Comparisons of degradation kinetics of chloroethenes in groundwater between microcosms and field scale

International audienceGroundwater pollution by chlorinated solvents is a major concern since several years. It has been demonstrated that in specific physicochemical conditions, microbial processes like direct reductive dechlorination allow contamination reduction at several sites. Therefore, determination of biodegradation kinetics of chloroethenes is crucial in applying Natural Attenuation protocols on contaminated sites and assessing the potential risks for human health and natural media Biodegradation of chlorinated solvents is effective in highly reduced conditions, which rarely concerns the whole contaminant plume. In this study, direct reductive dechlorination of chloroethenes was studied on two different scales, on microcosms in the laboratory and at the real scale that corresponds to the contaminated site Microcosms studies were conducted in three different ways. (1) sediments sampled from the site and mixed with groundwater modified or not by a synthetic electron donor (Na propionate, Na lactate, toluene), (2) composite sediments coming from several places of the site mixed with groundwater modified or not by a synthetic electron donor; (3) autoclaved sediments and groundwater modified or not by synthetic organic matter. Studies on the real scale were conducted by the achievement of a synthesis of historical data (hydrogeological, geological and physicochemical data) of a polluted site. The synthesis of physicochemical data and then modelling the real site revealed the presence of degradation products of chloroethenes in the plume : cis-1,2-DCE and VC The results of comparisons of degradation kinetics obtained on the laboratory and the field under the same physicochemical conditions showed significant differences. Indeed, biodegradation of chlorinated solvents were faster in lab studies than in the field at the global scale. The existence of chlorinated ethenes biotransformation in microcosms confirmed the presence of a bacterial population able to catalyse reductive dechlorination reaction until CV. It is also likely that the bacterial consortium permitted to degrade other species like electron acceptors; detection of sulphide ions and Fe(II) and the presence of a black precipitate of FeS are proofs of sulphate reducing, ferro reducing and dechlorinating activities. The clear difference that there is between kinetics of degradation on microcosms and field scale could be explained by differences in chemical conditions that are not optimal everywhere in the plume of pollutants. The differences of chemical conditions (electron acceptors, type of natural organic matter, pH, redox potential...) are investigated in details to explain the differences in kinetic constant

Application of an iterative source localization strategy at a chlorinated solvent site

This study presents an inverse modeling strategy for organic contaminant source localization. The approach infers the hydraulic conductivity field, the dispersivity, and the source zone location. Beginning with initial observed data of contaminant concentration and hydraulic head, the method follows an iterative strategy of adding new observations and revising the source location estimate. Non-linear optimization using the Gauss-Levenberg-Marquardt Algorithm (PEST++) is tested at a real contaminated site. Then a limited number of drilling locations are added, with their positions guided by the Data Worth analysis capabilities of PYEMU. The first phase of PEST++, with PYEMU guidance, followed by addition of monitoring wells provided an initial source location and identified four additional drilling locations. The second phase confirmed the source location, but the estimated hydraulic conductivity field and the Darcy flux were too far from the measured values. The mismatch led to a revised conceptual site model that included two distinct zones, each with a plume emanating from a separate source. A third inverse modelling phase was conducted with the revised site conceptual model. Finally, the source location was compared to results from a Geoprobe@ MiHPT campaign and historical records, confirming both source locations. By merging measurement and modeling in a coupled, iterative framework, two contaminant sources were located through only two drilling campaigns while also reforming the conceptual model of the site. © 2021 The Author

CAGIRE: a wide-field NIR imager for the COLIBRI 1.3 meter robotic telescope

The use of high energy transients such as Gamma Ray Bursts (GRBs) as probes of the distant universe relies on the close collaboration between space and ground facilities. In this context, the Sino-French mission SVOM has been designed to combine a space and a ground segment and to make the most of their synergy. On the ground, the 1.3 meter robotic telescope COLIBRI, jointly developed by France and Mexico, will quickly point the sources detected by the space hard X-ray imager ECLAIRs, in order to detect and localise their visible/NIR counterpart and alert large telescopes in minutes. COLIBRI is equipped with two visible cameras, called DDRAGO-blue and DDRAGO-red, and an infrared camera, called CAGIRE, designed for the study of high redshift GRBs candidates. Being a low-noise NIR camera mounted at the focus of an alt-azimutal robotic telescope imposes specific requirements on CAGIRE. We describe here the main characteristics of the camera: its optical, mechanical and electronics architecture, the ALFA detector, and the operation of the camera on the telescope. The instrument description is completed by three sections presenting the calibration strategy, an image simulator incorporating known detector effects, and the automatic reduction software for the ramps acquired by the detector. This paper aims at providing an overview of the instrument before its installation on the telescope.Comment: Accepted by Experimental Astronom

Comparaison de cinétiques de biodégradation du PCE déterminées sur 3 échelles différentes : étude d'une pollution mixte PCE/huiles minérales

National audienceLa contamination des eaux souterraines par les chloroéthènes, dont le PCE (tétrachloroéthylène) et le TCE (trichloroéthylène), est devenue une préoccupation majeure ces dernières années, du fait de leur caractère cancérigène avéré. Parmi toutes les stratégies de réhabilitation de sites pollués, l'Atténuation Naturelle (A.N.) semble être une technologie prometteuse, de part son efficacité dans certaines conditions ainsi que son faible coût. Entre tous les mécanismes régissant l'A.N., la déchloration réductrice directe est le processus prépondérant et permettant la biodégradation des chloroéthènes la plus significative. L'A.N. a cependant ses limites ; il est très fréquent d'observer au droit et en aval des sites pollués en chloroéthènes, l'accumulation de sous-produits de biodégradation comme le cis-1,2-dichloroéthylène et le Chlorure de Vinyle (CV). La présente étude décrira des comparaisons de cinétiques de dégradation du PCE selon différentes échelles. Par ces diverses expériences, nous pourrons ainsi évaluer les différents mécanismes qui sont susceptibles de ralentir voire d'inhiber la biodégradation du PCE sur un site d'étude. La détermination de constantes cinétiques permettra de donner quelques éléments de réponse sur l'origine de la conversion incomplète du PCE observée sur différentes zones du site contaminé

14. Pollutions et transferts à travers les sols

Le sol est un élément fondamental des écosystèmes, car il est le support principal de la croissance végétale terrestre. Sa position superficielle en fait aussi le premier réceptacle des rejets d’origine anthropique. De par sa composition, le sol peut jouer le rôle de filtre ou d’accumulateur d’éléments, il est donc indispensable d’étudier les transferts d’éléments au sein de celui-ci pour déterminer le devenir des éléments vers les autres réservoirs, que sont notamment les eaux souterraines e..

Réponse d'un système aquifère multicouche aux variations paléoclimatiques et aux sollicitations anthropiques (approche par modélisation couplée hydrodynamique, thermique et géochimique)

Le fonctionnement hydrogéologique des grands bassins sédimentaires est souvent analysé sur le court terme avec l'hypothèse que le régime hydrodynamique était stabilisé avant l'exploitation humaine. Le comportement du système multicouche sud-aquitain, et en particulier de l'aquifère des Sables Infra-Molassiques, est étudié ici sur le long terme afin d être intégré dans un modèle hydrodynamique. 'interprétation des données de la chimie de l eau a montré que des transferts verticaux associés ou non à un flux horizontal pouvaient expliquer ces résultats et a permis d'exposer l'idée d'une phase de recharge importante à la fin du Pléistocène. Un scénario des phases potentielles de recharge sur 40000 ans été proposé à partir d une reconstitution paléoclimatique. n modèle couplé hydrodynamique et thermique comportant onze couches sur une extension de 32000 km a été réalisé. La calibration des paramètres hydrodynamiques à l aide de variations induites par deux stockages de gaz et des conditions thermiques a été effectué. La simulation des variations eustatiques sur les derniers 100000 ans a montré la faible influence de ces fluctuations. La simulation des variations de recharge sur 40000 ans a été effectuée et a permis d établir trois principaux axes d'écoulement en cohérence avec les contraintes géochimiques. L'importance des transferts verticaux et horizontaux a pu être approchée à l'aide de bilans de flux. Enfin, la baisse piézométrique actuellement observée serait la conséquence principale des prélèvements de ces trente dernières années et de la vidange naturelle du système amorcé il y a 150 ans, suite à une recharge importante au cours du Petit Age Glaciaire.The hydrogeologic behaviour of large sedimentary basins is often analysed on the short term on the assumption that the hydrodynamic state was stabilized before the human exploitation. The behaviour of the south-Aquitaine multi-layered system, and in particular of the Infra-Molassic Sands aquifer, is studied here on a long term period in order to be integrated in a hydrodynamic model.The interpretation of the data of the water chemistry showed that vertical flows combined or not with a horizontal flow could explain these results and it made it possible to present the idea of a major phase of recharge at the end of the Pleistocene. A scenario of the potential phases of recharge over 40,000 years was realized starting from a paleoenvironmental reconstitution.A hydrodynamic and thermal coupled model including eleven layers on an extension of 32,000 km was carried out. The calibration of the hydrodynamic parameters using variations induced by two storages of gas and of the thermal conditions was done. The simulation of the eustatic variations on the last 100,000 years showed the weak influence of these fluctuations. The simulation of the variations of recharge over 40,000 years was carried out and enabled to establish three principal axis of flow in coherence with the geochemical constraints. The importance of the vertical and horizontal transfers could be done using water budget. Finally, the piezometric fall currently observed would be the principal consequence of the pumping of the last thirty years and of the natural drainage of the system started 150 years ago, following an important refill during the Little Ice Age.BORDEAUX3-BU Lettres-Pessac (335222103) / SudocSudocFranceF

Fast semi-analytical approach to approximate plumes of dissolved redox-reactive pollutants in heterogeneous aquifers. 1. BTEX

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