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V Tartre
Q Q
G

Where
G = Generation rate
Q = Flow rate, or extraction rate
V = Volume
Figure 19.1. Basic concept for the induced In Situ flux method : concentrations fluctuate depending on
generation rates and extraction rates

foundation as well-established procedures for assessing the ventilation efficiency
within buildings. The procedure involves locally ventilating the UST backfill with
fresh atmospheric air over a range of flow rates, while concurrently measuring
increases or decreases in gas or vapor concentrations due to flux from the surrounding
environment. Fluctuations in concentrations under these imposed dynamic conditions
permit an estimate of gas or vapor flux within the backfill.
When a closed (or reasonably confined) space is ventilated, gas concentrations
fluctuate depending on both the generation rate and the applied ventilation flow
rate (extraction rate) (National Safety Council, 1995). This relationship between
ventilation flow rates and gas generation is illustrated in Figure 19.1. If the generation
rate is greater than the extraction rate, gas concentrations increase. Conversely, if the
flow rate is greater than the generation rate, gas concentrations decrease.

19.3 DESCRIPTION OF THE TECHNIQUE IN THE VADOSE ZONE
The proposed technique purges the vadose zone in the vicinity of a sampling probe
with a non-contaminated gas during soil-gas investigation. The equipment used to
perform this task is presented in Figure 19.2. Purging the soil gas with nitrogen
or other inert compounds affects the gas-liquid-soil equilibrium, causing compounds
(analytes) that are present in sorbed or dissolved phases to partition into the gas phase.
During the ventilation period, chemical equilibrium among soil phases is disrupted
such that the rate at which vapor contaminants and biogenic gases are transferred to
the soil gas may be estimated. We define this mass transfer as a generation rate. When
a void is created in situ by the probe, this mass transfer is divided by the exposed
surface of the void to obtain the flux of gas and vapors immediately available for
migration. We term this flux “advective flux.”
After conditions begin to stabilize, the ventilation is significantly reduced or
stopped altogether. Subsequent increases in analyte concentrations, as well as the
length of the lag time between purge cessation and concentration rebounds, are used