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

©2001 CRC Press LLC
6.7 LEVEL II COMPUTER CALCULATIONS
As with Level I calculations, it is desirable to reduce the tedium of calculations
by using the computer. Figure 6.2 gives an illustrative fugacity form calculation,
a blank form being provided in the appendix. Computer programs that conduct
Level II calculations are available from the Internet. The input data include the
properties of the environment, the chemical properties, input rates by emission
and advection, and information on reaction and advection rates. The fugacity is
calculated, followed by a complete mass balance. Since equilibrium is assumed
to apply within the environment, it is immaterial into which phase the chemical
is introduced.
The user is encouraged to test the environmental behavior of some of the chemicals
introduced earlier, assuming or obtaining literature data on reaction rates.
Worked Example 6.7
Calculate the partitioning of the hypothetical chemical in Figure 5.6 assuming
that the rate constants for reaction are 0.001 h –1 in water, 0.01 h –1 in soil, and
0.0001 h –1 in sediment, and with no reaction in air. Assume advective inputs in air
at 10 –6 mol/m 3 (flow 10 7 m 3 /h) and in water at 0.01 mol/m 3 (flow 1000 m 3 /h). The
emission rate is 100 mol/h.
The hand calculation is fairly tedious and is reproduced in Figure 6.2. It involves
calculation of the total inputs of 100 mol/h (emission), 10 mol/h (advection in air),
and 10 mol/h (advection in water) totaling 120 mol/h (I). The reaction and advection
D values are then deduced and added to give a total (SD) of approximately
10,390 mol/Pa h. The fugacity is then I/SD or 0.0115 Pa.
Concentrations, amounts, and process rates can then be deduced and added to
check the mass balance. The computed output is given in Figure 6.3. Note that it is
not possible to input an infinite half life in air to give a zero rate of reaction. A
fictitious, large value of 10 11 h is used instead.
6.8 SUMMARY
In this chapter, we have learned to include advection and reaction rates in
evaluative Level II calculations. These calculations can be done using concentrations
and partition coefficients or fugacities and D values. The concepts of residence time
and persistence have been reintroduced. These are invaluable descriptors of environmental
fate. We have briefly reviewed the essential environmental chemistry of
biodegradation, photolysis, hydrolysis, and other reactions, and provided references
to studies, reviews, and estimation methods. Critics will be eager to point out a
major weakness in these calculations. Environmental media are rarely in equilibrium;
therefore, a use of a common fugacity or the use of equilibrium partition coefficients
to relate concentrations between phases or media is often not valid. Treating nonequilibrium
situations is the task of Chapter 7.