Vapor Intrusion - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 438

Fig. 10.8 Simulated normalized (i.e., C/Cmax) oxygen ( top ) and hydrocarbon ( bottom ) vapor con-
centrations and comparison between near-slab and sub-slab sampling (Johnson and Abreu 2003 )
of O 2 (upper image) and a typical hydrocarbon (lower image). Based on these sim-
ulations, a soil gas sample collected outside the building at a depth just slightly
below the foundation could have a concentration a few orders of magnitude lower
than the concentrations beneath the building (these two locations are shown as open
circles with solid lines above them in the lower image of Fig. 10.8 ). Soil gas probes
should generally be installed to depths 1-2 m (3-6 ft) below the foundation to avoid
a false negative bias in the screening (failing to identify a potential vapor intru-
sion condition), although this may depend on the site-specific soil conditions, depth
to the groundwater table, and whether the contaminants of concern are aerobically
degradable.
Field sampling and analysis to verify the model simulations is ongoing. EPA
recently completed a sampling program to assess horizontal and vertical profiles
of vapor concentrations adjacent to a concrete slab, and found significantly lower
concentrations beside the slab than those beneath the slab (EPA 2009 ).
Spatial variability may also need to be considered and some practitioners have
suggested that soil gas samples should be collected from adjacent to all four walls of
a residence, for example. In general, sample spacing should be proportional to the