text: The effectiveness of SVE, that is, the rate and degree of mass removal, depends on a number of factors that influence the transfer of contaminant mass into the gas phase.  The effectiveness of SVE is a function of the contaminant properties (e.g., Henry’s Law constant, vapor pressure, boiling point, adsorption coefficient), temperature in the subsurface, vadose zone soil properties (e.g., soil grain size, soil moisture content, soil permeability, soil carbon content), subsurface heterogeneity, and the air flow driving force (applied pressure gradient).  As an example, a residual quantity of a highly volatile contaminant (such as trichloroethene) in a homogeneous sand with high permeability and low carbon content (i.e., low/negligible adsorption) will be readily treated with SVE.  In contrast, a heterogeneous vadose zone with one or more clay layers containing residual naphthalene would require a longer treatment time and/or SVE enhancements.  SVE effectiveness issues include tailing and rebound, which result from contaminated zones with lower air flow (i.e., low permeability zones or zones of high moisture content) and/or lower volatility (or higher adsorption).  Recent work at U.S. Department of Energy sites has investigated layering and low permeability zones in the subsurface and how they affect SVE operations.