Vapor Intrusion - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 432

land Exposure Assessment algorithm (CLEA) model. Evans et al. ( 2002 ) concluded
that the Risc model was relatively easy to use and appeared to be mathemati-
cally correct and robust. In addition, the model is also typically conservative in
its predictions, which makes it a suitable model for first tier Risk Assessments.
The study stated that screening models that calculate indoor air concentrations
as a result of groundwater contamination need further verification (Evans et al.
2002 ).
Several other models have been developed for assessing subsurface vapor
transport and/or vapor intrusion to indoor air. Krylov and Ferguson ( 1998 )cre-
ated a model that is designed for buildings with suspended timber floors or
crawlspaces. Lowell and Eklund ( 2004 ) used mathematical modeling to assess
the distance from a source within which vapor intrusion could occur at levels of
concern.
10.4.2.3 Dilution Factor Models
In Sweden, a multi-media exposure model for contaminated sites is used within
a decision framework for Risk Assessment and remediation of contaminated
areas (Naturvårdsverket 1996 ). The Swedish Environmental Protection Agency
has developed a spreadsheet-based model for the calculation of guideline val-
ues and to perform site specific Risk Assessments (Naturvårdsverket 2005 ).
Soil and chemical-specific properties are used to estimate the soil air concen-
tration. The model calculates the indoor air concentration from the soil air
concentration via a standard dilution factor (DF) of 1:20,000. The DF is empir-
ically derived from measured data. A standard value is applied for volume
and ventilation rate of the house, which is assumed to be a one-compartment
house.
In Norway, contaminated sites are managed according to preliminary guidelines
published by the Norwegian Pollution Control Authority (SFT). The guidelines
were published in 1999, and incorporated a three-tiered approach (SFT 1999 )to
assess risk at contaminated sites in relation to land use and receptor (child and
adult). The generic assumption used for vapor intrusion is a DF between soil gas
and indoor air of 1:2,000. Soil and chemical specific properties are used to estimate
the soil gas concentration. A standard value is applied for volume and ventilation
rate of the one-compartment house (SFT 1995 ).
The US EPA recently conducted a very detailed analysis of empirical data (mea-
sured concentrations of VOCs in groundwater, soil gas, sub-slab-soil gas and indoor
air), and performed a detailed analysis to establish the order statistics for the AF
(inverse of DF) values for each media to indoor air (US EPA 2008 ). The sub-slab to
indoor air DF values compared favourably to information from the study of radon
dilution factors (Little et al. 1992 ), with an average value of about 1:500, and an
upper estimate of about 1:50. Groundwater to indoor air DF values were about an
order of magnitude greater, with a slightly larger range, which is to be expected
considering the range of geologic materials, water table depths, and other factors
involved.