Vapor Intrusion - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 424

input data are available, screening level models will generally provide results that
are similar to measured conditions, but considering the spatial, temporal, sampling
and analytical variations, uncertainty in model inputs, and limitations of simplistic
model formulations, it should not be surprising if measurements and model outputs
show discrepancies of approximately one order of magnitude (Hers et al. 2000 ;
Johnson 2002 ; Johnson et al. 2002 ; Tillman and Weaver 2007b ).

10.4.1 Mathematical Model Formulation

As illustrated in the conceptual model, the vapor intrusion pathway commonly
involves a phase partitioning component (transfer of contaminants from a source
to the soil gas phase), diffusive vapor transport through the water-unsaturated zone,
convective/diffusive transport through a building's foundation, and mixing within
the building. Mathematical models for vapor intrusion all have incorporated one or
more of the formulations and the general applied mathematical formulation of each
of these components are described below.

10.4.1.1 Phase Partitioning

Phase partitioning calculations are typically performed to estimate the concentration
of a particular contaminant in soil gas from its concentration in any other phase (i.e.,
groundwater, bulk soil, or non-aqueous phase liquid).

Groundwater to Soil Gas

The concentration in soil gas overlying contaminated groundwater can be estimated
as follows:

C sg = H × C gw (10.1)

μ

where C sg is the vapor concentration in soil gas ( g/L), H Henry constant (-) and

C gw the concentration in groundwater ( μ

g/L).
There may be rate-limiting mass transfer from groundwater to soil gas, so this
equation will overestimate the soil gas concentration in some cases; however, that
would tend to overestimate the potential for health-risks, which is conservative and
therefore appropriate for a screening level assessment. It will generally be valuable
to confirm soil gas concentrations by direct sampling and analysis if the calculated
values pose a potentially unacceptable risk.

NAPL to Soil Gas

The concentration of a contaminant in soil gas in contact with a NAPL (occurring
either as a residual phase in soil or as a lens on top of the water table) can be esti-
mated, assuming that the soil air above the NAPL is saturated with the contaminant
and related to the maximal contaminant vapor density (Vd) at 283 Kelvin (Fetter
1993 ):