Vapor Intrusion - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 426

to which of the compounds present are most likely to be significant contributors to
the total risk, in order to select appropriate sampling and analytical methods for their
characterization.

Bulk Soil Concentration to Soil Gas

The concentration of a contaminant in soil gas in contact with contaminated soil
(i.e., a bulk soil sample) can be estimated as follows:

C bs

f oc ×

H

C sg = (10.6)

1
H ×

1
ρ b ×

K oc + ( n w +

×

n g )

where:

C sg : concentration in the soil gas ( μ g/L)

C bs : concentration in the bulk soil ( g/kg dw )

H : the contaminant-specific Henry's Law constant [(

μ

μ g/L-vapor)/( μ g/L-H 2 O)]

f oc : fraction of organic carbon (-)
K oc : organic carbon partition coefficient (mL/g)
ρ b : soil bulk density (g/mL)
n w : the volumetric moisture content [L-H 2 O/L-soil]
n g : the volumetric gas content (

= n T -n w ) [L-vapor/L-soil]

In practice, there is often a poor correlation between measured soil concentra-
tions and measured soil gas concentrations, so the partitioning calculations should
be used with caution. The United States Environmental Protection Agency (US
EPA) conducted a study that showed soil sampling and analysis of volatile con-
taminants by SW846 Method 8240 may have negative biases of a factor of 10 to
1,000 because of loss of volatiles (EPA 1993a). However, others have observed a
tendency for partitioning calculations to overestimate the soil vapor concentrations
(Provoost et al. 2009 ). The fraction of organic carbon in some soils is highly vari-
able, and the bulk soil concentration can be significantly higher or lower if there is
more or less organic matter present, respectively. Research is required to improve
the understanding of this topic.

10.4.1.2 Transport Through a Porous Media

Vapor transport through water-unsaturated soils is typically dominated by diffusion
through the vapor phase, because for most VOCs the gas phase diffusion coeffi-
cient is relatively high (on the order of 10,000 times higher than aqueous diffusion
coefficients).
The vapor mass flux is calculated by Fick's law, as follows: