Environmental Engineering Reference
In-Depth Information
2.6.4 Predicting Contaminant Transport
There are many models that purport to describe the movement of contaminants, solutes,
etc., in soil. For convenience in discussion, we will use the term
solutes
to mean contam-
inants and all other kinds of solutes found in the porewater. Most of the models deal
with movement of solutes in saturated soils and is best applied to inorganic contaminants
such as heavy metals. Application of the models for determination of transport of organic
chemicals has been attempted by some researchers—with varying degrees of success.
So long as the movement of the contaminants is governed by Fick's law, and instanta-
neous equilibrium sorption processes occur, some success in prediction of transport can
be obtained. The ability to properly model partitioning of the contaminants remains as
one of the central issue in determination of success or failure of predictions. The problem
is complicated by the fact that a reasonably complete knowledge of the initial and bound-
ary conditions is not always available. Additionally, the presence of multicomponent con-
taminants and their individual and collective reactions with the soil fractions will make
partitioning determinations dificult. Detailed discussions of the modeling problems and
the physicochemical interactions and partitioning of contaminants in soils can be found in
textbooks dedicated to the study of contaminant fate and transport in soils, e.g., Knox et al.
(1993), Fetter (1993), Huang et al. (1995a,b), Yong (2001), Yong and Mulligan (2004). The most
common and widely used transport model has the relationship shown as:
2
∂
∂
c
t
∂
∂
c
x
∂
∂
c
xn
ρ
ρ
∂
∂
c
t
*
=
D
−
v
−
(2.2)
L
2
w
where
c
is the concentration of solutes or contaminants,
t
is the time,
D
L
is the longitudinal
dispersion coeficient,
v
is the advective velocity,
x
is the spatial coordinate, ρ is the bulk
density of soil, ρ
w
is the density of water,
n
is the porosity of soil, and
c*
is the concentration
of solutes or contaminants adsorbed by soil fractions (see Figure 2.15). If we assume a slope
constant
k
d
=
k
1
(as shown in Figure 2.15) for the constant adsorption isotherm, the concen-
tration of solutes sorbed by the soil fractions
c*
can be written as
c* = k
d
c
. The slope constant
k
d
is deined as the distribution coeficient and is meant to indicate the manner of distribu-
tion of the solutes being transported in the porewater of a soil-water system. Equation 2.2
can be written in a more compact form to take into account the distribution coeficient as
follows:
2
∂
∂
c
t
∂
∂
c
∂
∂
c
x
R
=
D
−
v
(2.3)
L
2
x
ρ
n
where
R
is the retardation factor,
1 +
k
.
d
w
See Chapter 9 for a detailed discussion of the transport and fate of contaminants.
2.7 Geoenvironmental Land Management
The major geosphere and hydrosphere features that constitute the geoenvironment com-
ponents for land management attention are shown in Figure 2.16. Land management, as
Search WWH ::
Custom Search