Environmental Engineering Reference
In-Depth Information
above, to relate indoor air concentrations to subsurface source concentrations. These
approaches are expressed as follows:
C
air
=
C
source
×
AF
Where
C
air is the indoor air concentration,
C
source is the concentration in the soil
gas at the source (or some specified depth in the subsurface), and AF is the attenua-
tion factor, which is a value less than one as defined here that accounts for dilution in
the building ventilation, degradation in the subsurface, resistance to mass transport
imposed by barriers, and similar mechanisms.
The attenuation factors can be estimated at least three different ways. They can
be:
1. Derived from empirical data (e.g., Johnson et al.
2002
; Mc Donald and Wertz
2007; US EPA
2009
);
2. Calculated using mathematical models (described in Section
10.4.2
), or
3. Derived using tracer chemicals or marker compounds (Section
10.5.2.4
).
10.4.2 Available Vapor Intrusion Models
Mathematical models for vapor intrusion can be used to estimate indoor air con-
centrations from subsurface vapor concentrations at some depth, perform sensitivity
analyses to assess probable ranges of conditions and identify important processes, or
provide an expectation of vapor behavior to guide strategies for site characterization
or mitigation. If the indoor air concentration can be calculated with reasonable accu-
racy (i.e., within approximately an order of magnitude), it may not be necessary to
collect indoor air samples, particularly where the predicted indoor air concentrations
are far below risk-based standards. Models are particularly useful for determining
potential indoor air concentrations in situations where a building is not present but
may be constructed in the future. Model results may also provide a line of evidence
that makes it easier to identify the contribution of vapors from sources inside the
building.
Different software models of vapor intrusion are available (commercially or in
the public domain). The models most frequently used in Europe and the US are dis-
cussed below. The discussion is not intended to express any partiality. It is up to the
user to select a model, verify that the formulation and assumptions are consistent
with the conditions at a particular site before use, and understand the model capa-
bilities, limitations and sensitivities to various input parameters prior to use. Also,
it is generally acknowledged that a qualified and experienced model user improves
the proper use of vapor intrusion models.
At present, there is considerable debate about the accuracy of vapor intrusion
models. The number of comparisons between carefully applied models and carefully
collected field data is limited (Abreu and Johnson
2005
). It appears that in most
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