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Recognizing Oil Spills

When oil enters the water, it initially spreads out and forms a continuous patch on the water’s surface. This layer or patch of oil spreads over the surface waves, making the area appear smoother or “slick” compared to the surrounding water.

As the oil layer becomes thinner, it is broken up by waves, wind, and current movements into smaller patches and narrow bands, or “windrows,” which are oriented in the direction of the wind or current.

Silvery sheen oil spill

Photo Credit: NOAA


Diesel, No. 2 fuel oil, and light crudes are categorized as “light oils.” When spilled into the water, these oils spread quickly and create a rainbow or silvery sheen on the water. A sheen is a thin layer of oil (less than 0.0002 inches or 0.005 mm) floating on the water’s surface.

Marine diesel fuel, commonly used on the water for recreational boats or smaller commercial vessels, will usually create a dull, dark colored film when spilled into the water.

Light oils either evaporate or dissipate quickly from the surface of the water and are easily mixed into the water column. Oil can spread out very thinly. In fact, a gallon of oil can cover more than a full square mile, forming the tiniest film on the surface at one-hundredth of a millimeter thick.

Light oil is not heavy enough to settle down to the seafloor, but it can remain in the environment when it is absorbed into sediments or when it is dispersed into small droplets that stay suspended in the water.

While a light oil spill might seem like it would disappear quickly, it can take months for the last traces of the oil to leave the environment.

Direct contact with diesel oil is toxic to fish, and it has been known to bioaccumulate within crabs and other shellfish, making then unsafe for human consumption.

Birds and mammals are affected by making direct contact with their feathers and fur or by ingesting the oil when they engage in preening. Even very thin sheens can do great harm to these animals.


Gasoline and jet fuels are very light oils. These oils can evaporate very quickly (within 1-2 days) and are extremely toxic when they come into contact with marine life. Very light oil spills are nearly impossible to clean up or contain. Once they enter the water, these oils are extremely dangerous to the environment.

Oil from Deep Water Horizon spill

Photo Credit: NOAA


Most crude, or naturally occurring oils, fall into the category of medium oils. The Deepwater Horizon spill of 2010 released 210 million gallons of medium crude oil into the Gulf of Mexico.

While roughly one-third of spilled crude oil will evaporate within 24 hours, medium oil spills can cause serious long-term contamination. Medium oil is heavy enough to settle down onto the seafloor, which means that it can take years for it to finally leave the environment.

Tar ball washed up on a beach

Photo Credit: NOAA


Heavy oils, such as bunker fuels, crude oil, and No. 6 fuel oil, are the most common type of ship fuel. When spilled, these oils create thick, dark slicks that do not evaporate when exposed to sunlight. They eventually spread out to form smaller, discrete patches or streaks.

The thickest oils can form into tar balls when mixed with the air and agitated by the movement of the waves in a process known as weathering. Tar balls can travel great distances and eventually wash up on shore.

Heavy oils impact every area of the marine environment. They can float at the surface, remain suspended in the water column, or sink to the seafloor. These oils can remain there for years.

Heavy oils can coat birds and fish, restricting their movement, covering their airways, and smothering them to death. When responders act quickly, heavy oils can be contained and collected by skimmers or vacuum pumps.