Environmental Engineering Reference
In-Depth Information
•
For some contaminants, Toxicological Reference Values are available in the form
of reference concentrations in air, RfC-values, in mg contaminant per m
3
,orasan
inhalation risk values, which can be used to give the concentration of contaminant
in air equivalent to an acceptable risk.
•
For other contaminants, Toxicological Reference Values are available only as
reference doses, RfD-values, or as risk-based values for oral intake, in mg
contaminant/kg
body weight
/d.
In the first case, exposure calculations therefore derive the contaminant con-
centration in air for comparison with the RfC-values. In the latter case, in some
countries, the daily intake of contaminants via inhalation of vapours is calculated
from the inhalation rate and the occupancy period and compared to RfD-values (see
Chapter 10
by McAlary et al., this topic).
11.5.3 Input Parameters
11.5.3.1 Diffusivities
The
diffusivity
of contaminants in the vapour phase in the air-filled soil pore volume
is a function of water saturation. The
effective diffusivity
in the vapour phase (
D
e,g
)
can be estimated according to Millington and Quirk (
1961
):
D
0,
g
a
1
3
ε
D
e
,
g
=
(11.59)
2
in which
the diffusivity of the contaminant in free air [m
2
s
−
1
]
D
0
,g
=
·
the total porosity [m
3
pore space per m
3
bulk soil]
ε
=
the soil air content [m
3
air filled pore space per m
3
bulk soil]
a
=
The effective diffusivity of dissolved contaminants in the water pore space (
D
e,w
)
can be calculated in a similar way as:
w
1
3
ε
D
e
,
w
=
D
0,
w
(11.60)
2
in which
the diffusivity of the contaminant in free water [m
2
s
−
1
]
D
0
,w
=
·
the soil water content [m
3
water filled pore space per m
3
bulk soil]
w
=
The total effective diffusivity (calculated assuming equilibrium contaminant in
air - contaminant in water) is then given by:
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