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Experimental and modeling studies were performed to investigate the simultaneous transport of trichloroethylene (TCE) and perchloroethylene (PCE) in the vadose zone of a large (25 by 12 by 3 m) well‐instrumented artificial aquifer called SCERES. The experimental facility, made up of a 1‐m‐thick saturated zone and a 2‐m‐thick unsaturated zone, allowed direct measurements of the contaminants in both the liquid and gas phases. Main objectives of the study were to obtain a better understanding of the fate and transport of chlorinated solvents in the subsurface and, more specifically, to compare simultaneously measured TCE and PCE volatilization rates from the soil surface with predictions obtained with both a comprehensive multiphase multicomponent numerical model (SIMUSCOPP) and a quasianalytical approach based on Fick's first law. The numerical and quasianalytical results generally agreed very well with the observed data. Transient PCE and TCE vapor phase concentrations calculated with the numerical model were found to be close to the observations, which indicated applicability of Raoult's Law. A comparison of observed and calculated TCE and PCE concentrations in the capillary fringe showed more impact of water infiltration on the simulations as compared with the observed data, which may reflect a lack of equilibrium between the gaseous and aqueous phase during leaching for the given experimental flow conditions. A sensitivity analysis showed that the adopted source boundary condition (a fixed nonaqueous phase liquid [NAPL] saturation distribution instead of an injected DNAPL source) did not have much influence on the concentration breakthrough curves, but that temperature can be an important factor influencing the results.

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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

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 quasianalytical 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.

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After the contamination of an aquifer by petroleum products, the residual oil trapped is a constant source of pollution by the entrainment of the most soluble hydrocarbons. By studying the exchanges of residual hydrocarbons between oil-water-air and soil, we pointed out that the liquid/gas exchange is the major factor of retention of soluble alkanes masking the adsorbing materials effects. For the soluble aromatic hydrocarbons, the main phenomenon observed is the liquid/solid exchange. The role of residual air is no more preponderant. The residual contamination of the vadose zone thus plays a preponderant role in the long-term pollution of a groundwater table. It is thus imperative to implement methods to prevent such harmful effects. The use of surfactants, by lowering the oil/water interfacial tension seems to be a new and effective method. Their adsorption into a natural matrix was studied with different porous substrates (sand, sand/silt). Their retention on sand is poor, but it increases with silt content. This is mainly due to a cationic exchange (Ca2+/Na+). In order to avoid this phenomenon a salt preflush by a 10 g/l Na Cl solution is effective. That allows a gas-oil recovery enhancement by reducing loss of surfactant in soil. Moreover, a surfactant partition between oil and water is underscored. A better understanding of these parameters would lead to the optimizing of the enhanced drainage technique for recovering residual oil trapped in an aquifer.

Présenté par Ghislain de Marsily RésuméLa caractérisation de zones sources de pollution par la méthode du traceur bisoluble nécessite la détermination du coefficient de partage des traceurs avec le polluant. Des essais réalisés en réacteur fermé (statiques) et en colonne de laboratoire (dynamiques) nous ont permis de mettre en évidence une cinétique de partage entre le traceur bisoluble (4-méthyl 2-pentanol) et la phase organique (trichloréthylène). Cette cinétique de partage est confirmée par l'interprétation des courbes de sortie des traceurs à l'aide d'un modèle de cinétique d'échange.