Environmental Engineering Reference
In-Depth Information
sensitive to water vapor, and since soil gas is generally humid, a water trap is recom-
mended, especially when the air temperature is lower than the ground temperature,
and the risk of condensation is increased. FIDs are not sensitive to water vapor, but
they require a source of hydrogen gas to fuel the flame, and are somewhat more
challenging to operate.
In general, hand-held PIDs provide reproducible total VOC readings within the
range of 1 to 10,000 ppmv. FIDs can provide reproducible readings at somewhat
lower concentrations, but still generally above 0.1 ppmv. Target indoor air concen-
trations are generally lower than these instruments are capable of detecting, but
they are very useful for soil gas screening, because soil gas quality standards are
generally about 100 times higher than indoor air risk-based quality standards.
10.5.4.2 Landfill Gas Meters for Oxygen, Carbon Dioxide and Methane
Concentrations
Landfill gas meters are available to quantify oxygen (O
2
), carbon dioxide (CO
2
) and
methane (CH
4
) concentrations to low percent levels. Landfill gas meters are useful
for field screening of soil gas prior to sample collection to confirm steady readings,
much as specific conductance, temperature, turbidity and dissolved oxygen are used
for monitoring groundwater purging prior to sample collection. At sites with aer-
obically degradable contaminants (especially hydrocarbons), it is common to find
soil gas with oxygen concentrations that are clearly lower than atmospheric lev-
els, and carbon dioxide concentrations that are clearly elevated above atmospheric
levels, which can be very informative for assessing the influence of biodegradation
on the potential for vapor intrusion. The detection limits for these instruments is
commonly in the low % range (rather than parts per million by volume [ppmv] or
ppbv), but this nevertheless provides ample resolution for field screening for these
parameters.
10.5.4.3 Hexafluoride and Helium Meters
Helium (He) and sulfur hexafluoride (SF
6
) are the most commonly used tracers for
air flow and building ventilation testing. Helium is inexpensive, readily available,
non-toxic, and easily detected in the range of 0.01 to 100% using portable helium
meters, which provides 4 orders of magnitude resolution for tracer testing. Helium is
very useful for leak testing, such as where a small volume of helium is sufficient to
test seals in sampling trains. SF
6
meters are sensitive to much lower concentrations
(
100 ppbv), so much less tracer gas is required. Therefore, SF
6
is a preferred
tracer when testing air flow in large volumes (e.g., entire building ventilation tests,
as described by Howard-Reed et al. (
2002
)).
∼
10.5.4.4 Mobile Laboratories
Mobile laboratories for soil gas surveys have been available in various forms for
almost two decades, however, in many cases, the reporting limits were in the range
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