Sources of Water Pollutants
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.