Theoretical investigation of pneumatic soil vapor extraction

Abstract EN

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).