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 Bioaccumulation
is the increase in the concentration of a chemical in a biological
organism over time. Compounds accumulate in living things when they
are absorbed and stored faster than they are metabolized or eliminated.
Understanding bioaccumulation is important in protecting humans and
other organisms from exposure to chemicals that may cause harm.
History
The
production and release of synthetic chemicals  began
at the end of WW II. The chemicals were primarily used to control
diseases,
such as malaria and yellow fever, and to increase food production.
However, in some cases, the release of these chemicals into the environment
had negative affects on biodiversity and human health. One of the
first chemicals found to have harmful affects on the environment
was DDT. By the 1950s, DDT was linked to the decline in
bald eagle and other bird populations due to the thinning of eggshells.
In 1962, the biologist Rachel Carson, wrote about the impacts of
pesticides on wildlife and human health in her book, Silent
Spring,
a landmark for the environmental movement.
Terms used in conjunction with bioaccumulation.
Uptake occurs
when an organism ingests a substance by breathing, swallowing, or
absorbing it through the skin. Storage refers to the presence
of a chemical in a body tissue or an organ. Bioconcentration is
when there is a net accumulation of a chemical in an organism as
a result of
intake
and elimination. Biomagnification is the increased
accumulation in the concentration of a chemical as it moves up through
the food chain.
For example, the following graphic illustrates a typical food chain:
Algae is eaten by the water flea eaten by a minnow eaten by a trout and
finally consumed by an osprey.
Examples of Chemical Bioaccumulation
Persistent
Organic Pollutants (POPs) are synthetic chemicals that resist
the normal processes of degradation. They are carbon-based chemical
compounds that share four characteristics: high toxicity, persistence,
fat solubility, and ability to evaporate and travel long distances.
There are 12 top POPs (called "the Dirty Dozen"  by
environmental groups) targeted for banning by an international community
of governments, scientists and
environmental groups. They include: 8 pesticides (aldrin,
chlordane, DDT, dieldrin, endrin, heptachlor, mirex, and toxaphene);
2 types of industrial chemicals (polychlorinated biphenyls or PCBs
and hexachlorobenzene); and 2 types of by-products of the manufacture,
use, and/or combustion of chlorine and chlorine-containing materials
(dioxins and furans).
Some
of these POPs, such as DDT, aldrin, dieldrin, toxaphene, chlordane,
and heptachlor, have been banned or restricted in the U. S. and some
other countries. However, because they are persistent and do not break
down
for a long time, scientists are still finding them in high concentrations.
Many developing countries are still using such POPs as DDT, and because
of their ability to travel far distances, these chemicals are still being
spread globally.
Focus on Pesticides
A pesticide is
any chemical used to repel, kill or control fungi, animals, or plants.
Pesticides are divided into different categories depending
on the kinds of organisms they are used to control. Insecticides are
used to control insect populations. Fungicides are used to kill
fungal growths. Mice and rats are killed by rodenticides and
unwanted plants are controlled with herbicides. Since many of
the POPs have been banned in the U. S., the chemical industry has developed
new pesticides called
organophosphates and carbamates. These pesticides break
down into harmless products in a few hours or days so they do not bioaccumulate.
These
new pesticides do have some side effects. They are not species-specific
and can kill beneficial species too. Since they do not last long in
the environment, more frequent applications must be performed and that
is more expensive. Also, these pesticides are much more toxic to humans
and other vertebrates. Common types of organophosphates are malathion,
parathion, and diazinon. Sevin, aldricarb and propoxur are examples
of carbamates.
WHAT CAN BE DONE
International Negotiations to Phase out POPs
The
phasing out of the use of POPs on a global scale began at the 1992
Earth Summit in Rio de Janeiro. Over 170 governments agreed to stop
using
synthetic chemicals that accumulate in the environment. The United Nations
Environment Programme took the lead in developing a plan for the phase-out
of a number of different chemicals. Banning harmful chemicals works!
In 1972, DDT was banned in the United States. Since then, the recovery
of impacted species such as the Bald Eagle and the Brown Pelican have
been well documented.
Alternatives to pesticide use on farms
In
order to eliminate the use of synthetic chemicals, some farmers are
returning to the traditional methods of farming now called "organic
farming." While they do experience smaller yields than farms that
use synthetic chemicals, on an average organic farms use only 40%
of the energy used on conventional farms. This reduces costs and raises
the profit margin.  Often
organic produce is more expensive to the consumer because the demand
is not great enough to influence pricing. Organic
farming uses methods such as crop rotation and planting a mixture of
crops to prevent pest problems. To replace the use of synthetic chemicals,
organic farmers, as well as many traditional farmers, also use Integrated
Pest Management (IPM) methods that take into consideration the entire
ecological aspects of the crop and the particular pest to which it
is susceptible. When chemicals must be used, technology can assist
the farmer in not overusing them. For example, GPS (Global Positioning
Systems) can help farmers target the areas that most need chemicals
(instead of spraying the entire field).
Personal use of synthetic chemicals
Each
person can do his or her part to reduce the use of synthetic chemicals.
Organic methods for plant and lawn care can be substituted for pesticides.
If organic methods are not feasible, the proper use of chemicals is
recommended. Many times people do not use the prescribed amount called
for on the directions of the chemicals. For example, they may think
that more is better and use 3 times the recommended amount, though
that is often not the case. (Refer to Additional Resources listed in
this activity plan for more information.)
Civic involvement
Students can get involved in their schools and communities to eliminate
or reduce the use of synthetic chemicals. See Lesson
2 on page 26.
NOTE:
To help students better understand bioaccumulation/biomagnification,
refer to the following sources:
• Enviro-Tacklebox™ Video on The Aggravation of Accumulation
•
Project WILD's "Deadly Links" activity.
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