Environmental Engineering Reference
In-Depth Information
TABLE 5.14.  Simplified Gasoline Mixture
c i =
( )( )(
1 1 1745
)
=
1745 mg/L
Concentration
(g/L)
This result verifies that for a spill of pure benzene,
the maximum concentration of benzene in the
groundwater is equal to the solubility of benzene in
water.
(b) For benzene in gasoline, all of the parameters in
Raoult's law are the same except that x i = 0.01
instead of x i = 1. Hence, the maximum concentra-
tion of benzene in the groundwater is given by
Constituent
Benzene
8.2
Toluene
43.6
Xylene
71.8
1-Hexene
15.9
Cyclohexane
2.1
n -Hexane
20.4
Other aromatics
74.0
Other paraffins (C 4 —C 8 )
336.7
145.1
c i =
0 01 1745
.
(
)
=
17 45
. mg/L
Heavy ends (>C 8 )
Total
717.8
In practice, the presence of benzene as a nAPL
would be indicated when the concentration of
benzene in groundwater samples exceed 1% of the
maximum concentrations calculated here. It is
important to note that as the nAPL is flushed with
water, the more soluble constituents are depleted,
thereby altering the mole fraction.
Source of data : Baehr and Corapcioglu (1987).
results in errors on the order of 10% in estimating equi-
librium aqueous solubility (Lee et al., 1992; macKay
et al., 1991; Ramaswami and Luthy, 1997). Errors of this
magnitude are often acceptable in comparison to mea-
surement errors or variability in transport parameters
that can result in uncertainties of one or more orders of
magnitude. Thus, the assumption of an ideal nAPL
yields a reasonable first estimate of the equilibrium
aqueous concentration of a contaminant released from
a multicomponent nAPL.
Raoult's law, Equation (5.83), is particularly useful in
the case of petroleum hydrocarbons, such as gasoline,
which consists of more than 100 chemical constituents.
A simplified mixture that has been used to represent
gasoline is given in Table 5.14.
nAPLs in the saturated zone of aquifers generally
occur in the surrounding groundwater at their effective
solubility levels. As a consequence, nAPLs are typically
found at locations where the (dissolved) contaminant
concentration in the groundwater exceeds 1% of the
effective contaminant solubility. The so-called 1% rule
is that the presence of pure product is indicated when
the observed concentration in the groundwater exceeds
1% of the solubility. Contaminants are seldom found at
their effective solubility levels because of the additional
dilution induced by monitoring wells with screen lengths
much longer than than thickness of the zone where the
concentration is at the solubility level.
EXAMPLE 5.16
5.6.2.3  Soil  and  Aquifer  Samples.  In addition to
considering effective solubilities when interpreting
contaminant concentrations in groundwater samples,
Raoult's law must also be considered in interpreting
contaminant concentrations in soil and aquifer samples.
In the case of soil and aquifer samples, contaminant
concentrations are expressed as the mass of the con-
taminant per unit mass of the sample, usually expressed
in units of mg/kg. For these samples, the presence of
residual nAPL in a soil or aquifer sample can be esti-
mated by calculating whether the amount of contami-
nant in the sample exceeds the amount required for the
contaminant to be at the solubility level in the aqueous
phase and adsorbed into the soil by equilibrium parti-
tioning. The steps to be followed in this analysis are:
(a) What is the maximum concentration of benzene in
groundwater resulting from the dissolution of a
lens of pure benzene floating on the water table?
(b) Contrast this with the maximum aqueous-phase
concentration of benzene resulting from the disso-
lution of a lens of nonaqueous gasoline containing
a mole fraction of 1% benzene.
Solution
(a) According to Raoult's law, Equation (5.83), the
maximum concentration of benzene in groundwa-
ter is given by
c
= γ
x
c
i
i
i
s i
Step 1. Determine the effective solubility, c 0 , and the
organic carbon sorption coefficient, K oc , of the
contaminant.
For pure benzene, x i = 1, γ i = 1, c s i = 1745 mg/L
(Appendix B.2), and hence Raoult's law gives
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