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Complementing approaches to demonstrate chlorinated solvent biodegradation in a complex pollution plume: Mass balance, PCR and compound-specific stable isotope analysis.
No full textInternational audienceThis work describes the use of different complementing methods (mass balance, polymerase chain reaction assays and compound-specific stable isotope analysis) to demonstrate the existence and effectiveness of biodegradation of chlorinated solvents in an alluvial aquifer. The solvent-contaminated site is an old chemical factory located in an alluvial plain in France. As most of the chlorinated contaminants currently found in the groundwater at this site were produced by local industries at various times in the past, it is not enough to analyze chlorinated solvent concentrations along a flow path to convincingly demonstrate biodegradation. Moreover, only a few data were initially available to characterize the geochemical conditions at this site, which were apparently complex at the source zone due to (i) the presence of a steady oxygen supply to the groundwater by irrigation canal losses and river infiltration and (ii) an alkaline pH higher than 10 due to former underground lime disposal. A demonstration of the existence of biodegradation processes was however required by the regulatory authority within a timeframe that did not allow a full geochemical characterization of such a complex site. Thus a combination of different fast methods was used to obtain a proof of the biodegradation occurrence. First, a mass balance analysis was performed which revealed the existence of a strong natural attenuation process (biodegradation, volatilization or dilution), despite the huge uncertainty on these calculations. Second, a good agreement was found between carbon isotopic measurements and PCR assays (based on 16S RNA gene sequences and functional genes), which clearly indicated reductive dechlorination of different hydrocarbons (Tetrachloroethene--PCE-, Trichloroethene--TCE-, 1,2-cisDichloroethene--cis-1,2-DCE-, 1,2-transDichloroethene-trans--1,2-DCE-, 1,1-Dichloroethene--1,1-DCE-, and Vinyl Chloride--VC) to ethene. According to these carbon isotope measurements, although TCE biodegradation seems to occur only in the upgradient part of the studied zone, DCE and VC dechlorination (originating from the initial TCE dechlorination) occurs along the entire flowpath. TCE reductase was not detected among the Dehalococcoides bacteria identified by quantitative PCR (qPCR), while DCE and VC reductases were present in the majority of the population. Reverse transcriptase PCR assays (rt-PCR) also indicated that bacteria and their DCE and VC reductases were active. Mass balance calculations showed moreover that 1,1-DCE was the predominant DCE isomer produced by TCE dechlorination in the upgradient part of the site. Consequently, coupling rt-PCR assays with isotope measurements removes the uncertainties inherent in a simple mass balance approach, so that when the three methods are used jointly, they allow the identification and quantification of natural biodegradation, even under apparently complex geochemical and hydraulic conditions
Les impacts potentiels des changements climatiques et des événements extrêmes sur la production agricole et effets réciproques
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Hydraulic and chemical pulse tests in a shut-in chamber imbedded in an argillaceous formation: Numerical and experimental approaches
No full textInternational audienceHydraulic and chemical pulse tests were performed in four shut-in chambers to obtain hydraulic and chemical parameters of the Callovo-Oxfordian shale. The osmotic tests, i.e., chemical pulses, were carried out by exchange of the formation water for a different solution either more or less concentrated. Two sets of two osmotic tests were done in March and October 2006. These osmotic tests are superimposed on a purely hydraulic response because of the drilling of the well causing a pressure drop. The pressure records (hydraulic and osmotic responses) were interpreted using a 2-D model to obtain the hydraulic and chemical parameters. The osmotic efficiency inferred for the Callovo-Oxfordian shale is on the order of 0.012 for a concentration of 0.43 mol L-1 and 0.12 for a concentration of 0.086 mol L-1. These results suggest that the Callovo-Oxfordian shale behaves as a weakly semipermeable membrane and only 0.1-0.15 MPa of the 0.2-0.5 MPa can be explained by these osmotic efficiency values
Justice et équité
No full textChapitre 6 d'un rapport de l’Académie des sciences qui analyse la question des évènements climatiques extrêmes, et leurs conséquences prévisibles sur les systèmes économiques, sociaux, sanitaires et de sécurité. Ce rapport est disponible dans : Rapport sur la Science et la TechnologieLa justice et l’équité ne peuvent se satisfaire de la plus grande vulnérabilité des pays pauvres aux variations climatiques, extrêmes ou non. Deux principes doivent soutenir une plus grande ambition dans la réduction de cette vulnérabilité : prévenir tout changement climatique dangereux et donner la priorité aux plus vulnérables. Des actions concrètes, impliquant les populations en place, sont en ce sens indispensables. L’exemple des sociétés pastorales sahéliennes illustre toute la complexité des problèmes de justice et d’équité pour des populations qui ont su répondre dans le passé aux contraintes spécifiques à leur milieu. Ce chapitre évoque diverses actions concrètes en cours dans les pays du Sahel, ainsi qu’au Bangladesh
What is the significance of pore pressure in a saturated shale layer?
No full textInternational audienceElectrostatic interactions, associated with negatively charged surfaces of clay minerals, produce a so-called "disjoining pressure" when diffuse layers overlap, i.e., at low porosity. Disjoining pressure is the pressure difference between the water in the clay pore space and that in a bulk solution at the same depth. Another widely used concept in clay-rocks is the "swelling pressure." It corresponds in fact to the macroscopic average of the disjoining pressure. This study proposes to determine the value of the swelling pressure of a natural material by a simple volume-averaging approach of the disjoining pressure, calculated for each clay mineral present in the material. The swelling pressure, which is dependent on the salinity of the pore fluid, is introduced into a hydrochemomechanical coupling, yielding a more general pressure diffusion equation. The results are compared to swelling pressure measurements for natural shale samples. The implications of this swelling pressure for water pressure measurements in natural formations are also discussed
