Soil flushing: a review of the origin of efficiency variability

Soil flushing using aqueous solutions is employed to solubilise contaminants. As water solubility is the controlling mechanism of dissolution, additives (surfactants, cosolvents, etc.) are used to enhance efficiencies and reduce the treatment time compared to the use of water alone. The use of surfactant alone gives efficiencies of about 80–85 % in laboratory experiments, but the amounts of product to be injected are very important, which does not seem to be economically sustainable. Studies indicate that when soil flushing is applied in the field, efficiency is very variable; it can vary from almost 0 % to almost 100 %. This illustrates the importance of knowledge of the field (soil heterogeneities, type of contamination, etc.). Using only one product (surfactant, cosolvent, cyclodextrin) often gives moderate efficiencies and needs very large amounts of products, with a product:pollutant ratio higher than 100:1. On the other hand, the use of more complex methods involving micro emulsions or several products with polymer injection lead to high efficiencies at first and a product:pollutant ratio that can be lower than 5. The importance of the initial saturation of the non-aqueous phase liquid is highlighted: the higher the initial saturation, the higher the efficiency. For initial saturations lower than 1 %, soil flushing may not be a very efficient technique. This paper provides an overview of recent studies in the area of soil and groundwater remediation, from laboratory columns scale to pilot and real sites. The research has focused on chlorinated solvents as they are extremely difficult to treat.