The Ocean Studies Board and Marine Board of the National Academy
of Sciences has published Oil
in the Sea (2003) which summarizes sources of oil pollution:
Nearly 85 percent of the 29 million gallons
of petroleum that enter North American ocean waters each year as
a result of human activities comes from land-based runoff, polluted
rivers, airplanes, and small boats and jet skis, while less than
8 percent comes from tanker or pipeline spills.
Average annual contribution to oil in the ocean (1990-1999) from
major sources of petroleum in kilotonnes.
In The Sea, Ocean Studies Board and Marine Board of the National
Academy of Sciences (2003).
The major sources of petroleum in the sea, in order of
- Natural seeps from rocks below the sea floor. Oil seeps are common
in many areas, including the Gulf of Mexico and offshore of Southern
California, and in other areas where oil is found beneath the continental
Radarsat image of oil seeps near Green canyon in the Gulf of Mexico.
The oil reaching the sea surface produces a slick that reflects little
radio energy, seen as black lines in this image. Radarsat is
a Canadian satellite that uses a synthetic-aperture
radar to map radio waves reflected from the
From MDA's Geospatial
- Consumption, which includes runoff from land and oil from marine
boating and jet skis in coastal waters. Most cars drip oil on streets
and highways which is washed off by rains. The millions of cars in
large coastal cities are important sources.
The dark band in the center of the lanes on
this highway are caused by oil dripping from countless cars that travelled
along the U.S. 49 northbound in the vicinity of Florence,
- Transportation, which includes spills from tankers and
pipelines as well as intentional discharge from ships at sea.
- Extraction, which includes spills from offshore platforms and blowouts
during efforts to explore for and produce oil and gas. An example
is the IXTOC
I oil well blowout in the Bay of Campeche, Mexico, June 1979 to
March 1980, which released 476,000 tonnes of crude oil into the Gulf
The Itox 1 exploratory well in the Bay of Camphece blew out on 3 June
1979 causing a major oil spill in the Gulf of Mexico.
By the time the well was brought under control
in 1980, an estimated 140 million gallons of oil had spilled into the
bay. The IXTOC I is currently number 2 on the list of largest oil spills
From Incident News.
Oil spills from oil tankers operating at sea world-wide account
for only 7.7% of oil in the ocean, yet large spills attract far more
attention than other much larger sources of oil pollution. The International
Tanker Owners Pollution Federation Historical
Data has information on all spills, large and small. They note
that "The average
number of large spills per year during the 1990s was about a third
of that witnessed during the 1970s."
Tobago, West Indies
nautical miles off Angola
Saldanha Bay, South Africa
nautical miles off Nova Scotia, Canada
nautical miles off Honolulu
nautical miles off Atlantic coast of Morocco
|| Off Kharg Island, Gulf of Iran
the Spanish coast
William Sound, Alaska, USA
Figure and table are from International
Tanker Owners Pollution Federation compilation
of historical data.
If you want to see images of some of the bigger spills, go to the NOAA
The 2010 oil spill in the Gulf of Mexico can be placed into context of other major oil-well blowouts. Three large spills include (Schenkman, 2010):
- IXTOC 1. It was the biggest spill witha daily outflow of 30,000 barrels per day initially, dropping to 10,000 barrels per day. The flow was stopped after eight months and 3.5 million barrels were released into the Gulf of Mexico about 80 km offshore of Yucatan, Mexico in 1979.
- Ekofisk. It released 202,000 barrels of oil about 250 km off the coast on Norway in the North Sea in 1977.
- Santa Barbara. This relatively small spill released 100,000 barrels of oil into the Santa Barbara Channel offshore of Santa Barbara, California in 1969, resulting in widespread efforts to ban drilling offshore of California and other coasts of the US.
Cleaning Up After Oil Spills
No two oil spills are the same because of the variation in oil types,
locations and weather conditions involved. However, broadly speaking,
there are four main methods of response.
1. Leave the oil alone so that it breaks down by natural means.
2. Contain the spill with booms and collect it from the water surface using
3. Use dispersants to break up the oil and speed its natural biodegradation.
4. Introduce biological agents to the spill to hasten biodegradation.
Petroleum Production and Exploration Association.
Oil spill have an immediate effect on marine life, and a longer term
effect. The International Tanker Owners Pollution Federation lists Effects
of Oil Spills. They include:
- Biological, including physical effects such as smothering and the
influence of toxic chemicals.
Simply, the effects of oil on marine life, are caused by either the
physical nature of the oil (physical contamination and smothering)
or by its chemical components (toxic effects and accumulation leading
to tainting). Marine life may also be affected by clean-up operations
or indirectly through physical damage to the habitats in which plants
and animals live.
The main threat posed to living resources
by the persistent residues of spilled oils and water-in-oil emulsions
one of physical smothering. The animals and plants most at risk are
those that could come into contact with a contaminated sea surface.
Marine mammals and reptiles; birds that feed by diving or form flocks
on the sea; marine life on shorelines; and animals and plants in
The most toxic components in oil tend to
be those lost rapidly through evaporation when oil is spilt. Because
of this, lethal concentrations of toxic components leading to large
scale mortalities of marine life are relatively rare, localized
and short-lived. Sub-lethal effects that impair the ability of
individual marine organisms to reproduce, grow, feed or perform
other functions can be caused by prolonged exposure to a concentration
of oil or oil components far lower than will cause death. Sedentary
animals in shallow waters such as oysters, mussels and clams that
routinely filter large volumes of seawater to extract food are
especially likely to accumulate oil components. Whilst these components
may not cause any immediate harm, their presence may render such
animals unpalatable if they are consumed by man, due to the presence
of an oily taste or smell. This is a temporary problem since the
components causing the taint are lost (depurated) when normal conditions
From Effects of Marine
Processes influencing weathering of oil in the sea.
of Oil at Sea.
- Destruction of coastal habitats.
- Damage to boats and gear used for fishing, and loss of market for
fish if buyers suspect the fish may be contaminated by the oil spill.
The long-term damages are more benign.
- Smaller, more volatile molecules in oil quickly evaporate or
oxidize. These molecules are the most toxic to life.
- Larger, less volatile molecules are not toxic. We put asphalt, which
is mostly the larger oil molecules, on roads and driveways and grass
grows through the cracks in the asphalt. And, the asphalt quickly oxidizes.
Few roads are useful after a decade because the asphalt oxidizes,
hardens, and breaks up. The asphalt must be replaced.
- Tar has been used for many purposes for centuries with little ill
We have learned much from previous oils spills. What can we do to minimize
environmental damage? Sometimes the clean up is worse than the spill.
The NOAA has been monitoring Prince William Sound, the location
of the spill, and they have amassed information on Results, Lessons
Learned, and Implications.
- Set aside areas that have not been cleaned to compare with cleaned
areas to assess usefulness of cleaning.
- High-pressure, hot-water cleaning causes short-term and long-term
- Stating that cleanup does "more harm than good" while to some
extent true, is a bit of an oversimplification. Still, we have learned
use of detergents, which are toxic to marine life, to disperse
- The use of steam and hot water to clean rocks, which kills
all organisms on the rocks.
Current evidence implies that oiled and
hot-water washed sites initially suffered more severe declines
in population abundance than oiled and not-washed sites.
- Any cleanup that changes the physical makeup of the area delays
recovery. In particular, Large scale excavation of gravel beaches,
which delays recovery for many years.
- Oil that penetrates deeply into sand or sediments can stay fresh
for years and be released slowly back into the water. Cleanup is
difficult because it disrupts the physical state of the area. Recovery
is delayed many years.
- Using water to flush away oil may remove fine sediment needed by
For more information, read the conclusions of the impact of the Exxon
Valdez oil spill in Alaska. David S. Page at Bowdoin College, Department
of Chemistry, and Edward S. Gilfillan at Bowdoin College, Environmental
Studies Program, have also summarized to effects
after 10 years. Results of studies of major
British oil spills at sea by the British
Marine Life Study Society. By 2001 a survey by Auke Bay Laboratory
found only 20 acres of beach contained oil residues, all buried below
the surface of the beach (Alaska Fisheries Science Center: The
Exxon Valdez Oil Spill: How Much Oil Remains?).
See the yearly photographs of Means
Rock to understand the difficulty
of determining if an area has returned to normal.
NOAA has a useful
site for students and teachers with much useful information.
Amaco Cadiz, the world's fourth largest largest oil spill, after
20 years. Very few problems: However,
this oil slick has become an old memory for the fishing and tourism
industries and for the other economical activities...The
shipwreck has become a new “home” for fishes and crustaceans
and a place to explore for experienced divers.
Ocean Studies Board and Marine Board (2003). Oil
in The Sea III: Inputs, Fates, and Effects. Washington DC:
The National Academies Press
Schenkman, L. (2010). Three Historic Blowouts. Science 328 (5979): 675.
7 July, 2010