ALBERT

Description: The often observed tailing of tracer breakthrough curves is caused by a multitude of mass transfer processes taking place over multiple scales. Yet, in some cases it is convenient to fit a transport model with a single-rate mass transfer coefficient that lumps all the non-Fickian observed behavior. Since mass transfer processes take place at all characteristic times, the single-rate mass transfer coefficient derived from measurements in the laboratory or in the field vary with time, ω ( t ). The literature review and tracer experiments compiled by Haggerty et al . [2004] from a number of sites worldwide suggest that the characteristic mass transfer time, which is proportional to ω ( t ) −1 , scales as a power law of the advective and experiment duration. This paper studies the mathematical equivalence between the Multi-Rate Mass Transfer Model (MRMT) and a time-dependent single-rate mass transfer model (t-SRMT). In doing this, we provide new insights into the previously observed scale-dependence of mass transfer coefficients. The memory function, g ( t ), which is the most salient feature of the MRMT model, determines the influence of the past values of concentrations on its present state. We found that the t-SRMT model can also be expressed by means of a memory function φ ( t,τ ). In this case though the memory function is non-stationary, meaning that in general it cannot be written as φ ( t-τ ). Nevertheless, the full behavior of the concentrations using a single time-dependent rate ω ( t ) is approximately analogous to that of the MRMT model provided that the equality ω ( t ) = - d In g ( t )/ dt holds and the field capacity is properly chosen. This relationship suggests that when the memory function is a power law, g ( t ) ∼ t 1- k , the equivalent mass transfer coefficient scales as ω ( t ) ∼ t −1 , nicely fitting without calibration the estimated mass transfer coefficients compiled by Haggerty et al . [2004]. This article is protected by copyright. All rights reserved.

Description: Aquifer hydraulic properties such as hydraulic conductivity ( K ) are ubiquitously heterogeneous and typically only a statistical characterization can be sought. Additionally statistical anisotropy at typical characterization scales is the rule. Thus, regardless of the processes governing solute transport at the local (pore) scale, transport becomes non-Fickian. Mass-transfer models provide an efficient tool that reproduces observed anomalous transport; in some cases though, these models lack predictability as model parameters cannot readily be connected to the physical properties of aquifers. In this study we focus on a multi-rate mass-transfer model (MRMT), and in particular the apparent capacity coefficient (β), which is a strong indicator of the potential of immobile zones to capture moving solute. We aim to find if the choice of an apparent β can be phenomenologically related to measures of statistical anisotropy. We analyzed an ensemble of random simulations of three-dimensional log-transformed multi-Gaussian permeability fields with stationary anisotropic correlation under convergent flow conditions. It was found that apparent β also displays an anisotropic behavior, physically controlled by the aquifer directional connectivity, which in turn is controlled by the anisotropic correlation model. A high hydraulic connectivity results in large β values. These results provide new insights into the practical use of mass-transfer models for predictive purposes.

Description: Optimizing an experimental design is a compromise between maximizing information we get about the target and limiting the cost of the experiment, providing a wide range of constraints. We present a statistical algorithm for experiment design that combines the use of linearized inverse theory and stochastic optimization technique. Linearized inverse theory is used to quantify the quality of one given experiment design while genetic algorithm (GA) enables us to examine a wide range of possible surveys. The particularity of our algorithm is the use of the multi-objective GA NSGA II that searches designs that fit several objective functions (OFs) simultaneously. This ability of NSGA II is helping us in defining an experiment design that focuses on a specified target area. We present a test of our algorithm using a 1-D electrical subsurface structure. The model we use represents a simple but realistic scenario in the context of CO 2 sequestration that motivates this study. Our first synthetic test using a single OF shows that a limited number of well-distributed observations from a chosen design have the potential to resolve the given model. This synthetic test also points out the importance of a well-chosen OF, depending on our target. In order to improve these results, we show how the combination of two OFs using a multi-objective GA enables us to determine an experimental design that maximizes information about the reservoir layer. Finally, we present several tests of our statistical algorithm in more challenging environments by exploring the influence of noise, specific site characteristics or its potential for reservoir monitoring.

Description: Article Ferroelectric organic materials can be used for tunnel barriers in memory devices as a cheaper and eco-friendly replacement of their inorganic counterparts. Here, Tian et al . use poly(vinylidene fluoride) with 1–2 layer thickness to achieve giant tunnel electroresistance of 1,000% at room temperature. Nature Communications doi: 10.1038/ncomms11502 Authors: B. B. Tian, J. L. Wang, S. Fusil, Y. Liu, X. L. Zhao, S. Sun, H. Shen, T. Lin, J. L. Sun, C. G. Duan, M. Bibes, A. Barthélémy, B. Dkhil, V. Garcia, X. J. Meng, J. H. Chu

Description: Myeloperoxidase, a heme enzyme stored in high amounts in neutrophils and monocytes, produces reactive oxygen species that are involved in the innate mammalian response to infection; among these, the most reactive is HOCl, generated by oxidation of Cl − in the presence of H 2 O 2 . This powerful oxidant reacts very fast with protein side chains and peptide bonds. Taurine ( − O 3 SCH 2 CH 2 NH 3 + ), present in high concentrations in human neutrophils, traps the HOCl to yield ( N -Cl)-Taurine, much less toxic and reactive; it is a long-lived oxidant which contributes to the killing of pathogenic organisms and plays a role in the inflammatory response. Kinetic data suggest that ( N -Cl)-Tau reacts with cysteine and glutathione , two relevant thiols in mammals, via two pathways: direct oxidation ( chlorination ) of the thiolate by ( N -Cl)-Tau involving general-acid catalysis , and, the major one, ( N -Cl)-Tau hydrolysis followed by fast chlorine transfer from the so-formed HOCl/ClO − mixture to the corresponding thiolate, despite the highly unfavorable hydrolysis equilibrium (K H =10 −8 M). Scavenging of HOCl by strong nucleophiles like thiolates opens an effective reactive channel. Theoretical calculations, at the B3LYP/6-311++G(d,p) level of theory and using the SMD model to simulate aqueous salvation, agree with the experimental behavior, predicting that both the chlorine and the proton are almost half transferred at the transition state in the direct, general-acid catalyzed, chlorination of the thiolate by ( N -Cl)-Tau. Under physiological conditions, the hydrolysis of ( N -Cl)-Tau is the main responsible of the oxidation of intracellular thiols far away from the site of HOCl enzymatic production. Copyright © 2013 John Wiley & Sons, Ltd. ( N -Cl)-Taurine reacts with cysteine and glutathione via two pathways: direct oxidation (chlorination) of the thiolate by ( N -Cl)-Tau involving general-acid catalysis, and the major one, in physiological conditions, involving ( N -Cl)-Tau hydrolysis followed by fast chlorine transfer from the so-formed HOCl/ClO − mixture to the corresponding thiolate, despite the highly unfavorable hydrolysis equilibrium (K H =10 −8 M). Theoretical calculations at the B3LYP/6-311++G(d,p) level agree with the experimental behavior, predicting the chlorine and the proton being almost half transferred at the transition state in the direct chlorination of the thiolate by ( N -Cl)-Tau.

Description: Most methods for hydraulic test interpretation rely on a number of simplified assumptions regarding the homogeneity and isotropy of the underlying porous media. This way, the actual heterogeneity of any natural parameter, such as transmissivity (), is transferred to the corresponding estimates in a way heavily dependent on the interpretation method used. An example is a long-term pumping test interpreted by means of the Cooper-Jacob method, which implicitly assumes a homogeneous isotropic confined aquifer. The estimates obtained from this method are not local values, but still have a clear physical meaning; the estimated represents a regional-scale effective value, while the log-ratio of the normalized estimated storage coefficient, indicated by , is an indicator of flow connectivity, representative of the scale given by the distance between the pumping and the observation wells. In this work we propose a methodology to use , together with sampled local measurements of transmissivity at selected points, to map the expected value of local values using a technique based on cokriging. Since the interpolation involves two variables measured at different support scales, a critical point is the estimation of the covariance and crosscovariance matrices. The method is applied to a synthetic field displaying statistical anisotropy, showing that the inclusion of connectivity indicators in the estimation method provide maps that effectively display preferential flow pathways, with direct consequences in solute transport. This article is protected by copyright. All rights reserved.