Geoscience Reference
In-Depth Information
12.3 SIMULATION OF SEDIMENT-BORNE
CONTAMINANT TRANSPORT
Sediment is a major source of pollutants in aquatic systems. Not only does sediment
itself significantly affect aquatic systems by erosion and deposition, but also it sorbs
contaminants that degrade the quality of receiving water bodies. Traditionally, mod-
eling of contaminant transport and water quality has mainly focused on the role of
water flow and paid less attention to the effect of sediment. It is necessary to establish
numerical models to investigate the transport of both water-borne and sediment-borne
contaminants and their impacts on water quality. This has been studied by Lang and
Chapra (1982) for lake systems. A generalizedmodeling framework for various aquatic
systems is presented in this section.
12.3.1 Sorption and desorption of contaminants
on sediment particles
Sorption is a process whereby a dissolved substance is transferred to and becomes asso-
ciated with solidmaterials (Chapra, 1997). It includes both adsorption and absorption.
Adsorption is a surface phenomenon in which the dissolved substance is accumulated
on the surface of solids, whereas absorption is a bulk phenomenon in which the dis-
solved substance interpenetrates or intermingles with solids. Desorption is the reverse
process of sorption, in which a sorbed substance is released from solid particles. Sorp-
tion and desorption are encountered in diverse situations of contaminant transport.
Many contaminant species, such as phosphorus, heavy metals, nuclides, bacteria, and
viruses, can be transferred from the dissolved phase to the sorbed phase associated
with sediment particles and then transported with sediment by the flow.
Consider a control volume consisting of a water and sediment mixture in the water
column or sediment bed, as shown in Fig. 2.4. A contaminant constituent is either
dissolved in water or sorbed on sediment particles. The concentrations of the dissolved
and sorbed parts are defined as
M
d
V
t
M
s
V
t
C
d
=
,
C
s
=
(12.93)
and the total contaminant concentration is
M
d
+
M
s
C
t
=
C
d
+
C
s
=
(12.94)
V
t
where
V
t
is the total volume of the water and sediment mixture (m
3
);
M
d
and
M
s
are the masses (mg) of the dissolved and sorbed contaminants in the control volume,
respectively;
C
d
and
C
s
are the concentrations (mg
m
−
3
) of the dissolved and sorbed
contaminants, respectively; and
C
t
is the total contaminant concentration (mg
·
m
−
3
).
Note that some constituents, such as phosphorus, also exist in a separate particulate
form (not necessarily sorbed to sediment particles). This may be considered by adding
a particulate component in Eq. (12.94). However, for simplicity, only the dissolved
·
