Environmental Engineering Reference
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or even model it without collecting some site-specific information regarding the
vertical profile of O 2 ,CO 2 and VOCs. Methods have been developed to model
vadose zone biodegradation beneath open surfaces (Lahvis and Baehr 1997 and
1996 ) and beneath buildings (Abreu and Johnson 2005 , 2006 ; Johnson and Abreu
2003 , Johnson et al. 1999 ). First order and Monod degradation kinetics have been
included in these models. Some of the more refined methods model the coupled
transport and reaction of hydrocarbons and oxygen because depletion of oxygen
in the subsurface may result from consumption resulting from the biodegradation
process.
10.3.3 Soil Gas Advection
Advection of soil gas into buildings occurs due to under-pressurization of the
building relative to pressure in shallow soil gas. The building under-pressurization
can be a result of the “stack effect” (warm air rising inside the building, exit-
ing through roof vents, and creating suction in lower levels), barometric pump-
ing, wind load on the side of the building, exhaust fans, clothes dryers, cen-
tral vacuums, or elevators (acting as a piston). The soil gas flow rate into a
building is a function of building pressure, permeability of soils immediately
beneath the foundation, and characteristics of vapor entry points through the
foundation.
The pressure differential between the building and the subsurface will often fluc-
tuate between positive and negative, in which case air will flow into and out of
the building, which can affect the soil gas in proximity to points of entry to the
building (e.g., floor cracks, drains, et cetera). This can result in dilution of sub-slab
concentrations, contribution of vapors from indoor sources to the sub-slab soil gas,
and/or addition of oxygen to the sub-surface, which may have a significant effect on
biodegradation of petroleum hydrocarbons. Pressure differences may also result in
indoor air flowing to the soil air as McHugh et al. ( 2006 ) demonstrated. Monitoring
the pressure differential with a digital micro-manometer and datalogger can help
to understand whether and to what effect pressure fluctuations are important. In
some cases, indoor air monitoring under positive and negative building pressure
can help to elucidate the contribution of vapors from the subsurface (Berry-Spark
et al. 2005 ).
Soil vapor intrusion to indoor air can occur regardless of whether a building
has a basement, slab-on-grade or crawlspace design. It can also occur even when
the building has a concrete floor that appears to be free of cracks. Vapor barriers
are placed under some buildings to attempt to inhibit subsurface vapor intrusion,
but the barriers are typically plastic sheets that can reduce advection of soil gas,
but may not significantly impede diffusion. If not properly designed and installed,
vapor barriers may contain inadvertent perforations, and air flow may be signifi-
cant even through a small perforation. Intrinsically safe buildings may be limited to
those constructed on stilts, such as may occur in lowland areas subject to regular
flooding.
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