When different kinds of oil enter the sea, many physical, chemical and biological degradation processes start acting on them. These processes change the properties and behaviour of the oil. Some processes cause the oil to "disappear", but the fact that it is no longer visible on the water surface does not necessarily mean that it is gone or has been rendered environmentally harmless.

Every time oil enters the sea, a number of factors will decide the physical, chemical and biological degradation of the oil, as well as the potential environmental damage (see effects on wildlife/species and effects on habitats) of the spill or discharge, in that particular area:

  • The composition and amount of oil discharged (see, e.g., NOAA).
  • The quantity and duration of the discharge/spill.
  • The time of year at which it occurs.
  • The temperature of the air and the receiving water body.
  • The weather conditions.
  • The species composition in the area affected.
  • The properties of the shore line (rocky, sandy, mud flats, mangroves, etc.)
  • The amount of oil-degrading micro-organisms in the area.
  • The supply of oxygen in the water
  • The following description (green text) of physical, chemical and biological degradation processes can be found on the site of the International Tanker Owners Pollution Federation Ltd (ITOPF).
  • Red text = complementing text from similar descriptions by other organizations/agencies (see below: EPA, AMSA, NOAA, Offshore environment). See also NOAA photos of different types and distributions of oil on water.
Fate of oil spilled, showing the main weathering processes. Graph courtesy of ITOP. © ITOPF
Weathering, sheen, windrows, spreading, dispersion, evaporation, dissolution, biodegradation, sedimentation, emulsification, dissolution, oxidation, aggregation ...

"Oil, when spilled at sea, will normally break up and be dissipated or scattered into the marine environment over time. This dissipation is a result of a number of chemical and physical processes that change the compounds that make up oil when it is spilled. The processes are collectively known as weathering. Oils weather in different ways. Some of the processes, like natural dispersion of the oil into the water, cause part of the oil to leave the sea surface, while others, like evaporation or the formation of water in oil emulsions, cause the oil that remains on the surface to become more persistent.

Weathering: Winds, waves and currents may result in natural dispersion, breaking a slick into droplets which re then distributed throughout the water. These droplets may also result in the creation of a secondary slick or thin film (sheen) on the surface of the water. (EPA) ••• A sheen is a very thin layer (less than 0.003 mm thick) floating on the water surface, and the most commonly-observed form of oil during the later stages of a spill. Depending on thickness, sheens range in colour from dull brown for the thickest ones to rainbows, gray, silvers, and near-transparency in the case of the thinnest sheens. Windrows are streaks of oil that line up in the direction of the wind (typically early during a spill, when the wind speed is at least 5 metres per second). Sheen is the form of oil that most frequently windrows. (NOAA)

The way in which an oil slick breaks up and dissipates depends largely on how persistent the oil is. Light products such as kerosene tend to evaporate and dissipate quickly and naturally and rarely need cleaning-up. These are called non-persistent oils. In contrast, persistent oils, such as many crude oils, break up and dissipate more slowly and usually require a clean-up response. Physical properties such as the density, viscosity and pour point of the oil all affect its behaviour.

Dissipation does not occur immediately. The time this takes depends on a series of factors, including the amount and type of oil spilled, the weather conditions and whether the oil stays at sea or is washed ashore. Sometimes, the process is quick and on other occasions it can be slow, especially in sheltered and calm areas of water.

Spreading: As soon as oil is spilled, it starts to spread out over the sea surface, initially as a single slick. The speed at which this takes place depends to a great extent upon the viscosity of the oil. Fluid, low viscosity oils spread more quickly than those with a high viscosity. Nevertheless, slicks quickly spread to cover extensive areas of the sea surface. Spreading is rarely uniform and large variations in the thickness of the oil are typical. After a few hours the slick will begin to break up and, because of winds, wave action and water turbulence, will then form narrow bands or windrows parallel to the wind direction. The rate at which the oil spreads is also determined by the prevailing conditions such as temperature, water currents, tidal streams and wind speeds. The more severe the conditions, the more rapid the spreading and breaking up of the oil.

Physical transport. Only ten minutes after a spill of 1 ton of oil, the oil can disperse over a radius of 50 metres, forming a slick 10 millimetres thick. The slick gets thinner (less than one mm) as oil continues to spread, covering an area of up to 12 sq.km. --- Further changes take place under the combined impact of meteorological and hydrological factors, and depend mainly on the power and direction of wind, waves and currents. An oil slick usually drifts in the same direction as the wind. While the slick thins, especially after the critical thickness of about 0.1 mm, it disintegrates into separate fragments that spread over larger and more distant areas. Storms and active turbulence speed up the dispersion of the slick and its fragments. A considerable part of the oil disperses in the water as fine droplets that can be transported over large distances away from the place of the spill. (Offshore Environment)

Evaporation: Lighter components of the oil will evaporate to the atmosphere. The amount of evaporation and the speed at which it occurs depend upon the volatility of the oil. An oil with a large percentage of light and volatile compounds will evaporate more than one with a larger amount of heavier compounds. For example, petrol, kerosene and diesel oils, all light products, tend to evaporate almost completely in a few days whilst little evaporation will occur from a heavy fuel oil. In general, in temperate conditions, those components of the oil with a boiling point under 200ºC tend to evaporate within the first 24 hours. Evaporation can increase as the oil spreads, due to the increased surface area of the slick. Rougher seas, high wind speeds and high temperatures also tend to increase the rate of evaporation and the proportion of an oil lost by this process.

Evaporation occurs when the lighter substances within the oil mixture become vapors and leave the surface of the water. This process leaves behind the heavier components of the oil, which may undergo further weathering or may sink to the ocean floor. For example, spills of lighter refined petroleum-based products such as kerosene and gasoline contain a high proportion of flammable components known as light ends. These may evaporate completely within a few hours, thereby reducing the toxic effects to the environment. Heavier oils leave a thicker, more viscous residue, which may have serious physical and chemical impacts on the environment. Wind, waves, and currents increase both evaporation and natural dispersion. (EPA)

Dispersion: Waves and turbulence at the sea surface can cause all or part of a slick to break up into fragments and droplets of varying sizes. These become mixed into the upper levels of the water column. Some of the smaller droplets will remain suspended in the sea water while the larger ones will tend to rise back to the surface, where they may either coalesce with other droplets to reform a slick or spread out to form a very thin film. The oil that remains suspended in the water has a greater surface area than before dispersion occurred. This encourages other natural processes such as dissolution, biodegradation and sedimentation to occur. The speed at which an oil disperses is largely dependent upon the nature of the oil and the sea state, and occurs most quickly if the oil is light and of low viscosity and if the sea is very rough. These factors led to the complete dispersion of the oil spilled from the Braer (Shetland Islands) in 1993. The addition of chemical dispersants (also surfactants) can accelerate this process of natural dispersion.

Emulsification: An emulsion is formed when two liquids combine, with one ending up suspended in the other. Emulsification of crude oils refers to the process whereby sea water droplets become suspended in the oil. This occurs by physical mixing promoted by turbulence at the sea surface. The emulsion thus formed is usually very viscous and more persistent than the original oil and is often referred to as chocolate mousse because of its appearance. The formation of these emulsions causes the volume of pollutant to increase between three and four times. This slows and delays other processes which would allow the oil to dissipate. Oils with an asphaltene content greater than 0.5% tend to form stable emulsions which may persist for many months after the initial spill has occurred. Those oils containing a lower percentage of asphaltenes are less likely to form emulsions and are more likely to disperse. Emulsions may separate into oil and water again if heated by sunlight under calm conditions or when stranded on shorelines.

Emulsification is a process that forms emulsions consisting of a mixture of small droplets of oil and water. Emulsions are formed by wave action, and greatly hamper weathering and cleanup processes. Two types of emulsions exist: water-in-oil and oil-in-water. Water-in-oil emulsions are frequently called "chocolate mousse", and they are formed when strong currents or wave action causes water to become trapped inside viscous oil. Chocolate mousse emulsions may linger in the environment for months or even years. Oil and water emulsions cause oil to sink and disappear from the surface, which give the false impression that it is gone and the threat to the environment has ended. (EPA) ••• Milk is an example of an oil-in-water emulsion; butter on the other hand is a water-in-oil emulsion. Both types of emulsification require wave action and occur only for specific oil compositions. Water-in-oil emulsions are extremely stable and may persist for months or years after a spill. Water-in-oil emulsions containing 50 to 80 per cent water are most common, and have a reddish-brown colour and grease like consistency. They are some times called "chocolate mousse" because of their pudding-like appearance. (AMSA)

Dissolution: Water soluble compounds in an oil may dissolve into the surrounding water. This depends on the composition and state of the oil, and occurs most quickly when the oil is finely dispersed in the water column. Components that are most soluble in sea water are the light aromatic hydrocarbons compounds, such as benzene and toluene. However, these compounds are also those first to be lost through evaporation, a process which is 10-100 times faster than dissolution. Oil contains only small amounts of these compounds making dissolution one of the less important processes.

Oxidation: Oils react chemically with oxygen either breaking down into soluble products or forming persistent compounds called tars. This process is promoted by sunlight and the extent to which it occurs depends on the type of oil and the form in which it is exposed to sunlight. However, this process is very slow and even in strong sunlight, thin films of oil break down at no more than 0.1% per day. The formation of tars is caused by the oxidation of thick layers of high viscosity oils or emulsions. This process forms an outer protective coating of heavy compounds that results in the increased persistence of the oil as a whole. Tar balls, which are often found on shorelines and have a solid outer crust surrounding a softer, less weathered interior, are a typical example of this process.

Oxidation occurs when oil contacts the water and oxygen combines with the oil to produce water-soluble compounds. This process affects oil slicks mostly around their edges. Thick slicks may only partially oxidize, forming tar balls. These dense, sticky, black spheres may linger in the environment, and can collect in the sediments of slow moving streams or lakes or wash up on shorelines long after a spill. (EPA) ••• Aggregation. Oil aggregates in the form of petroleum lumps, tar balls or pelagic tar, can be found both in the open and coastal waters and on beaches. They derive from crude oil after the evaporation and dissolution of its relatively light fractions, emulsification of oil residuals, and chemical and microbial transformation. - - - Oil aggregates look like light gray, brown, dark brown, or black sticky lumps. They have an even shape and vary from 1 mm to 10 cm in size, sometimes reaching up to 50 cm. Their surface serves as a substrate for developing bacteria, unicellular algae and other microorganisms. Besides, many invertebrates, like gastropods, polychaetes and crustaceans, resistant to oil impact often use them as shelter. Oil aggregates can exist from a month to a year in enclosed seas and up to several years in the open ocean. They complete their cycle by slowly degrading in the water column, on the shore if washed their by currents, or (if they lose their floating ability) on the sea bottom. (Offshore Environment)

Sedimentation/Sinking: Some heavy refined products have densities greater than one and so will sink in fresh or brackish water. However sea water has a density of approximately 1.025 and very few crudes are dense enough or weather sufficiently, so that their residues will sink in the marine environment. Sinking usually occurs due to the adhesion of particles of sediment or organic matter to the oil. Shallow waters are often laden with suspended solids providing favourable conditions for sedimentation. Oil stranded on sandy shorelines often becomes mixed with sand and other sediments. If this mixture is subsequently washed off the beach back into the sea it may then sink. In addition, if the oil catches fire after it has been spilled, the residues that sometimes form can be sufficiently dense to sink.

Biodegradation: Sea water contains a range of micro-organisms or microbes that can partially or completely degrade oil to water soluble compounds and eventually to carbon dioxide and water. Many types of microbe exist and each tends to degrade a particular group of compounds in crude oil. However, some compounds in oil are very resistant to attack and may not degrade.
The main factors affecting the efficiency of biodegradation, are the levels of nutrients (nitrogen and phosphorus) in the water, the temperature and the level of oxygen present. As biodegradation requires oxygen, this process can only take place at the oil-water interface since no oxygen is available within the oil itself. The creation of oil droplets, either by natural or chemical dispersion, increases the surface area of the oil and increases the area available for biodegradation to take place."

Many species of marine micro-organisms or bacteria, fungi and yeasts feed on the compounds that make up oil. Hydrocarbons (oil) consumed by these micro-organisms can be partially metabolized or completely metabolized to carbon dioxide and water. The rate of biodegradation depends on the temperature of the oil and water mixture. (AMSA). ••• A wide range of micro-organisms is required for a significant reduction of the oil. To sustain biodegradation, nutrients such as nitrogen and phosphorus are sometimes added to the water to encourage the micro-organisms to grow and reproduce. Biodegradation tends to work best in warm water environments. (EPA) ••• The addition of nutrients to speed up the process is also referred to as bioaugmentation, biostimulation, bioremediation, seeding, or fertilization.