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

that all water will spend 100 days in the lake. Some will bypass in only 10 days,
and some will persist in backwaters for 1000 days but, on average, the residence
time will be 100 days.
The reason that this concept is so important is that chemicals exhibit variable
lifetimes, ranging from hours to decades. As a result, the amount of chemical present
in the environment, i.e., the inventory of chemical, varies greatly between chemicals.
We tend to be most concerned about persistent and toxic chemicals, because relatively
small emission rates (E) can result in large amounts (M) present in the
environment. This translates into high concentrations and possibly severe adverse
effects. A further consideration is that chemicals that survive for prolonged periods
in the environment have the opportunity to make long and often tortuous journeys.
If applied to soil, they may evaporate, migrate onto atmospheric particles, deposit
on vegetation, be eaten by cows, be transferred to milk, and then consumed by
humans. Chemicals may migrate up the food chain from water to plankton to fish
to eagles, seals, and bears. Short-lived chemicals rarely survive long enough to
undertake such adventures (or misadventures).
This lengthy justification leads to the conclusion that, if we are going to discharge
a chemical into the environment, it is prudent to know
1. how long the chemical will survive, i.e., t, and
2. what causes its removal or “death”
This latter knowledge is useful, because it is likely that situations will occur in
which a common removal mechanism does not apply. For example, a chemical may
be potentially subject to rapid photolysis, but this is not of much relevance in long,
dark arctic winters or in deep, murky sediments.
In the process of quantifying this effect, we will introduce rate constants, advective
flow rates and, ultimately, using the fugacity concept, quantities called D values,
which prove to be immensely convenient. Indeed, armed with Z values and D values,
the environmental scientist has a powerful set of tools for calculation and interpretation.
It transpires that there are two primary mechanisms by which a chemical is
removed from our environment: advection and reaction, which we discuss individually
and then in combination.
©2001 CRC Press LLC
6.2 ADVECTION
Strangely, “advection” is a word rarely found in dictionaries, so a definition is
appropriate. It means the directed movement of chemical by virtue of its presence
in a medium that happens to be flowing. A lazy canoeist is advected down a river.
PCBs are advected from Chicago to Buffalo in a westerly wind. The rate of advection
N (mol/h) is simply the product of the flowrate of the advecting medium, G (m3 /h),
and the concentration of chemical in that medium, C (mol/m3),
namely,
N = GC mol/h