Environmental Engineering Reference
In-Depth Information
that the worms in the sediments had become highly tolerant
to cadmium. Some years after the pollution was cleaned up, as
required by the Environmental Protection Agency (EPA), the
scientists revisited the site and found that the worms had lost
their cadmium tolerance over relatively few generations.
How is the degree of toxicity measured?
“The dose makes the poison.” It is important to have accu-
rate measurements of how much of a given chemical causes a
given effect. Contaminants generally occur in low concentra-
tions, but small concentrations such as parts per million and
parts per billion can cause effects. A  part per million (ppm)
seems like a very small amount—and it is. One ppm (or mg/l)
is equivalent to one drop of a substance in about 13.2 gallons of
water. One ppb (or μg/l) is one part in 1 billion—much smaller
than a ppm. One drop in one of the largest tanker trucks used
to haul gasoline would be 1 ppb. Some chemicals, including
dioxin and tributyltin are toxic at levels below 1 ppb. It is
difficult and expensive to measure these low concentrations
of contaminants. Sophisticated equipment such as atomic
absorption spectrophotometers or gas chromatograph/mass
spectrometers is needed.
How can field studies be used to understand toxicity?
Integrated field approaches are important, along with labora-
tory studies to provide insights into effects at the population
and community level. Field experiments can investigate con-
taminated environments—but hardly ever, only under very
restricted conditions, may scientists release known amounts
of chemicals in the field to observe effects in controlled experi-
ments. Attempts to bring the field closer to the lab include stud-
ies on multiple species placed together in microcosms (small
containers) or mesocosms (large containers), which can be used
to investigate community level effects of contaminants. They
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