Oil Spill in the Gulf of Mexico
Oil and Nature
Available from the U.S. Fish and Wildlife Service,
New England Field Office, 1998
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Oil — when it heats our
homes and powers our
vehicles, it is a necessity.
When it spills into
our waters and coats
our shores, it becomes a
Oil spills along coasts affect many parts
of the environment, both non-living –
such as water, ocean bottom, and
shoreline; and living – like sea birds,
marine mammals, shellfish, and people.
Major oil spills most commonly involve
oils shipped in large quantities at sea,
such as crude petroleum, No. 1 and No.
2 fuel oils, diesel oil, Bunker C oil,
kerosene, and jet fuel. Oils are compounds,
complex mixtures that vary
widely in composition.
Oils can be described as belonging to
one of five groups:
- very light oils
(diesel, No. 2 fuel oil, light
crude, home heating);
- medium oils(most crude oils);
- heavy oils (heavy crude oils, No. 6 fuel oil, Bunker C);
- group V
(very heavy oils).
The different types of oils behave in
different ways during a spill, so the
response to a spill varies, depending on
the type of oil and quantity released.
Other important factors in a spill are:
- weather and season (for
example bird migration, nesting, or fish
- type of shoreline (such as sand
beach, tidal flat, rocky shore);
- exposure to wave and tidal
- types, abundance, and sensitivity
of living resources.
Most oil has a density less than water
and floats. The natural tendency of oil
is to spread in a thin layer on the
surface of the water as a sheen or film.
Such sheens are extremely difficult to
recover and do not remain for long
periods, however they do represent a
continued threat to fish and wildlife,
particularly nesting birds. Under
turbulent conditions, oil is more likely
to disperse into the upper layers of the
Oil changes rapidly once it is spilled
into water. These changes are enhanced
by the processes of evaporation,
dilution and emulsification (when water
incorporates into the oil, forming a
stable mixture). Some changes help
dissipate spilled oil, but others can
make it linger in the water, on the
bottom, or on the shore. Evaporation
tends to remove the more toxic components
and reduces the toxicity of spilled
oil. Emulsification, on the other hand,
can slow degradation of spilled oil.
“Weathering” describes the physical,
chemical, and biological changes that happen to crude oil and refined petroleum
products once they begin to
interact with the watery environment.
Ultimately, the more toxic elements of
oil products spilled in the marine,
estuarine, or freshwater environment
are broken down. Exposure to air,
sunlight, wave and tidal action, and
certain microscopic organisms degrades
and/or disperses oil. The rate of degradation
and dispersion depends on many
factors like the type of oil, weather,
temperature, and the type of shoreline
Very light oils are highly volatile,
which means they evaporate quickly,
usually completely within one to two
days after a spill. These oils are also
flammable and contain high concentrations
of soluble toxic compounds. Very
light oils can mix with water and kill
aquatic life that lives in the upper
layers. Cleanup is usually not necessary,
or possible, with spills of very
Light oils are moderately volatile, but
can leave a residue of up to one-third of
the amount spilled after a few days.
These oils contain moderate concentrations
of soluble toxic compounds. Light
oils leave a film or layer on intertidal
resources with the potential of longterm
contamination. Cleanup can be
very effective on spills of light oil.
Medium oils are less volatile, leaving a
residue of about two-thirds of the
amount spilled after 24 hours. These
oils are less likely to mix with water,
and oil contamination of intertidal areas
can be severe and long-term. The
impact of medium oils on waterfowl
and fur-bearing mammals can also be
severe. Cleanup is most effective with
spills of medium oil if conducted
Heavy oils have far less evaporation or
dilution potential, and they weather
more slowly. These oils do not readily
mix with water. Spills of heavy oils can
cause severe contamination of intertidal
areas and possible long-term contamination
of sediments. Heavy oils have
severe impacts on waterfowl and furbearing
mammals. Shoreline cleanup in
spills of this type is difficult and longterm
under most conditions.
Group V oils, mostly very heavy oils,
can float, sink, or hang in the water.
These oils can become oil drops and
mix in the water, or accumulate on the
bottom, or mix with sand and then sink.
As a rule, these oils are less toxic than
lighter oils, however they pose significant
problems to responders because
they are extremely difficult to track or
predict. In addition, particularly in the
Northeast, they may impact many
commercially important species like
flounder and lobster.
Spill response teams of federal, state
and local agencies, organizations and
industry representatives have prepared
contingency plans for oil spill emergencies.
The teams swing into action using
the plans when a spill occurs.
The U.S. Fish and Wildlife Service is
part of the spill response team, with
responsibility for managing and protecting
our living environment -- migratory
birds, anadromous fish (fish that begin
their lives in fresh water and live part of
their lives in the ocean), certain marine
mammals (sea otters, polar bears,
walrus, and manatees), sea turtles
(when they are on shore), and other
reptiles, and aquatic and terrestrial
The U.S. Fish and Wildlife Service
continues its work long after a spill
event, assessing the damage to habitat
and wildlife and finding ways to
minimize the long-term effects on new
generations of living creatures.