Soil flushing is a technology used for extracting contaminants from the soil. It works by applying water to the soil. The water has an additive that enhances contaminant solubility. Contaminants that are dissolved in the flushing solution are leached into the groundwater, which is then extracted and treated. In some cases, the flushing solution is injected directly into the groundwater. This raises the water table into the capillary fringe just above the surface of the water table, where high concentrations of contaminants are typically found. In many instances, surfactants (i.e., detergent-like substances) or solvents are used as the additive. The effectiveness of this process is dependent on hydrogeologic variables (e.g., type of soil, soil moisture) and the type of contaminant.
Limitations and Concerns
Flushing additives may leave small residuals in the soil or groundwater, and they should be evaluated on a site-specific basis.
Flushing may spread the contaminant beyond the capture zone or to introduce surfactants to the subsurface, so the technology should be used only where flushed contaminants and soil-flushing fluid can be contained and recaptured.
Additives must be recovered from the underlying aquifer and, when possible, should be recycled.
Treatment of the recovered fluids forms residual sludges that must be treated or disposed.
Recovered groundwater may need treatment to meet appropriate discharge standards.
Low permeability soils, such as clays, are difficult to treat with this method.
Surfactants may reduce soil porosity, and therefore they should only be used on a case-by-case basis.
Technology Development Status
Soil flushing is proven technology. Typically, laboratory and field treatability studies must be performed under site-specific conditions before soil flushing is selected as the remedy.
See http://www.clu-in.org/products/isf/ for a project database.