Environmental Engineering Reference
In-Depth Information
E
XAMPLE
6.29 P
ESTICIDE
V
OLATILIZATION
R
ATE FROM A
S
OIL
Estimate the rate of release (
μ
g/s) of dieldrin (a pesticide, molecular weight 381)
applied to a soil at 100
μ
g/g concentration. The soil has a total porosity of 0.5, an
f
oc
of
1%, and a water saturation of 5%. The soil density is 2 g/cm
3
and area of application is
1 ha. Assume a surface mass transfer coefficient of 0.1 cm/s.
The effective diffusivity
D
g
in a partially saturated soil is given by
D
A
(
ε
10
/
3
a
T
)
,
where
D
A
is the molecular diffusivity in air and
ε
a
is the air-filled porosity. Note that
ε
a
=
(
1
− θ
w
)
ε
T
, where
θ
w
is the water saturation. For dieldrin,
D
A
=
0.028 cm
2
/s.
Since
θ
w
=
0.05,
ε
a
=
0.57,
ε
w
= θ
w
ε
T
=
0.03. Hence
D
g
=
0.012 cm
2
/s.
R
F
= ε
g
+
(
ε
w
/K
aw
)
+ ρ
b
K
SA
·
K
aw
for dieldrin is 8.1
×
10
−
6
. From Chap-
ter 4, we have
K
SA
=
K
sw
/K
aw
=
f
oc
K
oc
/K
aw
. With log
K
ow
=
5.48, log
K
oc
=
(
0.92
)(
5.48
)
−
0.23
=
4.81. Hence
K
SA
=
7.9
×
10
7
P/kg and
R
F
=
6.8
×
10
7
.
[A]
0
= ρ
b
W
i
/K
SA
=
1.2
×
10
−
6
/
ε
μ
g/cm
3
. Hence
F
A
=
(
0.1
)(
1.2
×
10
−
6
)
[
exp
(
1.88
10
−
5
)
. erfc
(
0.0137
)
]=
10
−
7
g/cm
2
s. Hence mass lost,
m
i
=
×
1.12
×
μ
1.12
×
10
−
7
10
8
×
=
11.2
μ
g/s.
6.4.2 S
OIL AND
G
ROUNDWATER
T
REATMENT
As depicted in Figure 6.51, surface spills of solvents can potentially contaminate two
zones. The discussion in the previous section reflects the different transport mech-
anisms that operate in the two zones. In the vadose zone, pore air and porewater
transport processes are operative, whereas in the saturated zone porewater transport
is dominant. The remediation strategy for contaminants in the two zones also dif-
fers, the underlying phenomena being driven by the respective operative transport
mechanisms.
In both zones, there are three major categories of remedial processes:
(a) Containment, whereby contaminants are prevented from further spreading
(b) Removal, whereby contaminants are extracted from the subsurface
(c) Treatment, wherein contaminants are separated and treated by appropriate
technologies.
Containment is required in cases where the movement of fluids is to be controlled
before adverse effects are manifest in nearby communities that depend on drinking
water supplies from the aquifer. Physical barriers such as slurry walls, grout curtains,
and sheet pilings are used for containment. Hydraulic barriers, that is, reversing the
hydraulic gradient by a series of pumps and drains, are effective in containing a
slow-moving contaminant plume in both zones, especially in the saturated zone.
Removal is the only option in some cases. Highly contaminated surface soils are
excavated and treated before disposal. However, this is often infeasible and expensive
forsurfacesoilsandgroundwaterinthesaturatedzone.Amorepracticalsolutionisthe
so-called P&T method.As the very name indicates, this entails bringing groundwater
tothesurface,separatingthecontaminants,anddischargingthewatertothesubsurface
or to lakes or rivers. A variation of P&T practiced in both the vadose zone and the
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