Vapor Intrusion - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 439

size of the contaminated area (i.e., large plumes may be adequately characterized
with larger spacing between samples than smaller plumes). At some contaminated
sites, VOCs in groundwater have been mapped over distances of up to several
miles/kilometers, in which case subsurface concentrations (groundwater or soil gas)
are unlikely to vary significantly over distances of 50 ft/15 m in the direction
of groundwater flow. Concentrations generally change more rapidly in the direc-
tion perpendicular to groundwater flow, because transverse dispersion is much less
significant than longitudinal dispersion. Sample spacing ultimately must be a site-
specific consideration. A comprehensive discussion of vertical profiles and transects
of soil gas data is provided by API ( 2005 ), which may help guide the selection of
an appropriate scope of soil gas data collection.
Temporal variability in deep soil gas samples tends to be low, so multiple samples
over time may not be necessary. However, shallower samples can show temporal
variability associated with seasonal changes in groundwater levels and infiltration
of recharge (McAlary 2008 ).
10.5.2.4 Indoor Air
Indoor air typically contains dozens of contaminants at detectable concentrations,
some of which may be the same contaminants that are present in the subsurface at
a particular site, and some of which may be released by indoor sources or present
in ambient outdoor air at levels above risk-based target levels. Indoor air sampling
will identify detectable contaminants from all three sources (subsurface, indoor and
outdoor), and the effort required to resolve the relative contribution from each is
usually not trivial. Access agreements will often be required for indoor air sam-
pling, along with a community relations plan, and may stimulate legal, news-media
or political issues. Therefore, it is usually preferable to assess subsurface concen-
trations (near-slab or sub-slab) first, and assess potential indoor air concentrations
through empirical or modeled attenuation factors. If the subsurface concentrations
are too low to pose a potential risk from subsurface vapor intrusion (and are ade-
quately represented by the available samples), it may be possible to avoid indoor air
sampling and the complexities associated with resolving background contributions.
Conversely, if the subsurface concentrations are very high, it may be appropriate
to proceed with implementing exposure controls or mitigation systems. In areas
where subsurface concentrations are neither too low to pose a potential risk, nor high
enough to justify preemptive mitigation, indoor air sampling is usually appropriate.
When collecting indoor air samples to evaluate the potential for subsurface
vapor migration to indoor air, it is important to consider the contribution of
background (indoor) sources and ambient (outdoor) sources, not related to the
contaminated site, to indoor air quality. Background or ambient sources may
contribute vapors to indoor air at detectable concentrations, or possible concen-
trations above target levels. There are many potential indoor sources of back-
ground contaminants, including: household activities (smoking, cleaning, hobbies);
consumer products (gasoline, heating oil, cleaning supplies, glues); and build-
ing materials (carpets, paints, glues). Specific contaminants found in household