Water pollutants result from many human
activities. Pollutants from industrial sources may pour out from the outfall
pipes of factories or may leak from pipelines and underground storage tanks.
Polluted water may flow from mines where the water has leached through
mineral-rich rocks or has been contaminated by the chemicals used in processing
the ores. Cities and other residential communities contribute mostly sewage,
with traces of household chemicals mixed in. Sometimes industries discharge
pollutants into city sewers, increasing the variety of pollutants in municipal
areas. Pollutants from such agricultural sources as farms, pastures, feedlots,
and ranches contribute animal wastes, agricultural chemicals, and sediment from
erosion.
The oceans, vast as they are, are not invulnerable to pollution. Pollutants
reach the sea from adjacent shorelines, from ships, and from offshore oil
platforms. Sewage and food waste discarded from ships on the open sea do little
harm, but plastics thrown overboard can kill birds or marine animals by
entangling them, choking them, or blocking their digestive tracts if swallowed.
Oil spills often occur through accidents, such as the wrecks of the tanker
Amoco Cadiz off the French coast in 1978 and the Exxon Valdez in Alaska in
1992. Routine and deliberate discharges, when tanks are flushed out with
seawater, also add a lot of oil to the oceans. Offshore oil platforms also
produce spills: The second largest oil spill on record was in the Gulf of
Mexico in 1979 when the Ixtoc 1 well spilled 530 million liters (140 million
gallons). The largest oil spill ever was the result of an act of war. During
the Gulf War of 1991, Iraqi forces destroyed eight tankers and onshore
terminals in Kuwait, releasing a record 910 million liters (240 million
gallons). An oil spill has its worst effects when the oil slick encounters a
shoreline. Oil in coastal waters kills tidepool life and harms birds and marine
mammals by causing feathers and fur to lose their natural waterproof quality,
which causes the animals to drown or die of cold. Additionally, these animals
can become sick or poisoned when they swallow the oil while preening (grooming
their feathers or fur).
Water pollution can also be caused by other types of pollution. For example,
sulfur dioxide from a power plant’s chimney begins as air pollution. The
polluted air mixes with atmospheric moisture to produce airborne sulfuric acid,
which falls to the earth as acid rain. In turn, the acid rain can be carried
into a stream or lake, becoming a form of water pollution that can harm or even
eliminate wildlife. Similarly, the garbage in a landfill can create water
pollution if rainwater percolating through the garbage absorbs toxins before it
sinks into the soil and contaminates the underlying groundwater (water that is
naturally stored underground in beds of gravel and sand, called aquifers).
Pollution may reach natural waters at spots we can easily identify, known as
point sources, such as waste pipes or mine shafts. Nonpoint sources are more
difficult to recognize. Pollutants from these sources may appear a little at a
time from large areas, carried along by rainfall or snowmelt. For instance, the
small oil leaks from automobiles that produce discolored spots on the asphalt
of parking lots become nonpoint sources of water pollution when rain carries
the oil into local waters. Most agricultural pollution is nonpoint since it
typically originates from many fields.
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