Human Exposure Pathways - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 491

d s ρ s
Q C · τ exp

VF = (11.51)

in which

d s = the thickness of the contaminated layer [m]

τ exp = the exposure duration [s].

The second approach is to calculate separately the vapour flux either due to dif-
fusion, as is used in the CSOIL model from the Netherlands (Brand et al. 2007 ),
or through the combined effect of diffusive transport and transport due to an evap-
orative flux, as in the Flemish model Vlier-Humaan, (OVAM 2004 ). In this case
the volatile contaminant has to be located at some depth below the soil surface.
Furthermore it is assumed that the source is infinite and that the soil concentration
is not affected by evaporation. The diffusive flux through the soil is given by:

D eff C vap,soil − C vap,amb

Q diff = (11.52)

L c

in which

m − 2 s − 1 ] Q diff = the diffusive flux from the soil surface [kg · ·

m − 3 ] C vap,soil = the concentration in soil air [kg ·

m − 3 ] C vap,amb = the concentration in the ambient air above the soil [kg ·

L c = the diffusion length in the soil [m].

This is simplified by assuming that the concentration in the soil air is much less
than the concentration in the ambient air above the soil. Furthermore, the diffusivity
is transformed to relate to the total concentration of contaminants in the soil, as
follows:

D soil C pore
L c

V w
P w

Q diff = · (11.53)

in which
D soil = the total diffusivity in the soil [m 2 s − 1 ]

the concentration in soil pore water [kg m − 3 ] C pore =

V w = the volume fraction of water in the soil [-]

P w = the mass fraction of contaminant in pore water [-].

In the case when the mass fraction in pore water is zero, the diffusivity is trans-
formed to relate to the concentration in pore water or the total concentration of
contaminants in the soil, as follows:

D soil C soil ρ s
L c

Q diff = (11.54)