Vapor Intrusion - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 437

which is unpleasant for property owners. Relatively little information is available to
demonstrate how sub-slab soil gas concentrations vary over time, or in response to
barometric pressure changes (Luo et al. 2006 ). Recent publications have shown that
soil gas measurements showed a high degree of spatial variability and expressed the
need for a better time resolution (Eklund and Simon 2007 ; Luo et al. 2006 ). Tillman
and Weaver ( 2007a ) investigated the effect of temporal moisture content variabil-
ity beneath and external to a building and its effect on vapor intrusion assessment.
Some regulatory guidance documents (e.g., US EPA 2006 ) recommend collect-
ing three sub-slab samples for a building the size of a typical domestic residence
in the US to account for spatial variability, so sub-slab sampling efforts are not
insignificant, especially if the vapor intrusion assessment includes a neighborhood
of residences. Sub-slab soil gas sampling methods may not be practical for buildings
with suspended floors and crawlspaces.
10.5.2.3 Soil Gas Samples Collected Adjacent to a Building
Soil gas samples collected beside a building are commonly used to assess vapor
intrusion, because they can be collected with less disruption to the building occu-
pants. However, experience to date has shown that vapor concentrations in soil gas
samples collected beside the building are not strongly correlated to indoor air con-
centrations or to sub-slab concentrations (US EPA 2008 ). There are two possible
explanations for this observation (spatial variability in soil vapor concentrations and
bias or variability imposed by soil gas sampling methods), described in the two
paragraphs below. Resolution of the relative contributions of these two issues is a
topic of on-going research.
Soil gas surveys have been commonly used for fast and inexpensive site-wide
screening to identify possible areas of VOC releases for decades. However, the qual-
ity assurance and quality control needed to achieve the data quality objectives for
vapor intrusion assessments is much more stringent. Published guidance for soil gas
sampling and analysis to address vapor intrusion data quality objectives is sparse
at this time. The ASTM Standard Guide for Soil Gas Monitoring in the Vadose
Zone (ASTMD-5314-92) was originally written in 1992, and re-authorized in 2001,
since which time there has been a rapid increase in interest in vapor intrusion.
The California Department of Toxics Substance Control (CalDTSC) and the Los
Angeles Regional Water Quality Control Board (LARWQCB) provides a soil gas
sampling advisory (CalDTSC & LARWQCB 2003), although this is being updated
at the time of this document's publication. Some non-government organizations
have developed good references for soil gas sampling specific for assessing vapor
intrusion that provide good information, but are not entirely consistent (e.g., API
2005 ;EPRI 2005 ; Geoprobe 2006; ISO 2004 ; VDI 1998 ). However, several reg-
ulatory guidance documents for vapor intrusion assessment provide no substantial
discussion of soil gas sampling methods (e.g., MADEP 1994 , 2002 ;USEPA 2002 ).
Johnson and Abreu ( 2003 ) showed that the depth of the soil gas sample is very
important. Figure 10.8 shows simulated normalized (i.e., C/Cmax) concentrations