Page 320 - gas transport in porous media

CHAPTER 19

IN SITU MEASUREMENT OF INDUCED
CONTAMINANT FLUX

ANDRE TARTRE
EcoRemediation Inc., 70 South Winooski, Suite 201, Burlington, VT 05401, USA

19.1 INTRODUCTION

Estimates of in situ migration of contaminants are typically inferred from discrete
samples of soil or groundwater concentrations. Based on the spatial distribution
of concentrations, migration of contaminants is then estimated based on transport
models and site-specific hydrogeologic parameters, which are obtained from lab-
oratory analyses or field tests (pump or tracer tests). This chapter introduces an
alternative method to directly measure the in situ mass flux of contaminant vapors
directly.
The proposed sampling technique starts by purging the subsurface with a non-
contaminated gas for several minutes. The same fluid is collected away from the
injection point, and concentrations are monitored. Stabilized concentrations reached
during the purge are used to estimate the flux of compounds (analytes) immediately
available for migration. We term this flux the “advective flux.” When concentrations
stabilize (i.e., reach a chemical equilibrium), the purge is stopped. Rebound con-
centrations are monitored over a short period of time to evaluate the migration of
compounds induced by the artificial chemical gradient. If there is no advective flux,
rebounds are used to estimate migration due to molecular diffusion. We define this
flux as a “ diffusive flux.” This sampling technique may be used as an extension of
conventional soil-gas investigations. These in situ flux measurements may, in turn,
be used to estimate contaminant migration or biodegradation rates.

19.2 THEORY

Development of the “induced flux measurement technique” occurred during two-
years of research conducted in Canada for investigating releases from underground
storage tanks (USTs) (Tartre, 1997). This technique is built on the same theoretical
319
C. Ho and S. Webb (eds.), Gas Transport in Porous Media, 319–330.
© 2006 Springer.