Vapor Intrusion - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 434

concentrations. Therefore, indoor air quality measurement is generally not simple
or unambiguous, and sampling should include ambient as well as indoor air.
Soil gas sampling and analysis faces different challenges, particularly related to
sample representativeness. When vapor intrusion is a bona fide concern, soil vapor
concentrations are typically higher than indoor air concentrations by a factor that
accounts for the amount of attenuation in concentrations that occurs as subsurface
vapors migrate to indoor air (generally at least a factor of about 100). Therefore, it
is generally easier to resolve soil gas concentrations of concern against background
concentrations and analytical detection limits.
Groundwater sampling and analysis methods are generally more mature than soil
gas sampling methods. However, groundwater is furthest removed from the receptor,
and the estimation of indoor air concentrations from groundwater data therefore
involves a substantial additional uncertainty.
It is important to design the site-specific assessment with consideration of the
challenges facing each sampling and analytical approach. In many cases, multiple
lines of evidence may be advisable to avoid potential biases inherent in any single
method. This section describes the theoretical considerations, which will help the
practitioner in study design.
10.5.2 Pros and Cons of Sampling for Various Soil Compartments
Samples for contaminant analysis may be collected from several different compart-
ments during the course of a vapor intrusion pathway evaluation, including indoor
air, soil gas (sub-slab or in native material), and groundwater. Samples from these
different compartments have their own particular uses, benefits, and cautions as
summarized in Table 10.2 . More detailed descriptions of the benefits and limitations
of the various sample collection options are included in this section.
10.5.2.1 Shallow Groundwater
Groundwater sampling data are likely to be available at many sites, even where soil
gas data have not yet been collected, so it is often the first available line of evi-
dence for screening the vapor intrusion pathway. Groundwater sampling protocols
have been developed and refined over the course of the past few decades to the
point where many historic causes of bias and variability have been resolved, so the
data quality tends to be good. Even where groundwater data are not already avail-
able, groundwater samples can often be collected as quickly and as easily as soil
gas samples. To the extent practicable, groundwater samples should be collected
over a narrow interval (a few ft/1 m or less) (Ter Meer et al. 1999 ) just below the
groundwater table when the data are to be used for assessing the potential for vapor
intrusion. Interpretation of groundwater data should consider that groundwater typ-
ically is the farthest removed of all compartments from the receptor, and for this
reason, should generally be considered a supporting line of evidence. However, for