Human Exposure Pathways - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 490

11.5.2.1 Calculation of Outdoor Air Concentration

Calculation of outdoor air concentrations contains two parts, i.e., estimating the flux
of volatile contaminant from the soil and estimating the dilution in the outdoor air.
The transport of volatile contaminants in the soil is calculated in a way similar to
that used when estimating indoor air concentrations, see Chapter 11 by McAlary
et al., this topic. Since the model for calculating the flux from the soil is closely
interlinked to the calculation of dilution in outdoor air it will also be described in
this section.
The different types of models used are based on the same principles, although
the actual calculation methods may differ. Furthermore, the terminology may differ
substantially between different models and may be quite confusing when a compar-
ison of different models is made. Sometimes similar entities have different names
and also similar names are used for different entities.
A commonly used approach (e.g., ASTM 2004 ; Environment Agency 2008 ;US
EPA 1996a , 2002a ) is to estimate a volatilisation factor relating the concentration
in the ambient air with the concentration in the surface soil. Thus, the calcula-
tions include both the transport in the soil and the dilution in the ambient air. The
contaminant is assumed to be located close to the ground surface, thus the loss of
contaminant will lead to a reduction of the vapour release from the soil, with time. In
order to handle this, the average vapour flux over the exposure duration is calculated
either by assuming a diffusion driven vapour transport from an infinite source where
the soil close to the surface is gradually depleted from contamination. This gives rise
to a time-dependent vapour flux, where the volatilisation factor is estimated as the
average flux over the chosen averaging time:

4 D eff
πτ

ρ s
Q C

H
K sw ρ s

VF = (11.50)

in which

the volatilisation factor [kg m − 3 ] VF =

the dry bulk soil density [kg m − 3 ]

ρ s =

the air dispersion factor [kg m − 2 s − 1 per kg m − 3 ] Q/C =

the effective diffusion coefficient in the soil [m 2 s − 1 ] D eff =

τ = the averaging time for surface emission vapour flux [s]

H the dimensionless Henry's Law constant [-]

K sw =

=

the total soil-water portioning coefficient [m 3 kg -1 ].

Alternatively, a constant release rate from a finite source is assumed to occur for
the duration of the exposure, i.e. is the total amount of the contaminant is assumed
to be volatilised during the exposure duration. In this case the volatilisation factor is
given by: