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

C F ª Z Ff W ª 1.608 mol/m 3 ª 402 g/m 3 (fish)
C B ª Z Ff S ª 2.988 mol/m 3 ª 747 g/m 3 (benthos)
Total Amounts
Water V WZ Wf W = 1072 mol (dissolved only)
Particles V PZ Pf W = 54 mol
Sediment V SZ Sf S = 14940 mol
Total = 16070 mol
Residence Times
Total input to water is 1.393 mol/h from emissions, advective inflow and the
atmosphere and 0.4 mol/h from sediment. Total input from sediment is by deposition
and diffusion from water.
Water (1072 + 54)/(1.393 + 0.4) = 628 h or 26 days
Sediment 14940/(0.643 + 0.054) = 21400 h or 893 days
Total 16070/1.393 = 11540 h or 481 days
©2001 CRC Press LLC
(does not include fish or benthos which are probably negligible)
Total inputs to and outputs from both water and sediment and the entire system
balance within round-off error.
This example, while tedious to do by hand, is readily implemented on a spreadsheet,
or the software available on the internet can be used. It gives a clear quantification
of all the fluxes and demonstrates which processes are most important. A
mass balance diagram such as Figure 8.6 illustrates the process rates clearly.
8.7 QWASI MODEL OF CHEMICAL FATE IN RIVERS
The QWASI lake equations can be modified to describe chemical fate in rivers
by one of two methods.
The river can be treated as a series of connected lakes or reaches, each of which
is assumed to be well mixed, with unique water and sediment concentrations. There
can be varying discharges into each reach, and tributaries can be introduced as
desired. The larger the number of reaches, the more closely simulated is the true
“plug flow” condition of the river. Figure 8.7 illustrates the approach.
The second approach is to set up and solve the Lagrangian differential equation
for water concentration as a function of river length, as was discussed when comparing
Eulerian and Lagrangian approaches in Chapter 2. This has been discussed