Page 293 - gas transport in porous media

CHAPTER 17

SUBSURFACE FLOW MEASUREMENTS

JOSEPH ROSSABI
Redox Tech, LLC. 1006A Morrisville Parkway, Morrisville, NC 27560, USA

17.1 INTRODUCTION

Gas movement in the subsurface is predominately driven by diffusion, but there are
many important situations during which diffusion is not the principal transport mech-
anism. Soil vapor extraction or injection systems designed to remove volatile organic
contaminants or provide nutrients to subsurface organisms generally use powerful
mechanical pumps that induce advective gas flow in the subsurface. Natural pump-
ing caused by pressure differences between the surface and subsurface as a result of
atmospheric pressure changes, wind and terrain, or even earth tides can also produce
flows on the order of, or significantly higher than diffusion. Biological activity, chem-
ical and thermal changes such as those that occur in landfills can also lead to localized
pressure differences that cause subsurface advective flow. During these times espe-
cially, knowledge of the gas flow is important for an understanding of the movement
of chemicals and contaminants.
The measurement of gas flow in the subsurface like measurement of liquid flow is
rarely made in situ or over the entire flow field, rather these measurements are made
at endpoints usually accessed via wells although emplaced sensors are occasionally
employed. At these endpoints, sensors are placed and are used to sample flow, or
tracer materials are introduced and collected. From these point measurements and a
conceptual model of the structure and parameters of the subsurface materials, flow
through the media is inferred. The range of flow in the subsurface can be very large
corresponding to the range of permeability found in subsurface media. The degree
of accuracy and precision needed for a particular monitoring situation will also vary.
As a result, different methods for measuring flow should be selected that are suitable
for the flow range of interest and that satisfy the appropriate data quality objectives.
For example, passive injection using barometric pumping operating at a nominal flow
rate of 10 to 50 liters per minute will likely require a different measurement technique
3
than a multi-well soil vapor extraction unit operating at 15 m per minute.
291
C. Ho and S. Webb (eds.), Gas Transport in Porous Media, 291–302.
© 2006 Springer.