Incidence of surfactant decontamination technique on the hydraulic conductivity of a controlled aquifer polluted by diesel oil
In case of groundwater contamination by hydrocarbon spills, one of the main problems is how to recover residual hydrocarbons trapped in the porous medium, forming a long term pollution source. In order to develop a better approach to such problems, experiments were conducted in a large experimental controlled site, called SCERES (Site Contrôlé Expérimental de Recherche pour la Réhabilitation des Eaux et des Sols) made of an impervious concrete basin (25 m x 12 m x 3 m) packed with two layers of quartz sand and fitted with specific instrumentation. This research site was experimentally polluted with diesel oil (476 litres), then the shape of the impregnation body was identified and oil saturation values were quantified thanks to a specific coring programme before and after each step of the experiments (water table fluctuations, hydraulic pumping, surfactant infiltration, etc.). The principle of the remediation technique is based on a surfactant flushing from the soil surface. This process was carried out when all of the removable diesel oil had been recovered by hydraulic pumping in the central recovery well. The surfactant infiltration allowed a displacement of the main part of the residual pollutant in the vertical zone of the impregnation body, in spite of a progressive reduction of hydraulic conductivity leading to a plugging of the porous matrix. This phenomenon has been observed by many scientists (Allred et Brown, 1994 ; Celik et al., 1982) and explained as being caused by a precipitation of the anionic surfactant in presence of calcium ions. This decrease of hydraulic conductivity, due to surfactant infiltration and its effects, was studied in the laboratory by the implementation of experiments using columns filled with the same sand as in the SCERES basin. The results showed that the reduction of the hydraulic conductivity of the porous medium cannot be due to the precipitation of the anionic surfactants in presence of calcium ions of the sand. If this were situation, all the porous medium in the column would have exhibited this loss of hydraulic conductivity. In our case, only the upstream part of the physical model was influenced by this phenomenon. Furthermore, the sand devoid of its clay minerals has nearly the same behaviour as a natural sand, with respect to the infiltration flowrate of the surfactants. This proves that the clay minerals, present in small proportion in this matrix, are not mainly responsible for the plugging problem. The influence of the hydraulic gradient, another studied parameter, was observed and permitted to apprehend the behaviour of the surfactant solution during its infiltration through the porous medium. . The increase of this parameter, even if it allows the injection of more surfactant, could not resolve the problem of the reduction of hydraulic conductivity. Finally, all the experiments indicated that the plugging effect (cancellation of the flowrate) happened after nearly 20 hours of surfactant infiltration through the porous matrix. The time parameter seems to be important since its variation can influence the stability of the surfactant solution. The results indicated that we can infiltrate a greater volume of the solution if it has been recently prepared. In other words, this is certainly due to the time necessary for the formation of liquid crystals obtained in presence of the calcium ions in the water used for the preparation of the solutions (tap water). After nearly 20 hours these crystals should have attained a size sufficient to be retained by a filtration effect, near the soil surface. We suggest that this filtration is the major factor responsible for the reduction of hydraulic conductivity and the plugging. The present study shows the feasibility of this process and it highlights the need to conserve the initial characteristics of the surfactants (especially the size stability of the colloidal particles) during all the treatment. Because of the agglomeration of the micelles, we have to search for means to ensure a good dispersion in the aqueous medium, for example by adding a solvent agent.