Soil Vapor Extraction (SVE) removes volatile and some semi-volatile contaminants from the vadose zone, through the application of a vacuum, using a system of well vents placed in the unsaturated zone. These systems are designed to remediate residual contaminants in unsaturated soils and can be combined with groundwater pumping wells to remediate soil previously beneath the water table. The gas removed from the soil may be then discharged directly to the air or, more likely, to an off-gas treatment process. The residual soil concentration gradually becomes richer in less volatile compounds. Consequently, as a result of compositional changes, the vapor concentrations of the contaminants and mass removal rates decrease with time 3. Water may be withdrawn from the subsurface in separate or combined extraction wells. The fundamental principles of contaminant equilibrium between soil and air are basically the same as those between soil and water. However, soil and water systems must consider hydrophobic or hydrophlic effects on the sorption process.