Vapor Intrusion - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 412

The approximate mass of the source should be considered, to assess whether it
is large enough to be a persistent source for a typical exposure scenario (for about
30 years), or not. A very small release can result in highly localized soil vapor
concentrations that may result in (temporary) high concentrations in the indoor air,
but may not be sufficient to sustain an unacceptable flux to indoor air over a 30-year
exposure period. Estimating the mass of a source is challenging, and the uncertainty
in the estimate should be considered as part of the conceptual model.
The source constituents (i.e. the contaminants in the soil and/or groundwater)
should be evaluated and identified to the extent practical. A solvent release may be
predominantly a single contaminant, although intrinsic biodegradation may gener-
ate daughter products that may be more or less mobile and/or toxic. By contrast,
a hydrocarbon release typically contains a mixture of hundreds of contaminants,
of which many may be non-toxic and readily degradable under aerobic conditions,
and only very few may be significant contributors to health risk. Waste disposal
or recycling facilities may have even more complex mixtures. For NAPL mixtures,
volatilization of individual contaminants is dependent on the composition of the
mixture and the fraction of each contaminant may change significantly as weather-
ing proceeds. Typically, lighter contaminants are more volatile and will decrease in
concentration with time, leaving a mixture with increasing relative proportions of
heavier, less volatile contaminants. The volatility, toxicity, mobility, degradability
and initial mass fraction may all need to be considered for multiple contaminants
to identify primary contaminants of concern and develop a practical list of target
contaminant to be included in sampling and analysis.
Some sources produce vapors only by volatilization from aqueous or non-
aqueous releases, but others actively produce gases such as methane, carbon dioxide
or other volatile metabolites as products of biodegradation. Gas produced as a
byproduct of microbiological activity can generate pressure gradients that enhance
subsurface vapor migration by advection, in addition to diffusion. Methane is not
usually a threat to indoor air quality, but in some circumstances, the indoor air con-
centration may exceed the lower explosive limit (5% by volume). Since methane
cannot be present in soil gas at concentrations above 100%, this circumstance can
only occur where the building air exchange rate is low, and is often associated
with falling barometric pressure, which can increase the flow of soil gas into a
building.
10.2.2 Pathway
The movement of contaminants from the source to the building is described in the
pathway component of the conceptual model. In some settings, the pathway might
include, for example, soil gas transport from a source above the water table to an
overlying building. In other settings, the pathway might include a combination of
groundwater transport from a primary source to an adjacent property, with sub-
sequent off-gassing and vapor transport through the water-unsaturated zone to an
overlying building. Alternately, the pathway may include lateral diffusion through