Page 282 - gas transport in porous media

CHAPTER 16

ANALYZING BAROMETRIC PUMPING TO
CHARACTERIZE SUBSURFACE PERMEABILITY

JOSEPH ROSSABI
Redox Tech, LLC, 1006A Morrisville Parkway, Morrisville, NC 27560, USA, rossabi@redox-tech.com

16.1 INTRODUCTION

The prevalence and persistence of subsurface contamination by volatile organic
compounds (VOCs) and the shortage of funds to remediate this contamination below
regulatory action levels has led to a determined search for low cost clean up strate-
gies. As an alternative to long term active soil vapor extraction (ASVE), passive soil
vapor extraction and injection using atmospheric pressure changes as a driving force
is being used at an increasing number of sites. Employing barometric pumping, either
for passive injection of nutrients to enhance bioremediation or passive extraction of
contaminated soil gas, is an easily verified low cost strategy for remediation. For
mass-transfer limited, or remote sites, and also as an interim measure while more
aggressive strategies are being selected, barometric pumping may be an appropriate
cleanup choice. To verify the viability of these passive techniques, subsurface flow
characteristics are vital yet an inexpensive characterization method for acquiring
these parameters must be employed to match the low cost of the passive remediation
systems.
Figure 16.1 illustrates the scenario for barometric pumping through a vadose zone
well. In this figure, the well is screened below a substantial clayey zone. The atmo-
spheric pressure signal will penetrate the subsurface but is delayed and damped by this
low permeability material and the other materials between the screen and the ground
surface. As a result, there will be a pressure difference between the subsurface at the
screen depth (P z ) and the surface (P atm ), hence, flow through the well. Primarily the
effective permeability of the sandy materials in the screen stratum combined with
the magnitude of the pressure difference will control the magnitude of the flow. As
the VOCs in the vicinity of the screened zone slowly diffuse out of the fine grain
materials into the sandy zone, they are removed during regular periods of flow out of
the well. As a result, the magnitude of VOC concentration in the gas phase changes
279
C. Ho and S. Webb (eds.), Gas Transport in Porous Media, 279–290.
© 2006 Springer.