McKay, Donald. "Front matter" Multimedia Environmental Models ...

Thus, if there is river flow of 1000 m3/h
(G) from A to B of water containing 0.3
mol/m3
(C) of chemical, then the corresponding flow of chemical is 300 mol/h (N).
Turning to the evaluative environment, it is apparent that the primary candidate
advective phases are air and water. If, for example, there was air flow into the 1
square kilometre evaluative environment at 109
m3/h,
and the volume of the air in
the evaluative environment is 6 ¥ 109
m3,
then the residence time will be 6 hours,
or 0.25 days. Likewise, the flow of 100 m3/h
of water into 70,000 m3
of water results
in a residence time of 700 hours, or 29 days. It is easier to remember residence
times than flow rates; therefore, we usually set a residence time and from it deduce
the corresponding flow rate.
Burial of bottom sediments can also be regarded as an advective loss, as can
leaching of water from soils to groundwater. Advection of freons from the troposphere
to the stratosphere is also of concern in that it contributes to ozone depletion.
6.2.1 Level II Advection Algebra Using Partition Coefficients
If we decree that our evaluative environment is at steady state, then air and water
inflows must equal outflows; therefore, these inflow rates, designated G m3/h,
must
also be outflow rates. If the concentrations of chemical in the phase of the evaluative
environment is C mol/m3,
then the outflow rate will be G C mol/h. This concept is
often termed the continuously stirred tank reactor, or CSTR, assumption. The basic
concept is that, if a volume of phase, for example air, is well stirred, then, if some
of that phase is removed, that air must have a concentration equal to that of the
phase as a whole. If chemical is introduced to the phase at a different concentration,
it experiences an immediate change in concentration to that of the well mixed, or
CSTR, value. The concentration experienced by the chemical then remains constant
until the chemical is removed. The key point is that the outflow concentration equals
the prevailing concentration. This concept greatly simplifies the algebra of steadystate
systems. Essentially, we treat air, water, and other phases as being well mixed
CSTRs in which the outflow concentration equals the prevailing concentration. We
can now consider an evaluative environment in which there is inflow and outflow
of chemical in air and water. It is convenient at this stage to ignore the particles in
the water, fish, and aerosols, and assume that the material flowing into the evaluative
environment is pure air and pure water. Since the steady-state condition applies, as
shown in Figure 6.1a, the inflow and outflow rates are equal, and a mass balance
can be assembled. The total influx of chemical is at a rate GACBA
in air, and GWCBW
in water, these concentrations being the “background” values. There may also be
emissions into the evaluative environment at a rate E. The total influx I is thus
©2001 CRC Press LLC
I = E + GACBA
+ GWCBW
mol/h
Now, the concentrations within the environment adjust instantly to values C
A
and C
W
in air and water. Thus, the outflow rates must be G
A
C
A
and G
W
C
W
. These
outflow concentrations could be constrained by equilibrium considerations; for
example, they may be related through partition coefficients or through Z values to
a common fugacity.