Page 324 - gas transport in porous media

Chapter 19: In Situ Measurement of Induced Flux
and more relevant measure of mass transfer and mass production rates for a limited
volume of soil, as small as few cubic feet. 323
19.4 EVALUATION OF ADVECTIVE FLUX

The proposed sampling technology is the only one so far that evaluates the potential
for migration directly, in situ, using the contaminant of interest. This technology
offers an interesting alternative method to standard sampling practices.
During the purging period, advective flux is quantified when stable concentrations
are reached due to the presence of free products or due to immediate desorption of
contaminants from the soil matrix.
In the vadose zone, advective flux of volatile contaminants correlated well with
concentrations of specific compounds in the surrounding soil (Tartre, 1997) (e.g., :
r = 0, 88 for BTEX). The proposed In Situ flux method may then be used in the
vadose zone for most of volatile compounds (petroleum and chlorinated) to identify
on site sources of contamination and plume extent. This method is particularly cost
effective with gas and vapor sampling comparing with standard “soil/groundwater”
sampling methods.
With a minimal cost, the proposed method can be used with direct reading instru-
ments as a preliminary investigative tool to delimit plume and to identify sources. For
instance, the difference between gas and vapor rebounds after the purging period is
obtained with specific reading instruments. This sampling method is a powerful tool
to delimit a plume of volatile compounds at low cost.
Currently, the method may be applied in two (2) ways. Clean gas may be injected,
along with a tracer, directly into the subsurface under positive pressure so that advec-
tive flux may be evaluated through porous material. Alternatively, clean gas may be
injected into a controlled volume at an inflow rate that equals the outflow rate for more
impervious material, which is used to evaluate advective migration via channeling.
The steady state concentration reached during the purging period (concentration
(A) in Figure 19.4) is multiplied with the effective flow rate of the clean gas in order to
evaluate the rate at which contaminants are partitioning (from non-aqueous or sorbed
phases). This rate divided by the surface of the soil void provides in situ flux of
compoundsimmediatelyavailableformigration, identifyasapotential advective flux.

19.5 EVALUATION OF DIFFUSIVE FLUX
The proposed sampling technique can also be used to directly evaluate diffusive flux.
When the effective flow rate of the purging fluid approaches the generation rate of
analytes, the purge is halted. Rebound concentrations are monitored within the first
few minutes. This monitoring permits a direct estimate of analytes migrating in soil
gas under an artificial chemical gradient. This monitoring provides a direct evaluation
of molecular diffusion for a specific subsurface location.
The diffusive flux is then estimated by considering the slope obtained at a specific
inducedchemicalgradient.Asanexample, theinduceddiffusivefluxmaybeevaluated