Geoscience Reference
In-Depth Information
be derived by summing Eqs. (12.104) and (12.105) as
DC
tw
Dt
−
∂(ω
s
f
sw
C
tw
)
=
q
tw
−
k
tw
C
tw
(12.110)
∂
z
where
f
sw
is the fraction of the sorbed contaminant over the total contaminant in the
water column. Thus, the following relations exist:
q
tw
=
q
dw
+
q
sw
(12.111)
k
tw
=
f
dw
k
dw
+
f
sw
k
sw
(12.112)
and
q
t
,
ex
=
q
d
,
ex
+
q
s
,
ex
C
tb
=
max
(
E
b
−
D
b
,0
)
p
m
1
−
C
tw
p
m
f
dw
1
f
sw
s
+
(
E
b
−
)
s
+
min
D
b
,0
(12.113)
1
−
p
m
−
The equilibrium partition model has been used in many studies (Thomann
et al
.,
1991; Schrestha and Orlob, 1996; Ji
et al
., 2002), because it is simpler than the
non-equilibrium one.
12.3.3 Contaminant transport in sediment bed
In the sediment bed, contaminants are transported by the pore water flow. In general,
the pore water flow and contaminant transport can be simulated using a subsurface
flow model. It is of great interest to couple the surface and subsurface flow models
to investigate the flux exchanges between the water column and the sediment bed.
However, because the pore water velocity in the sediment bed is much slower than
the flow velocity in the water column, the transport of contaminants in the sediment
bed is often simulated using the multiple-layer flux model described below, which is
generalized from those of Lang and Chapra (1982) and DiToro (2001).
The sediment bed is divided into several layers. Definition of the layers in sedi-
ment and contaminant transport models may be different. However, for convenience,
the definition in the sediment model, which is shown in Fig. 2.9, is used directly in
the contaminant model. In the first layer, which is the mixing layer in the sediment
model and the aerobic layer in the contaminant model, the variation in concentration
of the dissolved contaminant is attributed to external loading, adsorption, desorp-
tion, decay, diffusional transfer at the bed surface, exchange due to sediment erosion
and deposition, diffusional transfer at the interface between bed layers 1 and 2, and
exchange due to burial and digging (rising and lowering) at the interface between bed
layers 1 and 2. The variation in concentration of the sorbed contaminant is attributed
to these effects, except for the diffusional transfers at the bed surface and between
