric transport and deposition. Here we show that this transport of polychlorinated biphenyls to the Arctic Ocean is strongly retarded by the oceanic biological pump. A simultaneous sampling of atmospheric, seawater and plankton samples was performed in July 2007 in the Greenland Current and Atlantic sector of the Arctic Ocean. The atmospheric concentrations declined during atmospheric transport over the Greenland Current with estimated half-lives of 1-4 days. These short half-lives can be explained by the high air-to-water net diffusive flux, which is similar in magnitude to the estimated settling fluxes in the water column. Therefore, the decrease of atmospheric concentrations is due to sequestration of atmospheric polychlorinated biphenyls by enhanced air-water diffusive fluxes driven by phytoplankton uptake and organic carbon settling fluxes (biological pump).