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

e achieved in 10 days. Detractors of steady-state models point with glee to situations
in which the modeler will be dead long before steady state is achieved.
Proponents of steady-state models respond that, although they have not specifically
treated the unsteady-state situation, their equations do contain much of the
key “response time” information, which can be extracted with the use of some
intelligence. The response time in the unsteady-state Example 6.5 was 93 hours,
which was SVZ/SD. This is identical to the overall residence time, t, in Example
6.2. The response time of an unsteady-state Level II system is equivalent to the
residence time in a steady-state Level II system. By inspection of the magnitude of
groups, VZ/D, or the reciprocal rate constants that occur in steady-state analysis, it
is possible to determine the likely unsteady-state behavior. This is bad news to those
who enjoy setting up and solving differential equations, because “back-of-theenvelope”
calculations often show that it is not necessary to undertake a complicated
unsteady-state analysis.
Indeed, when calculating D values for loss from a medium, it is good practice
to calculate the ratio VZ/D, where VZ refers to the source medium. This is the
characteristic time of loss, or specifically the time required for that process to reduce
the concentration to e –1 of its initial value if it were the only loss process. In some
cases, we have an intuitive feeling for what that time should be. We can then check
that the D value is reasonable.
6.6 THE NATURE OF ENVIRONMENTAL REACTIONS
The most important environmental reaction processes are biodegradation, hydrolysis,
oxidation, and photolysis. We treat each process briefly below with the view
to establishing methods by which the rate of the reaction can be characterized, and
giving references to authoritative reviews.
6.6.1 Biodegradation
Microbiologists are usually quick to point out that the process of microbial
conversion of chemicals in the environment is exceedingly complex. The rate of
conversion depends on the nature of the chemical compound; on the amount and
condition of enzymes that may be present in various organisms in various states of
activation and availability to perform the chemical conversion; on the availability
of nutrients such as nitrogen, phosphorus, and oxygen; as well as pH, temperature,
and the presence of other substances that may help or hinder the conversion process.
Virtually all organic chemicals are susceptible to microbial conversion or biodegradation.
Notable among the slowly degrading or recalcitrant compounds are highmolecular-weight
compounds such as the humic acids, certain terpenes that appear
to have structures that are too difficult for enzymes to attack, and many organohalogen
substances. Generally, water-soluble organic chemicals are fairly readily
biodegraded. Over evolutionary time, enzymes have adapted and evolved the capability
of handling most naturally occurring organic compounds. When presented
with certain synthetic organic compounds that do not occur in nature (notably the
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