Vapor Intrusion - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 423

10.3.4 Mixing Inside the Building
Once subsurface vapors migrate to the building, air circulation within the build-
ing will result in a mixing of the contaminant through the interior. Air exchange
rates within buildings depends on the design of the building heating and ventilation
systems and can range from 0.1 to 10 per h. (ASHRAE 1985 ). In some cases,
the air exchange rate is specified by building codes, or other standards (e.g.,
ANSI/ASHRAE 2004 ). Forced-air systems generally circulate air at a rate suffi-
cient to maintain well-mixed conditions within a given air-zone (building or part of
a building which is serviced by a single ventilation system). Buildings heated with
baseboard heaters or radiators tend to have less circulation, limited to convection
and currents imposed by wind-load on the building, leaking doors, and windows
and other openings. In very large buildings, there are often a series of air-handling
units, and different air-zones, which may be well-mixed internally, but may be rel-
atively isolated from one another. Considering the range of possible configurations,
there is a knowledge gap related to assigning a representative number of indoor air
samples as a function of the building design and ventilation system design.
The characterization and modeling of contaminants in indoor air has been exten-
sively studied in the fields of industrial hygiene and energy efficiency, but vapor
intrusion guidance documents typically assume a very simple scenario where the
entire indoor space is perfectly and instantaneously well-mixed and data from base-
ment levels (or even crawl spaces beneath buildings) represents potential exposures
in living areas. In reality, concentrations will be higher in rooms with limited venti-
lation (e.g., cellars) or immediately adjacent to points of entry (sumps, floor cracks,
et cetera) to the point where concentrations may be detectable with field instruments.
In large buildings, the ventilation system design and operation data may be avail-
able from mechanical engineers or building custodians, but in most single-family
dwellings, the ventilation rate and mixing between air-spaces will be unknown.
10.4 Mathematical Modeling
Mathematical modeling can help develop an understanding of the significance of
various fate and transport processes, compare expected performance of various
remedy designs (if needed), and possibly act as an additional line of evidence or
interpretive tool. In general, a mathematical model of vapor intrusion predicts the
indoor air concentration that may result through vapor transport from a subsurface
contaminant source into a building. Screening level models for a given set of site
conditions can be performed quickly, so it is usually appropriate to conduct a few
simple bounding calculations to assess “best-case” and “worst-case” conditions. If
vapor intrusion is significant in both cases, it may be preferable to proceed toward
remedy design than an expensive investigation. Conversely, if vapor intrusion is
insignificant in both cases, it may be possible to select a focused scope of investiga-
tion, and use the model calculations as a supporting line of evidence. If appropriate