Oil is a mixture of thousands of molecules, mainly hydrocarbons, which are mostly insoluble and lighter than water. When oil is spilled, it immediately begins to spread. In calm seas, it tends to form an oily film several tens of millimetres thick.
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Evolution of oil in water
A litre of oil can spread over a surface area the size of half a football pitch.
(Source: WALKER J., MAWET D.-P. Les marées noires : leurs origines et leurs effets sur l’environnement et l’homme)
Water movement quickly breaks this film up into slicks, which drift on the water surface, separated by areas of open water, and, for some of the oil, into droplets which are dispersed in the first few metres of the water column. The air, wind, light, swell and the water itself affect these slicks by a combination of physical and chemical processes: evaporation, emulsification, dissolution, oxidation, sedimentation. Aquatic organisms biologically break up the molecules of certain hydrocarbons, a process known as biodegradation.
Evaporation affects volatile compounds, generating the oily smell which accompanies an oil spill. These are mainly well-known substances such as gases (methane, ethane, propane, butane, etc.) and solvents (benzene, toluene, etc.). After several hours, a quarter, a third, or sometimes as much as half of the volume spilled is released into the atmosphere, in the form of gases and vapours. The higher the temperature and the stronger the wind, the quicker the evaporation process occurs. The mass of hydrocarbons in the water is reduced, whilst the density of the remaining pollutant increases and significant atmospheric pollution is produced.
Only a very small part of the components of oil dissolve, mainly the aromatic elements (benzene, toluene, etc.). This process is 10 to 100 times slower than evaporation. The amount of evaporable compounds which dissolve is therefore generally very low.
Recovery of “chocolate mousse”
Depending on the oil’s viscosity and the meteorological conditions, the motion of the sea mixing the oil and water can lead to water being incorporated in the oil. This can happen from a few hours and up to a few days after the spill. This reverse emulsification forms a stable substance, with a consistency comparable to mayonnaise. The amount of water incorporated varies from a fifth to four fifths of the total volume of the emulsion, considerably increasing the volume of pollutant that needs to be recovered. The resulting product, which is often referred to as “chocolate mousse” because of its appearance, is no longer dispersible. After several weeks of physical and chemical break down of its components, this “mousse” will form residues which may drift at sea for months before being deposited on beaches in the form of tar balls.
Oil, water and chocolate mousse: the title of a document published by Environment Canada on their website, providing in particular explanations on how to simulate an ecological spill experiment.
Evolution of oil spilled in water
The oxidation process comes into play at the water surface on slicks and in open water on droplets. In this case the process is more specifically that of photo-oxidation. This is a slow process, directly linked to light intensity and slick thickness, and breaks down less than one hydrocarbon chain in a thousand per day in intense sunlight. As a consequence, it has little part to play in the short term fate of an oil spill.
Weathering of oil in Cedre’s polludrome (flume tank): formation of a “chocolate mousse”
Sedimentation affects a small fraction encompassing
the heaviest products, including sludge
deposits, which settle into the depths of the sea. This phenomenon occurs
over a long period of time and concerns generally only a small part (less
than a twentieth) of the spill, except for very thick and heavy oils. Sedimentation
is of great significance if a spill occurs in a shoreline environment where
the oil can mix with sand or sediments, causing it to sink, such as happened
in the Braer, Erika and Prestige incidents, making the oil very difficult
to detect and recover.
Biological degradation or biodegradation is much slower than the previous processes. Biodegrading organisms are mainly bacteria (micro-organisms) which are naturally present in the marine environment. They gather and multiply on the surface of droplets, slicks and “chocolate mousse”. The bacteria absorb certain hydrocarbons and use them as a source of metabolic energy. Living in water and using oxygen, they act at the oil-water interface. They are therefore far more efficient in reducing the remaining mass of oil when they occur on widely spread, fine droplets than on thick “chocolate mousse” or heavy products which settle on the seafloor and become incorporated in the sediments. Their widespread multiplication at the time of a spill can occur at the expense of other species, thus altering the ecological balance in the affected areas.
Biodegradation of hydrocarbons