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Chapter 19: In Situ Measurement of Induced Flux
19.7
OPTIMIZATION OF EXTRACTION PROCESSES
The induced in situ flux sampling technique may also be used to optimize extraction
processes during remediation efforts. Often, extraction processes are limited after
a short period by diffusion of the contaminant from less permeable zone to more
permeable zone. Knowledge on exact locations of high advective flux is helpful to
target more precisely subsurface location where extraction will be driven mainly by
advection. The proposed equipment for existing well may be used to define clearly
subsurface zone with highest advective flux.

19.8 CASE STUDIES
19.8.1 Case Study “1”

An initial investigation to characterize subsurface conditions was conducted beside
a gas station where a gasoline leak was suspected. Some smells and vapors were
previously detected in subsurface utilities under the street just beside this gas station.
The preliminary investigation consisted to dig beside the gas station nearby the street
to localize potential contamination. At one location, fresh gasoline was observed at
about 8 feet below grade surface (BGS). A soil gas survey was then proposed to the
city to determine the extent of the contamination and possible migration pathways
under the street.
The “In Situ flux method” was conducted on few sampling points during this soil
gas survey. The result at point “1” where fresh gasoline was observed is presented
in Figure 19.7. The presence of fresh gasoline beside the sampling probe induced an
immediate rebound of vapor concentrations after the purge with nitrogen. Considering
other results from adjacent sampling locations, it was clear during investigation that
the source of the contamination under the street was originating from location “1”.
The results at point “16” is presented also in Figure 19.7. This sampling location
was someway apart from the plume. Obviously, rebound of vapors is less important,
which indicates clearly that dissolved or adsorbed gasoline is at some distance from
this sampling location.
A few months after this sampling effort, an extensive investigation with standard
soil/groundwater samples shows exactly the same extent of contamination with the
same migration pathways than the soil gas survey and the “In Situ flux method”.

19.8.2 Case Study “2”
The “New Hampshire Department of Environmental Services” wanted to determine
if the contamination at a site was spread uniformly, or if some hot spot could be
determined to reduce cost of the remediation effort. A soil gas survey was conducted
and few “in situ flux measurement” were performed.
The site was mainly contaminated with a petroleum product, nearby a former
“Varsol” distilling process used since World War Two. During the investigation,