Environmental Engineering Reference
In-Depth Information
in which
the total concentration in the soil [kg kg 1 ]
C soil =
In the Flemish model (Vlier-Humaan), the release due to the evaporative flux
of water and due to diffusion over the boundary layer at the soil surface is also
considered. The evaporative flux is given as:
Q evap =
C pore E v
(11.55)
in which
the evaporative flux from the soil surface [kg m 2 s 1 ]
Q evap =
the evaporation rate [m 3 m 2 s 1 ].
E v =
The diffusion over the boundary layer is given by:
D eff ( C vap , surf
C vap , amb )
Q vapour =
(11.56)
X a
in which
m 3 ]
C vap,surf =
the concentration in soil air at the soil surface [kg
·
X a =
the thickness of the boundary layer [m].
If the sum of the diffusive flux, Qdiff, and the evaporative flux, Qvap, is greater
than the diffusion over the boundary layer, Qvapour, it is assumed that the boundary
layer will limit the release of volatile contaminants into the ambient air. In other
cases, the release will be determined by the sum of the diffusive flux, Qdiff, and the
evaporative flux, Qvap.
In both approaches outlined in Eqs. ( 11.50 )to( 11.51 ) and Eqs. ( 11.52 )to( 11.56 ),
respectively, the dilution in the ambient air is described by an air dispersion factor
or a dilution velocity (Q/C). These are equivalent entities defined as the quotient
of the mass flux per unit area and unit time from the soil and the concentration in
the air at the receptor height. The air dispersion factor is often given in the units
g
m 2
s 1 per kg m 3 , while for the dilution velocity the units are shortened to
·
·
h 1 .
Various methods are used to determine the dilution velocity ranging from:
s 1 or m
cm
·
·
Assuming mixing in the air volume is defined by a specified wind speed and
mixing height.
Calculation of the mixing height from boundary layer theory.
Using external atmospheric dispersion models to calculate site-specific and area
specific dilution velocities.
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