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

elatively high concentrations. This arises because, in principle, the amount in the
environment (kilograms) can be expressed as the product of the emission rate into
the environment (kilograms per year) and the residence time of the chemical in the
environment (years). Persistence also retards removal from the environment once
emissions are stopped. A legacy of “in place” contamination remains.
This is the same equation that controls a human population. For example, the
number of Canadians (about 30 million) is determined by the product or the rate at
which Canadians are born (about 0.4 million per year) and the lifetime of Canadians
(about 75 years). If Canadians were less persistent and lived for only 30 years, the
population would drop to 12 million.
Intuitively, the amount (and hence the concentration) of a chemical in the
environment must control the exposure and effects of that chemical on ecosystems,
because toxic and other adverse effects, such as ozone depletion, are generally a
response to concentration. Unfortunately, it is difficult to estimate the environmental
persistence of a chemical. This is because the rate at which chemicals degrade
depends on which environmental media they reside in, on temperature (which
varies diurnally and seasonally), on incidence of sunlight (which varies similarly),
on the nature and number of degrading microorganisms that may be present, and
on other factors such as acidity and the presence of reactants and catalysts. This
variable persistence contrasts with radioisotopes, which have a half-life that is
fixed and unaffected by the media in which they reside. In reality, a substance
experiences a distribution of half-lives, not a single value, and this distribution
varies spatially and temporally. Obviously, long-lived chemicals, such as PCBs,
are of much greater concern than those, such as phenol, that may persist in the
aquatic environment for only a few days as a result of susceptibility to biodegradation.
Some estimate of persistence or residence time is thus necessary for priority
setting purposes. Organo-halogen chemicals tend to be persistent and are therefore
frequently found on priority lists. Later in this book, we develop methods of
calculating persistence.
3.2.3 Bioaccumulation
The third factor is potential for bioaccumulation (i.e., uptake of the chemical by
organisms). This is a phenomenon, not an effect; thus bioaccumulation per se is not
necessarily of concern. It is of concern that bioaccumulation may cause toxicity to
the affected organism or to a predator or consumer of that organism. Historically, it
was the observation of pesticide bioaccumulation in birds that prompted Rachel
Carson to write Silent Spring in 1962, thus greatly increasing public awareness of
environmental contamination.
As we discuss later, some chemicals, notably the hydrophobic or “water-hating”
organic chemicals, partition appreciably into organic media and establish high concentrations
in fatty tissue. PCBs may achieve concentrations (i.e., they bioconcentrate)
in fish at factors of 100,000 times the concentrations that exist in the water in
which the fish dwell. For some chemicals (notably PCBs, mercury, and DDT), there
is also a food chain effect. Small fish are consumed by larger fish, at higher trophic
levels, and by other animals such as gulls, otters, mink, and humans. These chemicals
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