Enhancements for improving the effectiveness of SVE can include directional drilling, pneumatic and hydraulic fracturing, and thermal enhancement (e.g., hot air or steam injection).  Directional drilling and fracturing enhancements are generally intended to improve the gas flow through the subsurface, especially in lower permeability zones.  Thermal enhancements such as hot air or steam injection increase the subsurface soil temperature, thereby improving the volatility of the contamination.  In addition, injection of hot (dry) air can remove soil moisture and thus improve the gas permeability of the soil.  Additional thermal technologies (such as electrical resistance heating, six-phase soil heating, radio-frequency heating, or thermal conduction heating) can be applied to the subsurface to heat the soil and volatilize/desorb contaminants, but these are generally viewed as separate technologies (versus a SVE enhancement) that may use vacuum extraction (or other methods) for collecting soil gas.