Water Pollution, contamination of
streams, lakes, underground water, bays, or oceans by substances harmful
to living things. Water is necessary to life on earth. All organisms
contain it; some live in it; some drink it. Plants and animals require
water that is moderately pure, and they cannot survive if their water is
loaded with toxic chemicals or harmful microorganisms. If severe, water
pollution can kill large numbers of fish, birds, and other animals, in
some cases killing all members of a species in an affected area.
Pollution makes streams, lakes, and coastal waters unpleasant to look
at, to smell, and to swim in. Fish and shellfish harvested from polluted
waters may be unsafe to eat. People who ingest polluted water can become
ill, and, with prolonged exposure, may develop cancers or bear children
with birth defects,
The major water pollutants are chemical,
biological, or physical materials that degrade water quality. Pollutants
can be classed into eight categories, each of which presents its own set
of hazards.
Petroleum Products
Oil and chemicals derived from oil are used for fuel,
lubrication, plastics manufacturing, and many other purposes. These
petroleum products get into water mainly by means of accidental spills
from ships, tanker trucks, pipelines, and leaky underground storage
tanks. Many petroleum products are poisonous if ingested by animals, and
spilled oil damages the feathers of birds or the fur of animals, often
causing death. In addition, spilled oil may be contaminated with other
harmful substances, such as polychlorinated biphenyls (PCBs).
Pesticides and Herbicides
Chemicals used to kill unwanted animals and plants, for
instance on farms or in suburban yards, may be collected by rainwater
runoff and carried into streams, especially if these substances are
applied too lavishly. Some of these chemicals are biodegradable and
quickly decay into harmless or less harmful forms, while others are
nonbiodegradable and remain dangerous for a long time.
When animals consume plants that have been treated with
certain non-biodegradable chemicals, such as chlordane and
dichlorodiphenyltrichloroethane (DDT), these chemicals are absorbed into
the tissues or organs of the animals. When other animals feed on these
contaminated animals, the chemicals are passed up the food chain. With
each step up the food chain, the concentration of the pollutant
increases. In one study, DDT levels in ospreys (a family of fish-eating
birds) were found to be 10 to 50 times higher than in the fish that they
ate, 600 times the level in the plankton that the fish ate, and 10
million times higher than in the water. Animals at the top of food
chains may, as a result of these chemical concentrations, suffer
cancers, reproductive problems, and death.
Many drinking water supplies are
contaminated with pesticides from widespread agricultural use. More than
14 million Americans drink water contaminated with pesticides, and the
Environmental Protection Agency (EPA) estimates that 10 percent of wells
contain pesticides. Nitrates, a pollutant often derived from fertilizer
runoff, can cause methemoglobinemia in infants, a potentially lethal
form of anemia that is also called blue baby syndrome.
Heavy Metals
Heavy metals, such as copper, lead, mercury, and
selenium, get into water from many sources, including industries,
automobile exhaust, mines, and even natural soil. Like pesticides, heavy
metals become more concentrated as animals feed on plants and are
consumed in turn by other animals. When they reach high levels in the
body, heavy metals can be immediately poisonous, or can result in
long-term health problems similar to those caused by pesticides and
herbicides. For example, cadmium in fertilizer derived from sewage
sludge can be absorbed by crops. If these crops are eaten by humans in
sufficient amounts, the metal can cause diarrhea and, over time, liver
and kidney damage. Lead can get into water from lead pipes and solder in
older water systems; children exposed to lead in water can suffer mental
retardation.
Hazardous Wastes
Hazardous wastes are chemical wastes that are either
toxic (poisonous), reactive (capable of producing explosive or toxic
gases), corrosive (capable of corroding steel), or ignitable
(flammable). If improperly treated or stored, hazardous wastes can
pollute water supplies. In 1969 the Cuyahoga River in Cleveland, Ohio,
was so polluted with hazardous wastes that it caught fire and burned.
PCBs, a class of chemicals once widely used in electrical equipment such
as transformers, can get into the environment through oil spills and can
reach toxic levels as organisms eat one another.
Excess Organic Matter Fertilizers
and other nutrients used to promote plant growth on farms and in gardens
may find their way into water. At first, these nutrients encourage the
growth of plants and algae in water. However, when the plant matter and
algae die and settle underwater, microorganisms decompose them. In the
process of decomposition, these microorganisms consume oxygen that is
dissolved in the water. Oxygen levels in the water may drop to such
dangerously low levels that oxygen-dependent animals in the water, such
as fish, die. This process of depleting oxygen to deadly levels is
called eutrophication.
Sediment Sediment,
soil particles carried to a streambed, lake, or ocean, can also be a
pollutant if it is present in large enough amounts. Soil erosion
produced by the removal of soil-trapping trees near waterways, or
carried by rainwater and floodwater from croplands, strip mines, and
roads, can damage a stream or lake by introducing too much nutrient
matter. This leads to eutrophication. Sedimentation can also cover
streambed gravel in which many fish, such as salmon and trout, lay their
eggs.
Infectious Organisms
A 1994 study by the Centers for Disease Control and
Prevention (CDC) estimated that about 900,000 people get sick annually
in the United States because of organisms in their drinking water, and
around 900 people die. Many disease-causing organisms that are present
in small numbers in most natural waters are considered pollutants when
found in drinking water. Such parasites as Giardia lamblia and
Cryptosporidium parvum occasionally turn up in urban water supplies.
These parasites can cause illness, especially in people who are very old
or very young, and in people who are already suffering from other
diseases. In 1993 an outbreak of Cryptosporidium in the water supply of
Milwaukee, Wisconsin, sickened more than 400,000 people and killed more
than 100.
Thermal Pollution Water
is often drawn from rivers, lakes, or the ocean for use as a coolant in
factories and power plants. The water is usually returned to the source
warmer than when it was taken. Even small temperature changes in a body
of water can drive away the fish and other species that were originally
present, and attract other species in place of them. Thermal pollution
can accelerate biological processes in plants and animals or deplete
oxygen levels in water. The result may be fish and other wildlife deaths
near the discharge source. Thermal pollution can also be caused by the
removal of trees and vegetation that shade and cool streams.
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Polluted River
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