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Chemical Heterogeneity (chemical + heterogeneity)

Distribution by Scientific Domains

Life Sciences	33%

Selected Abstracts

Effect of Heterogeneity on Radionuclide Retardation in the Alluvial Aquifer Near Yucca Mountain, Nevada

GROUND WATER, Issue 3 2001
S. Painter
The U.S. Department of Energy is currently studying Yucca Mountain, Nevada, as a potential site for a geological high-level waste repository. In the current conceptual models of radionuclide transport at Yucca Mountain, part of the transport path to pumping locations would be through an alluvial aquifer. Interactions with minerals in the alluvium are expected to retard the downstream migration of radionuclides, thereby delaying arrival times and reducing ground water concentrations. We evaluate the effectiveness of the alluvial aquifer as a transport barrier using the stochastic Lagrangian framework. A transport model is developed to account for physical and chemical heterogeneities and rate-limited mass transfer between mobile and immobile zones. The latter process is caused by small-scale heterogeneity and is thought to control the macroscopic-scale retardation in some field experiments. A geostatistical model for the spatially varying sorption parameters is developed from a site-specific database created from hydrochemical measurements and a calibrated modeling approach (Turner and Pabalan 1999). Transport of neptunium is considered as an example. The results are sensitive to the rate of transfer between mobile and immobile zones, and to spatial variability in the hydraulic conductivity. Chemical heterogeneity has only a small effect, as does correlation between hydraulic conductivity and the neptunium distribution coefficient. These results illustrate how general sensitivities can be explored with modest effort within the Lagrangian framework. Such studies complement and guide the application of more detailed numerical simulations. [source]

Effect of chemical heterogeneity on adsorbed solute dispersion at column scale

AICHE JOURNAL, Issue 4 2008
Safia Semra
Abstract Chemical heterogeneity seems to be responsible for spreading increase of adsorbed solute breakthrough curves. Adsorption in fixed beds assumes chemically homogeneous media. However, this is not always true, in particular when natural sands or mixed adsorbent filters are used in drinking water purification. Neglecting eventual effect of chemical heterogeneity may engender false modeling bases. So, considering homogeneous grain size distribution, the effect of chemical heterogeneity on global dispersion in porous media has been investigated experimentally in this article at column scale. Breakthroughs of adsorbed solute showed a visible effect of chemical heterogeneity on solute global dispersion increasing. The more heterogeneous the medium, the more spread the adsorbed solute breakthrough. Reduced variance showed a linear variation with the chemical heterogeneity scale at closely constant media global capacity. A pseudo-homogeneous model has been developed to simulate experimental data by increasing dispersion parameter. © 2008 American Institute of Chemical Engineers AIChE J, 2008 [source]

Effect of Heterogeneity on Radionuclide Retardation in the Alluvial Aquifer Near Yucca Mountain, Nevada

Cadmium accumulation by invertebrates living at the sediment,water interface

ENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 4 2001
Landis Hare
Abstract Benthic animals can take up trace metals both from the sediment compartment in which they burrow and from the water column compartment above their burrows (we define both compartments as containing water and particles). If criteria for the protection of benthic animals are based on metal concentrations in one of these two compartments, then it should first be demonstrated that the majority of the metal taken up by these animals comes from the given compartment. To determine whether benthic animals take up the majority of their cadmium (Cd) from the sediment compartment, we created a Cd gradient in lake sediment and compared Cd accumulation by the invertebrates colonizing these sediments with Cd concentrations in the sediment compartment. On the basis of this relationship and using a bioaccumulation model, we estimate that indigenous benthic invertebrates take up the majority of their Cd from the water column compartment. The results of our experiment are similar to those from a previous study conducted on a different benthic community in a larger lake. Taxa common to both lakes obtained similar proportions of their Cd from the water column compartment, suggesting that Cd accumulation by the same species will be constant across lakes of differing size and chemistry. Our results strengthen the argument that the protection of benthic communities from metal pollution should consider metal in both the water column and sediment compartments. In this regard, the AVS model, which considers only sedimentary metals, was more effective in predicting Cd concentrations in pore waters than those in most animal taxa. We suggest that measurements of vertical chemical heterogeneity in sediments and of animal behavior would aid in predicting the bioaccumulation and effects of sedimentary pollutants. [source]

Cell wall hemicelluloses as mobile carbon stores in non-reproductive plant tissues

FUNCTIONAL ECOLOGY, Issue 5 2007
G. HOCH
Summary 1As essential compounds of plant cell walls, hemicelluloses account for about a quarter of all plant biomass worldwide. 2In seed cotyledons and endosperm of species from several plant families, hemicelluloses are used as mobile carbon reserves. Whether cell wall hemicelluloses of non-reproductive plant tissue are multifunctional molecules, which can also serve as carbon sources during periods of enhanced carbon demand, is still equivocal. 3This review summarizes the current understanding of a possible reserve function of hemicelluloses. Although several descriptive and experimental studies suggested at least partial mobility of cell wall polysaccharides in mature, non-reproductive plant tissues, there is still a need for a broad-scale, ecophysiological exploration of the actual nature of hemicelluloses beyond their structural function. 4The chemical heterogeneity of hemicelluloses may be the major problem for precise quantitative analyses on a large, comparative scale. 5Because of the abundant distribution of hemicelluloses in plants, the existence of a significant mobile carbohydrate pool in cell walls of non-reproductive organs would shed rather new light on plant carbon relations in a source-sink context. 6Consequently, a reserve function of hemicelluloses questions the conventional division of cell compounds into structural (i.e. immobile) and non-structural (i.e. mobile) compounds. [source]

Trends in aquatic macrophyte species turnover in Northern Ireland , which factors determine the spatial distribution of local species turnover?

GLOBAL ECOLOGY, Issue 5 2004
Einar Heegaard
ABSTRACT Aim, The study examined qualitative predictions of ecological theories in relation to the spatial distribution of species turnover of aquatic macrophytes, through the following parameters: (1) distance between lakes (2) chemical conditions of the lakes (3) chemical differences between the lakes, and (4) the lake size. Location, 562 lakes dispersed throughout Northern Ireland were analysed. Methods, To obtain species turnover estimates independent of richness, the average distance between focal lakes and their five nearest neighbours in ordination space (DCA) was standardized by the species richness in a Generalized Additive Model (GAM). The relationships between species turnover and ecological (chemical condition, chemical difference, distance between lakes, and lake-size) and geographical parameters (latitude, longitude, and altitude) were analysed using GAM. Results, The results indicate that the pattern in species turnover is a combination of the chemical conditions and the distance between the lakes, including the interaction term. The effects of chemical heterogeneity and lake size parameters were both positive but weak. In general, increased distance and decreased ionic concentration contribute to increased turnover. The influence of distance on species turnover is strongest at low and high altitude, and at mid-elevation the species turnover is mainly driven by the chemical conditions. Towards the north there is an increasing influence of distance, whereas in the south the chemical conditions have their strongest influence. Conclusions, There is a need for components from several established ecological theories to explain the spatial trends in species turnover within Northern Ireland. Central theories in this particular study are the population/metapopulation dynamics, the continuum concept, and the species-pool concept. [source]

Effect of chemical heterogeneity on adsorbed solute dispersion at column scale

Pharmacological profile of the new antihistamines

CLINICAL & EXPERIMENTAL ALLERGY REVIEWS, Issue 1 2005
J.-P. Tillement
Summary The second-generation H1 antihistamines are a highly efficacious drug class in the treatment of allergic rhinitis (AR) and dermatitis, and distinct from the first-generation H1 antihistamines, predominantly because of their non-sedating nature at recommended dosages. Despite a marked chemical heterogeneity, the non-sedating H1 antihistamines have many similarities, in particular, high affinity for H1 receptors, high efficacy, anti-inflammatory effects, which may be independent of direct H1 -receptor function, and lack of central nervous system side-effects. Some studies have suggested that differences in the chemical structures of these compounds generally lead to differences in the pharmacokinetic properties, which determine their overall clinical usefulness. In particular, it has been demonstrated that there are differences in selectivity for H1 receptors, the apparent volume of distribution, metabolism and elimination and interaction with other drugs. A comparison of levocetirizine, fexofenadine, desloratadine and mizolastine (some of the most commonly prescribed drugs in the treatment of AR and dermatitis) has demonstrated that, unlike levocetirizine and fexofenadine, desloratadine and mizolastine can bind to muscarinic receptors and cardiac K+ channels, and therefore have both lower selectivity and the potential to induce muscarinic and serious cardiac side-effects. However, this is noted at higher than recommended doses. Similarly, desloratadine and mizolastine undergo extensive metabolism and, together with fexofenadine, have the potential to interact with other drugs, in turn increasing the potential for severe toxic effects. In contrast, levocetirizine is not metabolized, is eliminated rapidly from the body, does not demonstrate any significant drug interactions and has the lowest volume of distribution. These findings suggest that levocetirizine is likely to be a safer drug than fexofenadine, desloratadine and mizolastine. [source]