La pollution des eaux souterraines : une problématique à l'interface des géosciences, des mathématiques, de la physique, de la chimie et de l'informatique

Cyril Fleurant est Professeur des Universités en géographie physique à l’université d’Angers. Il est l’auteur de nombreuses publications internationales. Il est également chargé de mission pour la liaison lycées-université. Son exposé a trait à la pollution des nappes souterraines et montre la nécessité d’associer des processus géologiques et physico-chimiques, de les mettre en équations mathématiques et d’utiliser des outils numériques pour in fine pouvoir comprendre le phénomène et ainsi répondre à des problématiques scientifiques

Inondation, catastrophe naturelle ou mauvaise gestion du territoire ?

Présentation sur les liens entre inondation, catastrophe naturelle ou mauvaise gestion du territoire

Dispersion of inert particles in a rough fracture.

International audienceThis paper aims to show a transport model of either smooth or rough fractures. These are numerically generated by means of a fractal surface method. In order to study dispersion processes, a 3D transport model is used. This model is based on a random walk that takes into account both the sizes of the particles and the size exclusion effect of particles on the fracture walls. Simulations are carried out to study dispersion processes in which transport cases in either smooth or rough fractures and transport of several sizes particles are involved. (C) 2001 Academie des sciences / Editions scientifiques et medicales Elsevier SAS

Gel Electrophoresis of DNA Knots in Weak and Strong Electric Fields

Gel electrophoresis allows to separate knotted DNA (nicked circular) of equal length according to the knot type. At low electric fields, complex knots being more compact, drift faster than simpler knots. Recent experiments have shown that the drift velocity dependence on the knot type is inverted when changing from low to high electric fields. We present a computer simulation on a lattice of a closed, knotted, charged DNA chain drifting in an external electric field in a topologically restricted medium. Using a simple Monte Carlo algorithm, the dependence of the electrophoretic migration of the DNA molecules on the type of knot and on the electric field intensity was investigated. The results are in qualitative agreement with electrophoretic experiments done under conditions of low and high electric fields: especially the inversion of the behavior from low to high electric field could be reproduced. The knot topology imposes on the problem the constrain of self-avoidance, which is the final cause of the observed behavior in strong electric field.Comment: 17 pages, 5 figure

A model of cockpit karst landscape, Jamaica

International audienceThis paper deals with a landscape evolution model to compute cockpit karsts landforms. The CHILD model is used to model geomorphic processes at regional scale. After examining briefly CHILD's principles and equations of limestone dissolution processes, the denudation model is detailed. The relation between subcutaneous dissolution and denudation of the topography is introduced by means of an empirical equation associated with epikarst processes: the denudation is taken to be proportional to the dissolution in the subcutaneous zone. The model takes into account an anisotropic dissolution in space according to what is observed in reality or described by scenarios of cockpit karst landscape evolution. Simulated cockpit karst terrains are compared with real landscapes by means of several morphometric criteria: slope, relative relief and scaling properties. Results confirm the importance of anisotropic dissolution processes and could provide a numerical validation of the epikarst processes to describe cockpit karst genesis

Polarization-dependence of anomalous scattering in brominated DNA and RNA molecules, and importance of crystal orientation in single- and multiple-wavelength anomalous diffraction phasing

In this paper the anisotropy of anomalous scattering at the Br K-absorption edge in brominated nucleotides is investigated, and it is shown that this effect can give rise to a marked directional dependence of the anomalous signal strength in X-ray diffraction data. This implies that choosing the correct orientation for crystals of such molecules can be a crucial determinant of success or failure when using single- and multiple-wavelength anomalous diffraction (SAD or MAD) methods to solve their structure. In particular, polarized absorption spectra on an oriented crystal of a brominated DNA molecule were measured, and were used to determine the orientation that yields a maximum anomalous signal in the diffraction data. Out of several SAD data sets, only those collected at or near that optimal orientation allowed interpretable electron density maps to be obtained. The findings of this study have implications for instrumental choices in experimental stations at synchrotron beamlines, as well as for the development of data collection strategy programs

Modelling cockpit karst landforms

International audienceThe purpose of this article is to present a model of the formation processes of cockpit karst landscapes. The CHILD software was used to simulate landscape evolution including dissolution processes of carbonate rocks. After examining briefly how the CHILD model operates, two applications of this model involving dissolution of carbonate rocks are presented. The simulated landscapes are compared with real landscapes of the Cockpit Country, Jamaica, using morphometric criteria. The first application is based on the hypothesis that dissolution of carbonate rocks is isotropic over time and space. In this case, dissolution is constant throughout the whole area studied and for each time step. The simulated landscapes based oil this hypothesis have morphometric features which are quite different from those of real landscapes. The second application considers that dissolution of carbonate rocks is anisotropic over time and space. In this case, it is necessary to take into account subsurface and under.-round processes, by Coupling surface runoff and water infiltration into the fractured carbonates

Modèle d'évolution de paysages, application aux karsts en cockpit de Jamaïque

International audienceCet article présente les résultats d'un modèle géomorphologique d'évolution des karsts en cockpit (Jamaïque). Dans un premier temps, nous expliquons comment les processus de dissolution des carbonates ont été implémentés dans le logiciel d'évolution géomorphologique CHILD. Puis nous donnons les détails de notre modèle d'érosion, basé sur les principes de la théorie de l'épikarst. Le modèle prend en compte l'anisotropie spatio-temporelle conformément à ce qui est observé empiriquement ou encore décrit par des scénarii de l'évolution des karsts en cockpit. Ce modèle nécessite d'abord d'introduire un réseau de fractures afin de prendre en compte les écoulements de sub-surface. Ensuite, la dissolution et donc l'ouverture des fractures sont calculées ce qui participe à un processus de feed-back positif entre dissolution et écoulement. La relation entre la dissolution des carbonates fracturés et l'évolution de la topographie est introduit