Environmental Engineering Reference
In-Depth Information
Since [A]
0
=
0, we have [A]/[A]
s
on the left-hand side of the equation. Using
U
D
for
U
,wehave
erfc
10
5
+
6.5erfc
10
5
,
10
−
6
t
10
−
6
t
[A]
[A]
s
=
1
2
−
1.45
×
−
1.45
×
0.51
√
t
0.51
√
t
where
t
is in seconds. If we use
t
=
500
×
3.17
×
10
7
s, we obtain [A]/[A]
s
=
0.336.
E
XAMPLE
6.28 T
IMEOF
T
RAVELFOR
A
DVECTIVE
T
RANSPORTOFA
C
ONTAMINANT
IN
G
ROUNDWATER
Estimate the time taken for a plume of chlorobenzene to reach a groundwater well 100 m
from the source if the Darcy velocity is 1
×
10
−
4
cm/s for the soil in the last example.
Assume only advective transport is significant.
Since advection is dominant over dispersion,
U/U
D
=
1
/R
F
=
0.23.
U
=
(
0.23
)(
1
×
10
−
4
)
=
2.3
×
10
−
5
cm/s.Hence
t
=
X
s
/U
=
10,000
/
2.3
×
10
−
5
=
4.3
×
10
8
s
=
13.7 y.
6.4.1.2
Sediment-Water Exchange of Chemicals
Compounds distribute between the various compartments in the environment. One of
the repositories for chemicals is the sediment. Chemical exchange at the sediment-
water interface is, therefore, important in delineating the fate of environmentally
significant compounds. Sediment contamination arose from the uncontrolled pol-
lutant disposal in lakes, rivers, and oceans. As environmental regulations became
stricter, most pollutant discharges to our lakes and waterways became controlled.
The contaminants in sediments bioaccumulate in marine species and exposure to
humans becomes likely. Hence the risks posed by contaminated sediments have to be
evaluated and sediment remediation strategies determined. The risk-based corrective
action (RBCA) is predicated upon knowledge of chemical release rate from sediment
and transport through air and water environments (Figure 6.55). The first step in this
process requires an understanding of potential release mechanisms.
In the case of the sediment environment, a number of pathways for chemical
exchange between the sediment and water can be identified. These are represented
schematically in Figure 6.56. For sediments that rest in quiescent environments, diffu-
sion (molecular) retarded by adsorption is the most ubiquitous of transport processes.
Advective transport is driven by the nonuniform pressure gradients on the rough sed-
iment terrain. Other transport processes include active sediment particle transport. A
comparison of characteristic times for a hypothetical scenario was made by Reible,
Valsaraj, and Thibodeaux (1991) and is shown in Table 6.14. Processes with small
half-lives are likely to be the most important transport processes.
Diffusion of compounds from sediment in the absence of advection or biodegrada-
tion can be represented by the equation derived in the previous section on groundwater
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