2. Oil Pollution

Evolution of oil at sea

When oil is spilled at sea it undergoes a number of physical and chemical changes known as weathering. Oils weather in different ways. Some of the processes cause part of the oil to leave the sea surface, whilst others cause the oil that remains on the surface to become more persistent.

Find out which processes are responsible for the break up and dissipation of oil at sea in a chapter about oil weathering.

Directly after the spill, the fresh oil spreads out over the water surface, initially as a single slick. After a few hours, the slick will begin to break up and will then form narrow bands parallel to the wind direction (see photograph below).

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recent oil spill
Recent spillage (a few hours old).
Photo: Rainer Reuter

After a few hours up to 1 day, single slick fragments and thicker patches are increasingly noticeable compared to the thin layers which have a rainbow, sheen or metallic appearance.

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oil spill after a few hours
In an accident in the Philippines, the vessel M/T Solar I, spilled more than 100,000 gallons of fuel [bunker] oil and sank in nearly 2,300 feet of water in the Panay Gulf, located off the west coast of the central Philippines near Nueva Valencia. An additional 450,000 gallons of fuel remain in the vessel risking continued leaching into the Panay Gulf
Photo: NOAA


With ongoing weathering processes, after a few days brick red patches of reversed emulsion (i.e. water-in-oil emulsion) may form that are surrounded by thinner layers of rainbow, sheen or metallic appearance

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oil spill after a few days
Appearance of an oil spill a few days after the spill: an oil slice approaching land, resulting from the Exxon Valdez Oil Spill in March 1989.
Photo: Erich Gundlach / EVOSTC

Subsequently, several days after the spill, the thinner films gradually disappear and eventually only patches or stripes of emulsion may remain, especially in a rough sea. Iridescences can however reappear later, even several weeks or months after the spillage, if the sea is very calm and the sun is warming up the spill.

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irridescence of an oil slick
Irridescence of an oil slick visible on the water's surface above the sunken battleship USS Arizona (BB-39) in Pearl Harbor, Hawaii. The USS Arizona sank on 7th December 1941 after it had been hit by a Japanese aircraft.
In this case the iridescence is caused by small amounts of oil that are still being released from the ship wreck.
Photo: James G. Howes, November, 2005