Environmental Engineering Reference
In-Depth Information
V
p
×
M
Vd
=
(10.2)
R
×
T
Where
Vd
is the vapor density (g/m
3
),
V
p
the vapor pressure (Pa),
M
the molecular
weight (g/mol),
R
the gas constant (8.3144 Pa m
3
/mol-K) and
T
the absolute tem-
perature (K). The concentration in soil air is considered equal to the vapor density
as given in the equation below:
C
sg
=
Vd
(10.3)
Where
C
sg
is the concentration in the soil gas (g/m
3
) and
Vd
the vapor density
(g/m
3
).
Another approach is the use of the Raoult's Law with the component mole
fraction (McQuarrie and Simon
1997
) and is presented below.
P
i
v
×
x
i
×
M
×
W
i
C
vs
=
(10.4)
R
×
T
Where
C
vs
is the vapor concentration at the source (g/m
3
),
x
i
the mole frac-
tion of component i in the hydrocarbon (mol/mol),
P
i
v
the pure component
vapor pressure of component i (atm),
M
the molecular weight of component i
(g/mol),
R
the universal gas constant (82.1) [cm
3
-atm/mol-K) and
T
the absolute
temperature (K).
The mole fraction,
x
i
, is calculated from:
C
T
C
mix
MW
mix
MW
x
i
=
(10.5)
where
x
i
the mole fraction of component
i
in the hydrocarbon (mol/mol),
C
T
the total soil concentration of contaminant
i
(mg/kg
dw
),
C
mix
the total soil con-
centration of the mixture (mg/kg
dw
),
MW
mix
the weighted average molecular
weight of the mixture [g/mol] and
MW
the molecular weight of component
i
(g/mol).
If the molecular weights of the component,
i
, and the mixture are similar, this
roughly translates to the concentration of the component,
i
, over the concentration
of total mixture in the source.
The actual concentration may vary depending on several factors. NAPL mix-
tures often include some compounds that are aerobically degradable, so if oxygen is
present, the concentrations in soil gas above the NAPL may be substantially lower
than calculated from the partitioning equations. For NAPL that is at or near the
water table and there are significant water table fluctuations, soil vapor concentra-
tions tend to increase when the water table is low and decrease when the water table
is high. Many compounds of potential concern for vapor intrusion would have con-
centrations several orders of magnitude above levels of potential concern for vapor
intrusion, so where NAPL occurs in the vadose zone in close proximity to build-
ings, it is a reasonable precaution to proceed with sampling and analysis of soil gas
and/or indoor air, and use the phase partitioning calculations only as an initial guide
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