Environmental Engineering Reference
In-Depth Information
Therefore, the concentration of atrazine in the pore
water is estimated as 19 mg/L. Since the solubility of
atrazine is 33 mg/L, application of Equation (6.37) is
validated. If the pore concentration calculated were
greater than the solubility, the actual pore water con-
centration of atrazine would be equal to the solubility,
some pure-phase atrazine would be in the soil, and
Equation (6.37) would not be valid.
It is interesting to note that the drinking-water stan-
dard for atrazine is 0.003 mg/L. Therefore, a pore water
concentration of 19 mg/L indicates a significant likeli-
hood of groundwater contamination.
1.03 × 10
7
, estimate the concentration of atrazine in the
pore water and assess the effect of atrazine volatization
on the aqueous concentration.
Solution
From the data given,
c
T
= 53 g/m
3
= 53 mg/L,
θ
= 0.15,
n
= 0.20,
K
H
= 1.03 × 10
7
,
ρ
b
= 1610 kg/m
3
, and
K
d
=
1.6 mL/g = 1.6 × 10
−3
m
3
/kg. Substituting these data into
Equation (6.40) yields
1
c
=
c
aq
T
n
K
−
θ
θ
+
K
ρ
+
d
b
The potential for volatization of a substance is related
to the saturation vapor pressure of the substance;
however, actual volatization from the soil is also affected
by many other factors, such as atmospheric air move-
ment, temperature, and soil characteristics. If a sub-
stance exists in a soil in the vapor phase in addition to
the liquid and solid phases, Equation (6.35) is expanded
to include the vapor phase as
H
1
=
(
53
)
0 20 0 15
1 03 1
.
−
×
.
0 15
.
+
( .
1 6 10
×
−
3
)(
1610
)
+
.
0
7
= mg/L
19
Therefore, the concentration of atrazine in the pore
water is estimated as 19 mg/L. Example 6.6 had exactly
the same parameters, neglected vaporization, and
yielded the same aqueous concentration of 19 mg/L.
This result indicates that atrazine volatization likely
has a negligible effect on the fate and transport of
atrazine in the water environment.
Biological degradation of a substance usually implies
a breakdown by living microorganisms to more simple
compounds, ultimately to carbon dioxide, water,
methane, ammonium, and possibly to other simple by-
products. Biotransformation of substances in soil is
accomplished by microorganisms or fungi, and biodeg-
radation may occur in both aerobic and anaerobic envi-
ronments. Biodegradation is commonly represented by
Monod's equation
,
c
=
θ
c
+
F
ρ
+
ac
(6.38)
T
aq
b
g
where
a
is the volumetric air content (
a
=
n
−
θ
, where
n
is the volumetric porosity) (dimensionless), and
c
g
is
the vapor density of the chemical (ML
−3
of soil air). The
relation between the vapor density and corresponding
concentration of the chemical in (pore) water solution
is given by
Henry's law
as
c
=
K c
(6.39)
aq
H g
where
K
h
is henry's constant for the chemical [dimen-
sionless]. Care should be taken in using
K
H
since it is
commonplace in technical references to define the
dimensionless henry's constant as the inverse of
K
H
.
Combining Equations (6.38) and (6.39) gives the follow-
ing relationship between the aqueous concentration,
c
aq
,
and the total concentration,
c
T
:
dc
dt
Xc
K c
µ
aq
m
aq
= −
(6.41)
+
s
aq
where
µ
m
is the maximum substrate utilization rate
(T
−1
),
X
is the microbial biomass per unit volume of pore
water (ML
−3
), and
K
s
is the half-saturation constant for
the chemical (ML
−3
). For small concentrations of chemi-
cals in the soil and a sufficient and constant microbial
population, biodegradation is commonly described by
the first-order reaction
1
c
=
c
aq
T
n
K
−
θ
(6.40)
θ
+
K
ρ
+
d
b
H
EXAMPLE 6.7
dc
dt
aq
(6.42)
= −
k c
b aq
A 1-m
3
sample of soil is found to contain 53 g of atrazine
and to have a water content of 0.15, a porosity of 0.20,
and a bulk (dry) density of 1610 kg/m
3
. If the soil has
an estimated distribution coefficient of 1.6 mL/g, and
the (dimensionless) henry's constant of atrazine is
where
k
b
is the decay constant (T
−1
). If
c
aq
is large rela-
tive to
K
s
, Equation (6.41) indicates that biodegradation
is described by the linear equation
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