Authors

Abstract

Pneumatic Soil Vapor Extraction (SVE) is a new remediation technique
targeting to improve removal of Volatile Organic Compounds from low permeable
areas 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 is
based on enforcing a sequence of large pressure drops on the system to enhance the
recovery from the low-permeable areas to enhance removal from areas subject to
diffusion limitation. This technique has been shown to be promising at laboratory
scale.
A one-dimensional mathematical model was used to study governing factors
and to clarify and quantify the mechanisms responsible for enhanced contaminant
removal during this process. From analytical solution it is clear that the gas phase
inside low permeable area moves with sinusoidal velocity whose amplitude decreases
with depth. Two zones can be distinguished. First in which the gas phase can reach the
high permeability area and continuously mixed with clean air, the enhanced removal
mechanism is advection. The depth of this zone may range from .05m to .6m. Second
is in which there is no net contaminant advection, the enhanced removal mechanism is
hydrodynamic dispersion. The hydrodynamic dispersion coefficient may reach a value
range from 7 to 700 times the effective molecular diffusion coefficient. In the absence
of non-aqueous phase liquid in the first zone, it can be considered a clean conductive
zone and impose no transport resistance on the second zone (i.e. mathematically, the
upper boundary can be lowered just below the first zone).
The model was tested by comparing its results with experimental results
published by a previous study. Overall, comparisons appear to be reasonably good.
Investigation shows that pneumatic SVE is promising at field setting. In order for this
technique has significant removal enhancement the gas phase permeability in the low
permeability region should be at least on order of 1*10-12 m2 (1 darcy).

Keywords