 The vacuum extraction of soil gas induces gas flow across a site, increasing the mass transfer driving force from aqueous (soil moisture), non-aqueous (pure phase), and solid (soil) phase into the gas phase.Air flow across a site is thus a key aspect, but soil moisture and subsurface heterogeneity (i.e., a mixture of low and high permeability materials) can result in less gas flow across some zones.In some situations, such as enhancement of monitored natural attenuation, a passive SVE system that relies on barometric pumping may be employed.