Visualizing Environmental Science

Movement and Fate of Toxicants 83

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The intended pathway of pesticides in the environment is shown
in the tan column to the right of the figure, and the actual
pathways are shown in the blue column to the left.

Aerial spraying of pesticide and evaporation

Gravitational

settling and

precipitation

Precipitation Precipitation

Runoff

and

seepage

Crops and livestock

Air

Intended pathway

for pesticide

Actual pathways

of pesticides in the environment

Humans

Aquatic

organisms

and fresh water

Precipitation

Erosion,

leaching

Groundwater

Animals

Harvest

Agricultural soil

Ocean, marine

organisms, and

ocean sediments

Ground

Food plants Target pest

Substantial evidence indicates that DDT causes these
birds to lay eggs with thin, fragile shells that usually
break during incubation, causing the chicks’ deaths. Af-
ter 1972, the year DDT was banned in the United States,
the reproductive success of many birds began to slowly
improve.
The impact of DDT on birds is the result of (1) its
persistence, (2) bioaccumulation, and (3) biological
magnification. Persistence means that the substance
is extremely stable and may take many years to break
down into a less toxic form. When an organism can’t
metabolize (break down) or excrete a toxicant, it is
simply stored, usually in fatty tissues. Over time, the
organism may accumulate high concentrations of the
toxicant.
The buildup of a persistent toxicant in
an organism is bioaccumulation ( Figure
{°n> and b). Organisms at the top of the
food chain tend to store greater concentra-
tions of bioaccumulated toxicants in their
bodies than those lower on the food chain.
As an example of biological m agnification,

consider a food chain studied in a Long Island salt

marsh that was sprayed with DDT over several years

for mosquito control: algae and plankton shrimp m

American eel m A tlantic needlefish m ring-billed gull

(ˆ}ÕÀiÊ{°nV and d). All top carnivores, from fishes to

humans, are at risk of health problems from biologi-

cal magnification. Scientists therefore test pesticides to

ensure that they do not persist and accumulate in the

environment.

Mobility in the Environment

Persistent toxicants tend to move through the soil,

water, and air, sometimes long distances. For exam-

ple, pesticides applied to agricultural lands

may be washed into rivers and streams by

rain, harming aquatic life ( ˆ}ÕÀiÊ{°™). If

the pesticide level in their aquatic ecosys-

tem is high enough, plants and a nimals may

die. At lower pesticide levels, aquatic life

may still suffer from symptoms of chronic

toxicity such as bone degeneration in fishes.

biological

magnification The

increase in toxicant

concentrations as

a toxicant passes

through successive

levels of the food

chain.