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Henry's Law constant is an indicator of how readily dissolved vadose zone contaminants will volatilize by a vapor extraction system Less volatile petroleum hydrocarbons may also be treated by DPE Introduction of oxygen into the subsurface during the vapor extraction process stimulates aerobic biodegradation of nonchlorinated(and ' some chlorinated)hydrocarbon compounds and can promote in situ remediation of soil contaminants that would not typically be volatilized and removed by the extraction system Biological processes have been shown to playa sigmficant role in remediation of petroleum hydrocarbons at sites employing DPE(Roth and others 1995) 4.22 Contaminant Phases ' DPE systems can be implemented to target all phases of contamination associated with a typical NAPL spall site These systems remove residual vadose zone soil contamination residing in soil gas, dissolved in soil pore-space moisture, and adsorbed to soil particles DPE also effectively removes dissolved and free- phase{both light and dense NAPL [LNAPL and DNAPL])contamination in groundwater Remediation capabilities of DPE in the vadose zone are similar to those of SVE Because it uses 1n-well ' groundwater extraction,however,higher vacuums can typically be applied at DPE sites without concerns related to groundwater upwelling As a result, DPE may also accelerate volatilization and removal of ' vadose zone contaminants over traditional SVE DPE can be implemented for remediation of the capillary fringe and smear zone VOC concentrations are typically highest in capillary fringe soils because of the tendency of LNAPL to accumulate at the water table Changes in water level move any accumulation of free product on the surface of the water table and create a smear zone of residual contamination SVE systems are typically ineffective at volatilizing contaminants in the capillary fringe and smear zone because of their high water content and low effective air-filled porosity of these soils In addition,water table upwellnng at the point of extraction in an SVE system can submerge residual contamination and prevent removal by the vapor extraction system Dewatenng from the extraction well itself not only counters upwellmg effects but results in a cone of groundwater depression A cone of depression allows soil vapor flow induced by the extraction well 4-3 1 1