Environmental Engineering Reference
In-Depth Information
Fig. 10.2
Lateral diffusion of vapors through the water-unsaturated zone (schematic)
the water-unsaturated zone without contact or interaction with groundwater, as
shown in Fig.
10.2
.
There may also be preferential soil gas flow through granular fill under a building
(Fig.
10.3
), especially in areas where the gas permeability of the surrounding soil
is very low. Floor drains, for example, are designed to allow water to drain away,
but are usually not designed or constructed to eliminate soil gas entry. The granular
materials surrounding a sewer pipe may or may not be very well compacted after
placement, so settlement over time may form air voids beneath the slab that are very
highly permeable. Foundation walls are usually constructed first, then floor slabs
are poured, often leaving a space between the floor slab and walls (i.e., perimeter
crack) for expansion and contraction. This perimeter crack is often obscured by
wall-coverings, and may not be accessible for inspection or direct testing. The same
may be true for other utility penetrations and homeowner modifications, which may
also create a pneumatic connection to granular fill.
Sumps or wet basements can also allow groundwater containing VOCs to enter
the building, or contact the building envelope directly (Fig.
10.4
). This scenario
can be especially problematic if the source of vapors is in the form of low-density
non-aqueous phase liquids (LNAPLs) floating on the water table.
Conceptualization of the pathway should also include potential vapor migration
barriers. A low-permeability layer in the water-unsaturated zone with high moisture
content or perched water may impede or prevent upward migration of vapors from
deeper sources. If the recharge rate is sufficient to cause perched water layers within
the water-unsaturated zone, there may be an effective vapor barrier. The seasonality
of infiltration would need to be considered in this scenario. Areas that receive regular
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