Comprendre les transferts de contaminants dans le sol et protéger nos ressources en eau

L’eau de boisson ou d’irrigation ouvre une voie privilégiée par laquelle les contaminants, issus de certaines activités industrielles, peuvent atteindre l’homme. Les scientifiques, cherchant à maîtriser les risques engendrés par ces activités, se sont donnés comme mission prioritaire de comprendre et de simuler les transferts de métaux lourds, radionucléides et autres composés organiques toxiques dans les sols

Incorporation et rôle des halogènes dans les silicates vitreux et fondus

International audienceCet article propose une synthèse sur la solubilité, la spéciation et la diffusion des halogènes (fluor, chlore, brome et iode) dans les silicates fondus ainsi que leur effet sur la viscosité. Le fluor est l'halogène le plus soluble dans les silicates fondus. En se substituantàsubstituantà l'oxygène et en s'associantàassociantà l'alumi-nium, il favorise la dépolymérisation du réseau, ce qui se traduit par une baisse de la viscosité du verre. La solubilité du chlore est favorisée par la présence d'alcalins dans la composition du liquide. Le chlore s'asso-cie aux cations modificateurs de réseau comme le calcium ou le sodium. Son effet sur la viscosité dépend de la composition chimique de la matrice vitreuse, notamment de la proportion d'alcalins et d'aluminium. La solubilité du brome est augmentée dans un liquide peralcalin riche en silice. La solubilité de l'iode peut atteindre 1 % en masse dans un verre borosilicaté. L'iode s'associe alors aux ions sodium. Mots clés : Silicates fondus / halogènes / solubilité / spéciation / viscosité Abstract-Incorporation and role of halogens in silicate melts and glasses. This article provides a review on the solubility, speciation and diffusion of halogens (fluorine, chlorine, bromine and iodine) in molten silicates and their effect on the viscosity. Fluorine is the most soluble halogen in silicate melts. By replacing oxygen in tetraedra and by coordinating with aluminum, it triggers depolymerization of the glassy network, which decreases the viscosity of silicate melt. The solubility of chlorine is favored by the presence of alkali in the melt composition. Chlorine shows affinity for the network modifiers cations such as calcium or sodium. Its effect on viscosity depends on the chemical composition, especially the proportion of alkali elements and aluminum. The solubility of bromine increases in silica-rich and alkali-rich melt. The solubility of iodine can reach 1wt.% in a borosilicate glass in which it is associated with sodium ions

What we can learn from measurements of air electric conductivity in ²²²Rn-rich atmosphere

International audienceElectric conductivity of air is an important characteristic of the electric properties of an atmosphere. Testing instruments to measure electric conductivity ranging from ~10-13 to 10-9 S m-1 in natural conditions found in the Earth atmosphere is not an easy task. One possibility is to use stratospheric balloon flights; another (and a simpler one) is to look for terrestrial environments with significant radioactive decay. In this paper we present measurements carried out with different types of Conductivity Sensors in two 222Rn-rich environments, i.e. in the Roselend underground tunnel (French Alps) and in the IRSN BACCARA chamber. The concept of the Conductivity Sensor is based on the classical time relaxation method. New elements in our design include isolation of the sensor sensitive part (electrode) from the external electric field and sensor miniaturization. This greatly extends the application domain of the sensor and permits to measure air electric conductivity when the external electric field is high and varies from few tens of V m-1 to up to few tens of kV m-1. This is suitable to propose the instrument for a planetary mission. Two-fold objectives were attained as the outcome of these tests and their analysis. First was directly related to the performances of the Conductivity Sensors and the efficiency of the Conductivity Sensor design to shield the external electric field. Second objective aimed at understanding the decay mechanisms of 222Rn and its progeny in atmosphere and the impact of the enclosed space on the efficiency of gas ionization. This article is protected by copyright. All rights reserved

Evidence of both M2 and O1 Earth tide waves in radon-222 air concentration measured in a subglacial laboratory

International audienceMany earthquake-related radon-222 temporal changes have been recorded since the 1960s and are frequently discussed, sometimes initiating a controversial debate on the relevance of radon-222 as an earthquake precursory signal. The diurnal S1-O1 and semidiurnal S2-M2 Earth tide signatures in radon signals are acquired in a natural context. This can be used to calibrate the radon changes under strain accumulation close to epicentral areas, which are often discussed but are rarely evidenced by experimental data. The analysis of a 10 month time series acquired in the subglacial laboratory of the Argentière glacier, Mont Blanc Massif, French Alps, demonstrates here the unambiguous episodic appearance of the M2-O1 waves in the radon signal with significant amplitudes of 36 and 50 Bq m 3, respectively. We thus prove that radon variations induced by gravitational M2 and O1 waves are detectable in a natural environment. In this particular place, the radon response is probably amplified by cyclic stress variations applied on the upstream side of the natural rock dam into which the laboratory is drilled. The amplification of the radon signal is induced by poroelastic deformation under this particular mechanical forcing. This can elucidate why most precedent studies failed to detect M2-O1 signatures in radon signals recorded in other underground laboratories

Metamorphism and tectonics in southern Madagascar: An overview

International audienceThe scope of this paper is to briefly summarize the general tectonic pattern and the metamorphic evolution of the continental crust from southern Madagascar. After a presentation of the main geophysical features of the Malagasy crust, the brittle and the ductile strain patterns are established at a crustal scale. The Pan-African metamorphic zonation is discussed and interpreted through a tectonic model related to a transpressive regime

Geophysical evidences for large-scale mullion-type structures at the mantle–crust interface in southern Madagascar: implications for Neoproterozoic orogeny

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Using Water Stable Isotopes in the Unsaturated Zone to Quantify Recharge in Two Contrasted Infiltration Regimes

A reliable estimate of recharge is needed for the sustainable management of groundwater resources. Water stable isotope (delta O-18 and delta H-2) profiles in the unsaturated zone are frequently used to quantify groundwater recharge based on the seasonality of water isotopic compositions in precipitation. A very simple approach consists of integrating the soil water content between peak values of soil water isotopic composition, typically corresponding to precipitation signatures from warm and cold seasons. When precipitation isotopic compositions are available, a conceptual surface water isotopes budget and lumped parameter dispersion model can be computed. These models were applied on two field sites with similar permeable soils with grass cover but contrasting recharge regimes and seasonality, one in the Paris Basin (France) with continuous recharge from autumn to spring and the other in the St. Lawrence Lowlands (Quebec, Canada) with episodic recharge in fall and after snowmelt. For the two sites, the peak-to-peak method and isotope surface budget led to comparable recharge intensities. At least at the Paris Basin site, evaporation was shown to slightly modify the average unsaturated zone and hence groundwater isotope composition. The proposed parameterization of isotope fractionation due to evaporation allows qualitative estimation of the fraction of evaporation, at least during the recharge seasons. In spite of its simplifications and limitations, the proposed parsimonious model can give estimates of recharge in a variety of sites even if they are not well characterized, as it benefits from the large availability of monthly isotopic compositions in precipitation