Remediation techniques

Soil vapour extraction (SVE) is a relatively simple process that physically separates contaminants from soil. As the name suggests, SVE extracts contaminants from the soil in vapour form. Therefore, SVE systems are designed to remove contaminants that have a tendency to volatilise or evaporate easily. SVE removes volatile organic compounds (VOCs) and some semi-volatile organic compounds (SVOCs) from soil beneath the ground surface in the unsaturated zone (that part of the subsurface located above the water table). By applying a vacuum through a system of underground wells, contaminants are pulled to the surface as vapour or gas. Often, in addition to vacuum extraction wells, air injection wells are installed to increase the air flow and improve the removal rate of the contaminant. An added benefit of introducing air into the soil is that it can stimulate bioremediation of some contaminants.

Used alone, soil vapour extraction cannot remove contaminants in the saturated zone of the subsurface (the water-soaked soil that lies below the water table). At sites where contamination is in the saturated zone, a process called air sparging may be used along with the SVE system. Air sparging means pumping air into the saturated zone to help flush (bubble) the contaminants up into the unsaturated zone where the SVE extraction wells can remove them (as show in the diagram below).

For air sparging to be successful, the soil in the saturated zone must be loose enough to allow the injected air to readily escape up into the unsaturated zone. So air sparging works fastest at sites with coarse-grained soil, like sand and gravel.

SVE and air sparging may be good choices at sites contaminated with solvents and other volatile organic compounds (such as trichloroethane, trichloroethylene, benzene, toluene, ethylbenzene, and xylene) and fuels. Because properties of the soil have such an important effect on the movement of soil vapours, the performance and design of SVE and air sparging systems depend greatly on the properties of the soil. SVE is best used at sites with loose unsaturated soil, such as sand, gravel, and coarse silt or fractured bedrock. However, it has been applied to sites with denser soils, although treatment may take longer.