Vapor Intrusion - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 441

10.5.2.5 Soil Sampling

Soil sampling is generally not recommended for assessing subsurface vapor intru-
sion to indoor air because there are no published studies that clearly show a
unique relationship between measured soil concentrations and measured soil gas
concentrations. The poor correlation may be attributable to inconsistent amounts
of volatilization losses during soil sample collection (EPA 1993a ). The Encore TM
Sampling device has been developed to manage the volatilization losses, but the
field preservation with methanol results in a solvent peak during analysis that results
in elevated detection limits, which may fail to identify the presence of volatile
contaminants at concentrations of potential concern for vapor intrusion.

10.5.3 Analytical Methods

The selection of appropriate laboratory analytical methods should be made by devel-
oping a list of volatile contaminants known or suspected to be present at a given site
in the subsurface at concentrations above regulatory screening levels, and discus-
sion with a laboratory skilled in analysis of air samples. Analytical reporting limits
for indoor air samples should be lower than target indoor air concentrations, unless
this is technically impracticable. Analytical reporting limits for soil gas samples can
be higher, because soil gas concentrations are always attenuated to some degree
by building ventilation and other processes. Target detection limits, possible back-
ground levels, and sample volumes may all require consideration in the process of
selecting the most appropriate sampling and analytical methods.
Vapor samples may be collected in a variety to devices, including rigid Summa TM
canisters, flexible Tedlar bags, and sorbent tubes. Summa TM canisters are stainless
steel containers with interior surfaces coated with glass. These containers are advan-
tageous because they are a whole air sample, and there is often sufficient volume in
the canister to repeat an analysis if there are questions. Holding times up to 30 days
are often acceptable.
Tedlar bags are convenient and cost-effective for soil gas and air sampling.
However, they have some limitations that are important to consider. In particular,
the holding time for Tedlar bags is shorter than for other containers (typically a few
days or less), recovery of certain contaminants is poor, and at very low levels there
may be detectable concentrations of some VOCs even in brand new Tedlar bags
(Hayes et al. 2006 ). Tedlar bags are preferred containers for field screening, and
VOC analysis with a mobile laboratory, and where reporting limits are not extremely
low (i.e., >5 parts per billion by volume [ppbv], or about 0.001 g/L).

Automatic thermal desorption (ATD) tubes or Volatile Organic Sampling Train
(VOST) tubes are cylinders packed with adsorptive media through which air is
drawn at a measured rate for a measured time. The mass trapped on the tube is
determined by laboratory analysis and the concentration is calculated by dividing
the mass by the product of the flow rate and time (i.e., volume of air passed through
the tube). ATD or VOST tubes are better suited to heavier contaminants, although

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