Mobility of soluble and non-soluble hydrocarbons in contaminated aquifer

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.