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

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

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

Dissolution geology of organic materials on Saturn’s moon Titan: alien analogs of terrestrial karst

Karst or dissolution geology can occur whenever a circulating fluid can dissolve a geological material. On Earth, the “classical” karst definition is for limestone (CaCO3) in water (H2O), but other material/solvent combinations can create terrestrial dissolution terrain as well. These include so-called “evaporite karst materials” such as halite (NaCl)/H2O or gypsum (CaSO4)/H2O, dolomite (CaMg(CO3)2)/H2O, and even silica (SiO2)/H2O [Ford and Williams, 2007].  On Mars, there has been the suggestion of kieserite (MgSO4)/H2O system that may have formed in an earlier, wetter environment [Baroni and Sgavetta, 2013]. Saturn’s moon Titan extends the definition of karst to include non-aqueous liquids dissolving a landscape made of organic materials. The Cassini mission has provided evidence that Titan’s 1.5 bar nitrogen atmosphrere and cryogenic 94 K surface temperature supports a hydrocarbon-based cycle on Titan similar to the terrestrial water cycle. These circulating liquids may be capable of dissolving some of the surface organic molecules derived from Titan’s complex atmospheric photochemistry. Although under a different gravity, temperature, materials and fluid regime, many of the features on Titan’s surface bear striking resemblances to terrestrial karst terrains. Our investigations have focused on the labyrinth terrains of Titan. These are elevated plateaux of organic materials that appear similar to polygonal karst, tower karst, and fluviokarst on Earth [Malaska et al., 2010; 2017]. Remote sensing data is consistent with these plateaux being constructed of low-dielectric organic materials [Janssen et al. 2009; 2016; Malaska et al, 2016b]. Theoretical calculations followed by cryogenic laboratory experiments have shown that organic materials found on Titan’s surface will dissolve when subjected to Titan’s rainfall of methane-rich fluids [Raulin, 1987; Lorenz and Lunine, 1996; Malaska et al., 2010; 2011; Malaska and Hodyss, 2014; Cornet et al., 2015] and preliminary modelling has been able to reproduce some of the morphologies observed on Titan [Cornet et al., 2017]. Titan’s labyrinth terrains may originate as mixed organic windblown sediments that are later lithified in a process similar to calcite-cemented sandstone on Earth. Organic molecules and sediments produced by Titan’s rich organic photochemistry include organic molecules such as acetylene (C2H2), ethylene (C2H4), hydrogen cyanide (HCN),  benzene (C2H6), acrylonitrile (C2H3CN), acetonitrile (CH3CN), cyanoacetylene (HC2CN), other alkynes and nitriles, and a complex refractory organic materials similar to laboratory tholins. Once uplifted, the saturation equilibrium and kinetics of dissolution for each material and fluid combination affecting the plateau may play key roles in determining how the karstic system will evolve [Malaska et al., 2011; Cornet et al., 2015]. Some of the Titan organic minerals will dissolve, while some will be left behind as an insoluble lag deposit. Advanced laboratory investigations of organic materials on Titan is underway to further understand how these geological structures evolve and compare them with the formation processes of terrestrial analogs. We suggest that karst is a general planetary process wherever circulating fluids are capable of dissolving materials and developing subsurface drainage

Cellular automata and Agent-based models in response to different environmental problems: a review on French research over the last ten years

Cellular Automata (CA) and Agent-Based Models (ABM) are used to better assess patterns and processes result-ing from environmental interactions using simple rules. Over the last ten years, the applications have been carriedout on various complex systems: flash floods, fire propagation, river meandering, landscape evolution... The fre-quent use these applications demand are supported by an array of advances in field outside of physical geographyas physics, computer sciences and mathematics. This communication presents common, singular and innovativeapplications observed in French research. On one hand, the CA RuiCells aims at understanding the spatial hy-drological behaviours in all points of one catchment, linking impacts due to basin forms and slopes through localto global scales (Delahaye et al., 2007). The CA Soda has been developed to measure erosion at fine scale, lessthan a few meters (Vallette, 2006). Smoothed-particules can also be implemented in CA to improve dynamics orhydrological fluxes (Drogoul, 1995). On the other hand, ABM appears in geomorphology after first initiatives inecology, sociology or human geography in the 1990's. Modules of CATCHSCAPE allow to simulate the hydrolog-ical system with its distributed water balance, irrigate schemes management, crop and vegetation dynamics (Bécuet al. 2008). For alluvial plains, ABM can also be used to simulate processes between independent interacting enti-ties which behave according to the local environment (Teles et al., 1999). At the opposite, many models have beendeveloped in other countries, as for simulating erosive thresholds (Favis-Mortlock, 1998), lava dynamics (Avolioet al., 2006), fluvial meandering (Coulthard and Van de Wiel, 2006), evolution of coasts (Dearing et al., 2005) anddunes (Thomas and Nicholas, 2007). Consequently, this communication addresses the debate on two questions:why French geomorphologist researchers are late in applying CA and ABMs, and is the simplification in processeshad led to change research questions or to offer new perspectives

Development of labyrinths on Titan: A numerical model based on surface dissolution

Titan is an Earth-like world with active erosion processes based on the interaction of liquid methane with solid organics and ices at the surface, which shapes the landscapes over geological timescales. The Cassini mission allowed to discover the so-called “labyrinthic terrain”, heavily dissected regions on Titan located at high latitudes and resembling terrestrial cockpit or polygonal karst terrain developed by rock dissolution, thanks to repeated Cassini/RADAR observations. In this work, we make use of a 3D Landscape Evolution Model (LEM) that includes karstic dissolution as the major geological process, coupled to a radar backscattering model able to generate te associated SAR images of the numerical lansdcapes, in order to infer the possible thickness and degree of maturation of the Titan karst

Dissolution on Saturn's Moon Titan: A 3D Karst Landscape Evolution Model

Titan is an Earth-like world possessing a nitrogen-rich atmosphere covering a surface showing signs of lacustrine (lakes and depressions), fluvial (channels, valleys), aeolian (longitudinal dunes) activity. The chemistry implied in the geological processes is, however, strikingly different from that on Earth. Titan’s extremely cold environment (T ~ -180°C) only allows water to exist under the form of an icy “bedrock”. The presence of methane as the second major constituent in the atmosphere, as well as an active nitrogen-methane photochemistry, allows methane and ethane to drive a hydrocarbon cycle similar to the terrestrial hydrological cycle. A plethora of organic solids, more or less soluble in liquid hydrocarbons, is also produced in the atmosphere and can lead, by atmospheric sedimentation over geological timescales, to formation of some kind of organic geological sedimentary layer. [figure_sikun2other] Based on comparisons between Titan’s landscapes seen in the Cassini spacecraft data and terrestrial analogues, karstic-like dissolution and evaporitic crystallization have been suggested in various instances to take part in the landscape development on Titan. Dissolution has been invoked, for instance, for the development of the so-called “labyrinthic terrain”, located at high latitudes and resembling terrestrial cockpit or polygonal karst terrain. In this work, we aim at testing this hypothesis by comparing the natural landscapes visible in the Cassini/RADAR images of Titan’s surface, with those inferred from the use of a 3D Landscape Evolution Model (LEM) based on the Channel-Hillslope Integrated Landscape Development (CHILD), modified to include karstic dissolution as the major geological process. Digital Elevation Models (DEMs) are generated from an initial quasi-planar surface for a set of dissolution rates, diffusion coefficients (solute transport), and sink densities of the mesh. The landscape evolves over millions of years. Synthetic SAR images are generated from these DEMs in order to compare with Titan’s landforms seen in the actual SAR images and infer the possible thickness and degree of maturation of the Titan kars