Transport avec échange gazeux du trichloroéthylène vers une nappe aquifère

Deux expériences ont été réalisées sur un site expérimental contrôlé de dimensions décamétriques reconstituant un aquifère alluvial. L’originalité de ce travail est basée sur le fait que cette plate-forme expérimentale permet de rendre compte du rôle de la frange capillaire dans les phénomènes de transfert, ce qui est difficilement accessible sur des systèmes réduits de laboratoire ou dans les investigations sur site réel. L’objectif principal est l’évaluation quantitative des mécanismes de transfert de Composés Organiques Volatils (COV) depuis la zone non saturée vers la nappe dans le cas d’une source de pollution localisée en zone non saturée. Le cas du transport du trichloroéthylène (TCE) a été abordé où une analyse comparative du transfert du TCE depuis la zone non saturée vers la nappe via la frange capillaire est présentée en étudiant les deux mécanismes : dispersion et dissolution. Dans la première expérience, la dispersion passive de la pollution par les vapeurs depuis la zone non saturée vers la nappe via la frange capillaire est étudiée. Dans la seconde expérience, l’impact sur la pollution de la nappe du lessivage des vapeurs par une pluie contrôlée est quantifié. Les résultats montrent que la dispersion passive des vapeurs peut causer une pollution significative de l’eau de la nappe, et ce, malgré la lenteur du processus de diffusion dans la partie inférieure de la frange capillaire suffisamment saturée en eau. Le lessivage des vapeurs par la pluie provoque une pollution de nappe plus importante et plus étendue. La quantification des flux de pollution partant de la zone non saturée vers la nappe a été réalisée dans la première expérience en se servant de la méthode de JOHNSON et PANKOW (1992), et du code de calcul (Hydrus) dans la seconde expérience. Les résultats expérimentaux et analytiques mettent en évidence, d’une part, le rôle d’écran joué par la frange capillaire contre le transfert de la pollution vers la nappe, et d’autre part, l’augmentation significative du degré et de l’étendue de la pollution de la nappe en cas de lessivage des vapeurs par les eaux de pluie.Two large-scale experiments were conducted on a controlled artificial aquifer referred to as SCERES (Site Contrôlé Expérimental de Recherche pour la Réhabilitation des Eaux et des Sols: 25 x 12 x 3 m). The experimental tool SCERES was completely buried in the subsurface in order to get stable temperature conditions in the aquifer. The hydraulic gradient, flow rate, visualization of the water table and water sampling were managed and monitored in two technical pits located at the upstream and downstream ends of the SCERES basin. It was packed with a main layer of uniform quartz sand and a 0.5 m-deep drainage layer at the bottom of the basin having hydraulic conductivities of 8 x 10‑4 and 6 x 10‑3 m/s respectively. The quartz sand had a mean grain diameter of 0.45 mm, a total porosity of 0.4 and a uniformity coefficient of 2.1; its longitudinal dispersivity was determined in laboratory column experiments to be approximately 1 mm. The aquifer is composed of a 1 m-thick saturated zone and a 2 m-thick unsaturated zone, making it possible to monitor the propagation of vapours in this zone. The hydraulic gradient of the groundwater was fixed at 0.003 m/m, which corresponded to a flow rate of 0.5 m3/h and an average velocity of approximately 0.4 m/day. The thickness of the capillary fringe was estimated to be approximately 0.25 m as deduced from water profile measurements by exploration with a TDR (Time Domain Reflectometry) probe. In the capillary fringe, water saturation at depths of 1.85 and 1.95 m was approximately 57% and more than 90%, respectively.The contaminant chosen for these experiments was trichloroethylene (TCE), because it is among the most frequently detected volatile organic compounds (VOCs) in subsurface environments. TCE as the pure phase was injected 0.35 m beneath the soil surface of SCERES with an experimental design that maintained a uniform infiltration of TCE in the vadose zone for 15 min. The chosen injection device was built from a stainless steel tank (0.58 m diameter and 0.15 m height), on which 31 screened brass rods were fixed at the bottom and separated by a distance of 0.1 m. Each rod contained four injection holes of 0.2 mm diameter in the lower extremity (JELLALI, 2000; JELLALI et al., 2001). In order to prevent TCE volatilization from the device during injection, the TCE volume in the injection tank was covered with a 0.02 m thick water layer.The resulting vapour and aqueous phases were monitored along with temperature and moisture content in order to investigate the mass transfer of VOC from the unsaturated zone to groundwater. The originality of this research was based on the fact that the controlled SCERES site allowed the study of transfer phenomena in the capillary fringe, which is difficult to reach in a reduced laboratory physical model or in a real contaminated site. This characteristic offers an exceptional opportunity for data acquisition in controlled conditions between laboratory and site scales. This aspect is of importance due to the fact that in this capillary zone, water content varies with depth and this situation causes changes in flow rate affecting the intensity of the pollution flux. This study aimed to quantify, in an experimental way, the pollution flux from the unsaturated zone towards the groundwater.In the first experiment, carried out in the summer (July-September), the infiltrated TCE volume was 5 L. In the second experiment, carried out in autumn (October-December), the TCE volume was 3 L in order to increase the distance between the pollution source and the water-table. These volumes were selected on the basis of previous knowledge of TCE residual saturations determined in laboratory column tests (JELLALI, 2000) and in order to obtain a pollution source limited to the unsaturated zone. The first experiment was conducted without water infiltration to study the dispersion of TCE vapours across the capillary fringe, while the second experiment was carried out with a limited rain infiltration in order to investigate the effect of vapour leaching on groundwater pollution. The transport of TCE was monitored in the vadose zone, the capillary fringe and the groundwater where a comparative analysis of two mechanisms of TCE transfer from the unsaturated zone to groundwater via the capillary fringe was carried out: dispersion and dissolution. In both cases, the coupling of measurements of pollutant concentrations in the unsaturated zone, the capillary fringe and the groundwater of SCERES allowed us to take into account the mechanisms intervening near the source area, on a scale close to that of a real pollution problem. The concentration of dissolved TCE was analyzed by a gas chromatograph equipped with a flame ionization detector (GC-FID) after liquid-liquid extraction with hexane. The online quantitative analysis of TCE vapours was performed by a multigas monitor equipped with a photoacoustic infrared detector.For the first experiment, the TCE mass fluxes from the vadose zone to groundwater were quantified using the method of JOHNSON and PANKOW (1992) where we applied the analytical solution of GRATHWOHL (1998) and incorporated parameters due to the capillary fringe. For the second experiment, we used the numerical code HYDRUS that allowed the simulation of one-dimensional flow in the unsaturated zone. In this example, the TCE mass fluxes leaving the unsaturated zone to the groundwater due to rain infiltration were obtained from the knowledge of the infiltration flow rate and the measured concentrations at the top of the capillary fringe.The observed results indicate that the hydrodynamic dispersion of TCE vapours within the capillary fringe (vertical dispersion) can cause significant groundwater pollution despite the slowness of the aqueous diffusion in the lower region of the highly water saturated capillary fringe. Vapour leaching due to controlled water infiltration causes more significant groundwater pollution in degree and extent than vertical dispersion. The experimental and analytical results demonstrate, on the one hand, the role of the capillary fringe as a barrier against pollution transfer to groundwater when the only mechanism is hydrodynamic dispersion, and on the other hand, significant enhancement of groundwater contamination due to the capture and leaching of vapours from the vadose zone by infiltrating water

Laboratory experiments on DNAPL gravity fingering in water-saturated porous media

International audienceLaboratory experiments were carried out at the Darcy scale to investigate the gravity-driven fingering phenomenon of immiscible two-phase flow of water and a dense nonaqueous-phase liquid (DNAPL) such as trichloroethylene (TCE). Rate-controlled displacement experiments were performed on a homogenous sand-filled column under various displacement conditions. Several system parameters (e.g. flow rate, flow mode (upward flow, downward flow) and mean grain-size diameter of the porous medium) were varied in the experimental programme. Optical fiber sensors were developed to quantify the spatial distribution of the advancing displacement front in a given control section of the experimental device. Following each experiment, multi-point measurements of the remaining TCE saturation were obtained by insitu soil sampling. The resulting DNAPL distribution was heterogeneous even though the medium was homogeneous sand. Higher DNAPL injection rates and lower medium permeability both reduced gravity fingering. This is because viscous forces stabilize the advancing front with pressure gradients increasing as function of the injection rate and decreasing as function of the permeability. Average residual TCE saturations obtained by mass-balance in the experiment after a complete drainage-imbibition cycle were influenced by the mean grain-size diameter of the porous medium but were not affected by the flow mode of the primary drainage process

Dissolution et retention selective d'hydrocarbures en milieu poreux sature. Impact de l'air residuel et role du materiau solide lors de leur propagation

INIST T 73074 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueSIGLEFRFranc

Approche expérimentale et numérique des instabilités de déplacement de DNAPL en milieu poreux saturé à l’échelle de Darcy

Les recherches portent sur une quantification des instabilités de déplacement d’un DNAPL en milieu poreux homogène et saturé. L’approche expérimentale repose sur la mesure des vitesses d’arrivées du DNAPL dans une colonne de laboratoire à différents points d’une section de contrôle au moyen des fibres optiques et sur la cartographie in situ des saturations locales. La simulation numérique du déplacement du front de polluant est abordée par une approche de type réseau des pores et capillaires

Evaluation of VOC fluxes at the soil-air interface using different flux chambers and a quasi-analytical approach

Dense nonaqueous-phase liquids (DNAPLs) spilled on the soil migrate vertically depending upon gravity and capillary forces through the unsaturated zone of the porous aquifer, forming a vapour plume. These volatile organic compounds (VOCs) can be transferred by advection-diffusion to the groundwater or to the atmosphere. Evaluating DNAPL vapour fluxes at the soil-air interface is one of the key challenges in the remediation of contaminated sites. This work discusses the results of a large-scale vapour plume experiment with a well-defined trichloroethylene (TCE) spill, including a sequential raising and lowering of the water table, where the TCE vapour fluxes at the soil surface were experimentally quantified in two ways: (i) directly, with measurements at the soil-air interface using different flux chambers at various operational modes under both transient and steady-state conditions of the vapour plume, and (ii) indirectly, using a quasi-analytical approach based on soil gas measurements. It was shown that upward displacement of the water-air front during the controlled raising of the water table (approximately 10 cm h−1) increased the TCE vapour flux measured at the soil surface by factors of 4 to 10. Under steady-state transport conditions, TCE vapour fluxes measured using five types of flux chambers and three operational modes were similar. The effects of the flux chamber geometry, the accumulation of TCE vapours in the chamber headspace or the air recirculation at a low flow rate on the measured TCE vapour fluxes were low. At steady-state transport conditions, TCE vapour fluxes measured with the flux chambers and estimated using the quasi-analytical approach were of the same order of magnitude. However, under transient conditions of the vapour plume, the TCE vapour flux predicted by the quasi-analytical approach greatly underestimated or overestimated the real TCE vapour flux at the soil-air interface