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Oil spill

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An oil spill is the release of a liquid petroleum hydrocarbon into the environment due to human activity, and is a form of pollution. The term often refers to marine oil spills, where oil is released into the ocean or coastal waters. The oil may be a variety of materials, including crude oil, refined petroleum products (such as gasoline or diesel fuel) or by-products, ships' bunkers, oily refuse or oil mixed in waste. Spills take months or even years to clean up.

Oil is also released into the environment from natural geologic seeps on the sea floor. Most man-made oil pollution comes from land-based activity, but public attention and regulation has tended to focus most sharply on seagoing oil tankers.

Environmental effects

Studies of the Exxon Valdez oil spill have shown that the environmental damage caused by oil spills can be greater than was previously thought. Petroleum-based hydrocarbons can negatively impact marine life at concentrations as low as one part per billion.

The lighter fractions of oil, such as benzene and toluene, are highly toxic, but are also volatile and evaporate quickly. Heavier components of crude oil, such as polycyclic aromatic hydrocarbons (PAHs) appear to cause the most damage. While they are less toxic than the lighter volatiles, they persist in the environment much longer. A heavy oil spill can also blanket estuaries and shoreline ecosystems such as salt marshes and tidal pools, preventing gas exchange and blocking light. The oil can mix deeply into pebble, shingle or sandy beaches, where it may remain for months or years.

Seabirds are severely affected by spills. The oil penetrates and opens up the structure of their plumage, reducing its insulating ability, and so making the birds more vulnerable to temperature fluctuations and much less buoyant in the water. It also impairs birds' flight abilities, making it difficult or impossible to forage and escape from predators. As they attempt to preen, birds typically ingest oil that coats their feathers, causing kidney damage, altered liver function, and digestive tract irritation. This and the limited foraging ability quickly causes dehydration and metabolic imbalances. Most birds affected by an oil spill die unless there is human intervention.

Marine mammals exposed to oil spills are affected in similar ways as seabirds. Oil coats the fur of Sea otters and seals, reducing its insulation abilities and leading to body temperature fluctuations and hypothermia. Ingestion of the oil causes dehydration and impaired digestion.

Largest oil spills

Oil Spills of over 100,000 tonnes or 30 million US gallons, ordered by Tonnes
Spill / Tanker	Location	Date	*Tonnes of crude oil	Reference
Gulf War oil spill	Persian Gulf	January 23 1991	136,000 - 1,500,000
Ixtoc I oil well	Gulf of Mexico	June 3 1979- March 23 1980	454,000 - 480,000
Atlantic Empress / Aegean Captain	Trinidad and Tobago	July 19 1979	287,000
Fergana Valley	Uzbekistan	March 2 1992	285,000
Nowruz oil field	Persian Gulf	February 1983	260,000
ABT Summer	off Angola	1991	260,000
Castillo de Bellver	Saldanha Bay, South Africa	August 6 1983	252,000
Amoco Cadiz	Brittany, France	March 16 1978	223,000
Amoco Haven tanker disaster	Mediterranean Sea near Genoa, Italy	1991	144,000
Odyssey	off Nova Scotia, Canada	1988	132,000
Sea Star	Gulf of Oman	December 19 1972	115,000
Torrey Canyon	Scilly Isles, UK	March 18 1967	80,000 - 119,000
Irenes Serenade	Navarino Bay, Greece	1980	100,000
Urquiola	A Coruña, Spain	May 12 1976	100,000

One tonne of crude oil is roughly equal to 308 US gallons, or 7.33 barrels.

Estimating the volume of a spill

By observing the thickness of the film of oil and its appearance on the surface of the water, it is possible to estimate the quantity of oil spilled. If the surface area of the spill is also known, the total volume of the oil can be calculated.

	Film Thickness		Quantity Spread
Appearance	in	mm	gal/sq mi	L/ha
Barely visible	0.0000015	0.0000380	25	0.370
Silvery sheen	0.0000030	0.0000760	50	0.730
First trace of color	0.0000060	0.0001500	100	1.500
Bright bands of color	0.0000120	0.0003000	200	2.900
Colors begin to dull	0.0000400	0.0010000	666	9.700
Colors are much darker	0.0000800	0.0020000	1332	19.500

Methods of cleaning

A sheen is usually dispersed (but not cleaned up) with detergents which makes oil settle to the bottom. Oils that are denser than water, such as Polychlorinated biphenyls (PCBs), can be more difficult to clean as they make the seabed toxic.

Methods for cleaning up include:

Bioremediation: use of microorganisms or biological agents to break down or remove oil
Controlled burning can effectively reduce the amount of oil in water, if done properly. But it can only be done in low wind, and can cause air pollution.
Dispersants act as detergents, clustering around oil globules and allowing them to be carried away in the water. This improves the surface aesthetically, and mobilises the oil. Smaller oil droplets, scattered by currents, may cause less harm and may degrade more easily. But the dispersed oil droplets infiltrate into deeper water and can lethally contaminate coral. Recent research indicates that some dispersants too are toxic to corals.
Watch and wait: in some cases, nautural attentuation of oil may be most appropriate, due to the invasive nature of facilitated methods of remediation, particularly in ecologically sensitive areas.
Dredging: for oils dispersed with detergents and other oils denser than water.
Skimming: Requires calm waters
Solidifying

Equipment used includes:

Booms: large floating barriers that round up oil and lift the oil off the water
Skimmers: skim the oil
Sorbents: large sponges that absorb oil
Chemical and biological agents: helps to break down the oil
Vacuums: remove oil from beaches and water surface
Shovels and other road equipments: typically used to clean up oil on beaches

Prevention

Secondary containment - methods to prevent releases of oil or hydrocarbons into environment.
Oil Spill Prevention Containment and Countermeasures (SPCC) program by the United States Environmental Protection Agency.
Double hulling - build double hulls into vessels, which reduces the risk and severity of a spill in case of a collision or grounding. Existing single-hull vessels can also be rebuilt to have a double hull.

References

External links

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Last updated on Thursday March 13, 2008 at 23:25:53 PDT (GMT -0700)
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