EXPERIMENTAL METHODS
probe was driven to 1 m. After this depth was
reached, the hammer cap was removed and lock-
Initially, a laboratory experiment was con-
ing pliers were clamped onto the top of the
ducted to see if VOC vapors could be quantita-
probe. Next the entire rod was rotated a half turn
tively transferred from the probe's inlet to a syr-
and raised approximately 2 cm. Lifting the probe
inge and then into a sealed VOA vial. A 22-mL VOA
separates it from its tip, creating approximately a
vial was attached to the tip of the probe using strips
3-mL void in the subsurface.
of Parafilm "M," and then VOCs were spiked into
Soil-vapor samples of 0.5 mL were obtained as
this empty chamber. The volume inside the VOA
follows. After 30 minutes was allowed for equili-
vial was estimated to be around 21 mL. Once the
vial was securely attached, a 2-L volume of a
bration, the transfer line was flushed by pulling a
3.5-mL volume of soil gas through it via a 3-mL
methanol (MeOH) stock standard, containing
Luer Lok syringe fitted with a 22-gauge stainless
trans-1,2-dichloroethene (TDCE), cis-1,2- dichlo-
steel needle (Becton Dickinson). Both longer and
roethene (CDCE), and TCE, was injected into the
vial using a 10-L syringe (26-gauge needle) that
shorter equilibration periods were studied to see
how long it takes for TCE vapor concentrations
punctured the Parafilm "M" wrapping. After a 5-
to stabilize and to see how they fluctuate with
minute wait for the MeOH and analytes to evap-
time. For this study, all soil-gas samples were col-
orate, a 3.5-mL volume of vapor was purged
lected in at least triplicates, using a 0.5-mL gas-
through the transfer line of the soil-gas probe
tight syringe with a 22-gauge needle (Hamliton),
with a 3-mL Luer Lok syringe (actual volume 4
and were immediately transferred to 22-mL VOA
mL). Next, three consecutive 0.500-mL volumes
vials sealed with Teflon-lined butyl rubber septa
were obtained with a gas-tight syringe (Hamil-
and aluminum crimp tops (Wheaton). Further-
ton) and each was transferred to a sealed VOA
more, the probe's transfer line was always
vial. For comparison, three working standards
purged before soil-vapor samples were collected.
were similarly prepared in 22-mL VOA vials. A
Before the soil-vapor sample was transferred
0.500-mL volume was removed from each and
to a VOA vial, a 22-gauge disposable needle was
transferred to a sealed VOA vial.
inserted through the septum to serve as a pres-
During the field trials, active soil-vapor and
sure vent for the sample container. When the
discrete soil samples were collected from 16 sep-
sample was injected into the VOA vial, the tip of
arate locations at CRREL in Hanover, New
the pressure vent needle was positioned near the
septum's Teflon face, while the tip of the sample
the vadose zone, ranging from less than 0.001
needle was placed near the middle of the vial. All
mg/kg to more than 100 mg/kg, attributable to
collections and transfers with these gas-tight syr-
accidental spills more than 20 years ago (Hewitt
inges were done cautiously and slowly, so their
1994). The site's typical soils at 1 m depth are
seals would not be bypassed. Furthermore,
silty-sands, with a moisture content of around 21
7% and an organic carbon content that is less
because needles can easily become clogged by a
piece of septum, they were frequently checked
than 0.5% (percent organic carbon anticipated on
by making them blow bubbles through water.
the basis of previous measurements of soils from
For example, samples of soil vapor were routine-
this depth). At each location, soil-vapor samples
ly taken by slightly over-filling the syringe, then
were collected first and then a discrete soil sam-
pushing one or more bubbles out prior to setting
ple was obtained.
the volume to 0.500 mL and transferring the soil-
The following is an outline for the sampling
vapor sample into a VOA vial. This procedure
protocol used to collect both the soil-vapor and
was also used with the Luer Lok syringe, espe-
matrix samples. Before the soil-vapor probe was
cially after a new septum was installed in the
pushed into the soil, a 0.32-cm-diam. solid steel
soil-gas probe.
rod was manually pushed approximately 60 cm
When the soil-vapor samples were completely
into the ground to check for interfering obstruc-
collected at a given location, the probe was
tions (rocks and cobble) close to the surface.
pulled out of the ground using the locking pliers
After this small-diameter rod was removed, the
as a handle. If the probe could not be easily freed
active soil-gas probe, with its disposable tip, was
from the substrate, the locking pliers were
pressed into the soil surface at the same penetra-
tapped in a upwards direction with the 2.5-lb
tion point. Once the probe was in place, the ham-
hammer. Once the probe was retrieved, a Veihm-
mer cap was put on top and repeatedly hit with a
eyer tube was inserted into its channel to make a
2.5-lb (approximately 1-kg) hammer until the
3