Vapor Intrusion - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 414

Fig. 10.3 Vapor transport through preferential pathways (schematic)
rainfall will be more likely to sustain layers of high moisture content in the water-
unsaturated zone than areas with wet and dry seasons. Also, there may be a “rain-
shadow” beneath a building where moisture contents are lower. This may depend
on soil type and the size of the building.
In humid climatic regions and areas with artificial recharge (e.g., irrigation,
storm-water retention ponds), a layer of clean water may accumulate above a plume
of VOCs in groundwater, the thickness of which would typically grow with increas-
ing travel distance and time from the point of release. This condition has been
referred to as a “fresh-water lens” (Fitzpatrick and Fitzgerald 1996 , 2002 ;McAlary
et al. 2004 ) or a “diving plume” (API 2006 ), and can act as an effective vapor barrier,
inhibiting off-gassing of VOCs from the water table sufficiently to protect overlying
Fig. 10.4 Vapor intrusion in a building with a wet basement (schematic)