Theoretical Investigation of Pneumatic Soil Vapor Extraction


Pneumatic Soil Vapor Extraction (SVE) is a new remediation techniquetargeting to improve removal of Volatile Organic Compounds from low permeableareas in heterogeneous soil settings in unsaturated zone. In contrast to traditional SVE,in which soil vapor is extracted continuously by a vacuum pump, pneumatic SVE isbased on enforcing a sequence of large pressure drops on the system to enhance therecovery from the low-permeable areas to enhance removal from areas subject todiffusion limitation. This technique has been shown to be promising at laboratoryscale.A one-dimensional mathematical model was used to study governing factorsand to clarify and quantify the mechanisms responsible for enhanced contaminantremoval during this process. From analytical solution it is clear that the gas phaseinside low permeable area moves with sinusoidal velocity whose amplitude decreaseswith depth. Two zones can be distinguished. First in which the gas phase can reach thehigh permeability area and continuously mixed with clean air, the enhanced removalmechanism is advection. The depth of this zone may range from .05m to .6m. Secondis in which there is no net contaminant advection, the enhanced removal mechanism ishydrodynamic dispersion. The hydrodynamic dispersion coefficient may reach a valuerange from 7 to 700 times the effective molecular diffusion coefficient. In the absenceof non-aqueous phase liquid in the first zone, it can be considered a clean conductivezone and impose no transport resistance on the second zone (i.e. mathematically, theupper boundary can be lowered just below the first zone).The model was tested by comparing its results with experimental resultspublished by a previous study. Overall, comparisons appear to be reasonably good.Investigation shows that pneumatic SVE is promising at field setting. In order for thistechnique has significant removal enhancement the gas phase permeability in the lowpermeability region should be at least on order of 1*10-12 m2 (1 darcy).