266 research outputs found
Assessment of uncertainty associated with the estimation of well catchments by moment equations
No full textNon-local stochastic moment equations are used successfully to analyze groundwater flow in randomly heterogeneous media.
Here we present a moment equations-based approach to quantify the uncertainty associated with the estimation of well catchments.
Our approach is based on the development of a complete second order formalism which allows obtaining the first statistical moments of the trajectories of conservative solute particles advected in a generally non-uniform groundwater flow. Approximate equations of moments of particles trajectories are then derived on the basis of a second order expansion in terms of the standard deviation of the aquifer log hydraulic conductivity. Analytical expressions are then obtained for the predictors of locations of mean stagnation points, together with their associated uncertainties. We implement our approach on heterogeneous media in bounded two-dimensional domains, with and without including the effect of conditioning on hydraulic conductivity information. The impact of domain size, boundary conditions, heterogeneity and non-stationarity of hydraulic conductivity on the prediction of a well catchment is explored. The results are compared against Monte Carlo simulations and semi-analytical solutions available in the literature. The methodology is applicable to both infinite and bounded domains and is free of distributional assumptions (and so applies to both Gaussian and non-Gaussian log hydraulic conductivity fields) and formally includes the effect of conditioning on available information
Impact of the choice of the variogram model on flow and travel time predictors in radial flows
No full textPrediction of hydraulic head, flux and contaminant travel time/trajectories in natural aquifers is uncertain due to the geologic media complexity and lack of information. Hence it is appropriate to cast the equations that govern groundwater flow and contaminant transport within a stochastic framework. The latter is oriented towards rendering ensemble moments of the analyzed quantities. In this view the spatial variable transmissivity is usually modeled as a Stochastic Continuum, characterized by a set of parameters (covariance shape, geometric mean, variance and correlation length). These are generally assumed to be known with certainty even though they are usually derived using a limited amount of experimental data, which are often not enough for a complete characterization.
Full-Bayesian approaches (e.g. Woodbury and Rubin 2000; Woodbury and Ulrych 2000) take into account the uncertainty in the knowledge of the variogram parameters (geometric mean, variance and correlation length). Feyen et al. (2002) illustrate an application of these methodologies to determine the uncertainty asso-ciated with the delineation of well capture zones. Hendricks Franssen et al. (2002) investigate the impact of the uncertainty of variogram parameters on the same topic using sequential Gaussian simulation (Gómez-Hernández and Journel 1993) to generate transmissivity fields and the sequential self-calibrated method for in-verse conditioning. In all these works the shape of the correlation structure of the natural logarithm of transmissivity is fixed and assumed known without uncertainty. Salandin and Rinaldo (1989) analyze the influence of the form of the log-conductivity covariance on dispersion coefficients in random permeability fields under mean uniform flow conditions.
Here, we focus on the impact of the choice of the functional form for the log-transmissivity variogram on (ensemble) moments of hydraulic head and contami-nant residence time under convergent flow conditions, such as those created by a single pumping well.
Although of high relevance in practical applications, problems associated to contaminant transport in the vicinity of extraction wells in heterogeneous media have been tackled only recently (e.g. Guadagnini and Franzetti 1999, Riva et al. 1999, Dagan and Indelman 1999, van Leeuwen et al. 2000, Feyen et al. 2002).
We perform a numerical Monte Carlo analysis of (a) the predictors of hydraulic head and residence time (rendered by their means) for conservative solute particles injected at various radial distances from the well, and (b) the associated prediction errors (rendered by the variance of the state variables investigated).
The natural logarithm of aquifer transmissivity, Y, is modeled as a statistically homogeneous Gaussian random field. Three functional forms of the variogram (namely Exponential, Gaussian and Spherical), chosen amongst the most common models used in the literature, are considered. The impact of the choice of the variogram model on flow and travel time predictors is analyzed for different domain sizes in terms of correlation scale of Y (i.e. extent of the aquifer within which the effects of pumping are not negligible) and degrees of heterogeneity (in terms of the variance of Y )
Genetic parameters estimation in an Italian horse native breed to support the conversion from agricultural uses to riding purposes
No full textHorses are nowadays mainly used for sport and leisure purposes, and several local breeds, traditionally used in agriculture, are exposed to the risk of extinction. The long-term survival of local horse breeds depends on strategies to both monitor their genetic diversity and to find their sustainable role in the equine market. Thus, several local horse breeds need to adapt their breeding objective to allow a modernization process. The Bardigiano is an example of such horse breeds; we, therefore, studied the existing evaluation protocol from a genetic standpoint to assess the protocol's suitability to convert the Bardigiano from an agricultural to a riding horse. To this end, we estimated genetic parameters for four conformation measurements, ten grading traits and 23 linear traits. For conformation measurements, the heritabilities ranged from 0.31 for cannon bone circumference to 0.63 for height at withers. For conformation and attitude grading traits, the highest heritability (0.34) was estimated for development and the lowest (0.09) for gaits. The heritabilities for linear traits ranged from 0.05 for the leg straightness to 0.32 for the coat colour. Genetic correlations between linear traits and corresponding grading traits varied considerably, ranging from −0.42 to 0.98. This study showed that the current evaluation protocol in the Bardigiano horse is appropriate for genetic evaluation. Genetic parameters estimation can, in turn, be used to develop novel breeding values to help this conversion. Our study paves the way to optimize the Bardigiano horse breeding programme, and it may help several other local horse breeds experiencing similar issues
Support Vector Machines for delineation of geologic facies from poorly differentiated data
No full textA procedure for the solution of multicomponent reactive transport problems
No full textModeling transport of reactive solutes is a challenging problem, necessary for understanding the fate of pollutants and geochemical processes occurring in aquifers, rivers, estuaries, and oceans. Geochemical processes involving multiple reactive species are generally analyzed using advanced numerical codes. The resulting complexity has inhibited the development of analytical solutions for multicomponent heterogeneous reactions such as precipitation/dissolution. We present a procedure to solve groundwater reactive transport in the case of homogeneous and classical heterogeneous equilibrium reactions induced by mixing different waters. The methodology consists of four steps: (1) defining conservative components to decouple the solution of chemical equilibrium equations from species mass balances, (2) solving the transport equations for the conservative components, (3) performing speciation calculations to obtain concentrations of aqueous species, and (4) substituting the latter into the transport equations to evaluate reaction rates. We then obtain the space-time distribution of concentrations and reaction rates. The key result is that when the equilibrium constant does not vary in space or time, the reaction rate is proportional to the rate of mixing. The methodology can be used to test numerical codes by setting benchmark problems but also to derive closed-form analytical solutions whenever steps 2 and 3 are simple, as illustrated by the application to a binary system. This application clearly elucidates that in a three-dimensional problem both chemical and transport parameters are equally important in controlling the process
Mean travel time of conservative solutes in randomly heterogeneous unbounded domains under mean uniform flow
No full textWe derive a closed-form expression for mean travel time of a conservative solute migrating under uniform in the mean flow conditions within an infinite stationary field with simple exponential correlation of the natural logarithm of hydraulic conductivity. Our
expression is developed from a consistent second-order expansion in sY (standard deviation of the log hydraulic conductivity) of the equations for moments of travel time and trajectories of conservative solutes in two-dimensional randomly nonuniform flows of Guadagnini et al. [2001]. As such, it is nominally valid for moderately heterogeneous fields, with sY < 1. Its validity for larger heterogeneity degrees is tested against numerical
Monte Carlo simulations. Our results clarify the nonlinear effect in the mean travel time with respect to distance that has been observed numerically (and modeled empirically) in
the literature
Oxidative stress in Alzheimer’s disease: are we connecting the dots?
No full textRedox impairment is a prominent feature of Alzheimer’s disease (AD). It has led to the “oxidative stress hypothesis”, which proposes antioxidants as beneficial therapeutic tools in AD treatment. To date, a wide variety of antioxidants have been examined as neuroprotectants. However, success has been elusive in clinical trials. Several factors have contributed to this failure, including the complexity of the redox system in vivo. Potentially
critical aspects include the fine-tuned equilibrium between antioxidant defenses and free radical production, the lack of specific antioxidant target(s), and the inherent difficulty in delivering antioxidants where they are needed. Herein, we highlight significant progress in the field.
Future directions of antioxidant research are also presented
Multi-target Design Strategies in the Context of Alzheimer's Disease: Acetylcholinesterase Inhibition and NMDA Receptor Antagonism as the Driving Forces
No full textIn recent years, the multi-target-directed ligand concept has been used to design a variety of molecules hitting different biological targets for Alzheimer's disease. We have sought to combine, in the same molecule, the neuroprotective action of N-methyl-d-aspartate receptor antagonism with the symptomatic relief offered by cholinergic activity through acetylcholinesterase inhibition. This strategy could potentially maintain the positive outcomes of memantine-acetylcholinesterase inhibitor combinations, but with the benefits of a single molecule therapy. Herein, we discuss selected examples of multifunctional compounds, which we rationally designed to simultaneously modulate these targets. We also examine the intertwined relationship between acetylcholinesterase, N-methyl-d-aspartate receptors, and other active players in the neurotoxic cascade
Pathophysiology of cerebral small vessels in vascular cognitive impairment
No full textCerebral small-vessel alterations play a central role in determining lesions of subcortical structures and eventually may lead to cognitive impairment. Small-vessel diseases are classified according to the pathological viewpoint. The most important ones are the changes in small arteries and arterioles caused by prolonged hypertension. These small-vessel changes may result in ischemic damage to the brain parenchyma and blood-barrier alterations. Both mechanisms are thought to contribute to the occurrence of white-matter changes and lacunar infarcts. Modern magnetic resonance techniques such as diffusion, perfusion, and spectroscopy may allow the in vivo study of the pathophysiology of small-vessel diseases and the consequences on the brain parenchyma
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