THE OIL IN BRIEF
According to the biogenic theory, petroleum is a non-renewable fossil fuel consisting primarily of hydrocarbons, derived from the decomposition of plant and animal organisms occurred in the anaerobic environment, following their continued accumulation in the underground for millions of years in rocks which gradually are formed. The oil has not always the same chemical-physical composition but varies depending on the field of origin.
The crude oil is what is extracted from the reservoir, while the products are the fuels and fuel oils obtained by refining crude oil. The commodity classification API (American Petroleum Institute), accepted internationally, divides crude oils into four classes according to their density °API. Combining the classification API to the empirical concept of persistence of oils at sea, these are mainly divided into persistent (crude oils, fuel oils and asphalt) and not persistent (gasoline, kerosene and diesel). On the base of this classification we distinguish 4 main groups of crude oils and products:
|GROUP||SPECIFIC GRAVITY||°API DENSITY||PERSISTENCE||EXAMPLE|
|group I||< 0.8||> 45||not persistent||gasoline, kerosene, naphtha|
|group II||0.8 – 0.85||35 – 45||not very persistent||diesel fuel, Abu Dhabi Crude|
|group III||0.85 – 0.95||17.5 – 35||average persistent||Arabian Light Crude|
|group IV||> 0.95||< 17.5||very persistent||Heavy Fuel Oil, Venezuelan crude oils|
From: Biliardo and Mureddu, 2005
The oil in contact with the sea water is subject to some chemical and physical changes which can lead to the disappearance of pollution and to others that determine its persistence in the marine environment.
The oil spilled in the sea, even in the absence of external mechanical actions, spreads on the surface under the influence of surface tensions acting along the development of the air-water-oil interface and other physical-chemical factors, assuming initially the form of a single spot, rarely uniform. The size of the created spot depends on the amount of oil spilled, the distance its source, its viscosity, the weathering conditions and the available surface area. Generally after some time the patch begins to break down because of the action of wind and sea turbulence.
Simultaneously with the spread on the sea surface the process of evaporation of the lighter components of oil spilled begins. The evaporation rate and the amount of evaporated oil depend on the volatility of the oil; light products such as gasoline, kerosene and fuel oil may completely evaporate within a short time while the heavy oils undergo a small evaporation because they contain few volatile elements. Intense evaporation is favoured by high wind, turbulence of the sea, high temperatures and large size of the slick; in this case fractions of residual crude oil are formed with higher values of density and viscosity having lost the light fraction.
This process can happens simultaneously to the diffusion and consists in the partial or total breaking of the slick into fragments and drops of different sizes forming a mixture in the upper part of the water column. This mixture can also regroup and reform the slick due to water turbulence. This process is faster if the spilled oil is of light with low viscosity and the sea is very rough.
When some types of oil are dispersed in the upper part of the water column, they may be subject to the process of dissolution of their water-soluble compounds which can however also be lost through evaporation.
Some viscous oils in the presence of turbulent wave motion tend to form very viscous emulsions and more persistent than the original oil. They are called chocolate mousse for their typical color, which varies from brown to yellow. They can contain up to 80% of water as tiny droplets. This phenomenon causes a substantial increase in volume of the pollution that is formed, which gradually assumes a semi-solid consistency and slows the other chemical and physical processes that would allow the dissipation of pollution.
After some time from the spill and if conditions permit, the oxidation process can start and lead to the formation of tarry masses (solid outer crust surrounding a softer core) that, through assimilation and under UV irradiation, break into smaller and smaller portions. Then, due to a storage of substances in suspension and a kind of biological colonization, these small portions slowly fall on the seabed or are found stranded along the coast. The processes of transformation and degradation of the tar may even last for decades.
Refined products that have higher density than water and sediment may lie on the seabed. Usually the sinking occurs through the attachment of particles or organic material to the substance, especially in coastal areas where the turbulence is high. If oil burns after being spilled, having lost the combustible fraction that is the lighter, residues can be formed sufficiently dense to sink, as happened in the case of the oil tanker HAVEN in 1991.
This process occurs when the oil is aged and through the metabolization of hydrocarbons by the marine microorganisms, such as bacteria, fungi, yeasts and algae with the final production of harmless compounds such as water and carbon dioxide. The processes of diffusion, dispersion, emulsification and dissolution are the first processes that occur in case of spill whereas oxidation, sedimentation and biodegradation are more important later on and determine the ultimate fate of the oil.
Amato E., (2003), La tutela degli ecosistemi e la valutazione dei danni ambientali negli interventi di risposta agli inquinamenti accidentali a mare. Corso di formazione Castalia Ecolmar-CoNISMa.
Biliardo U. and Mureddu G., (2005), Traffico petrolifero e sostenibilità ambientale: dimensione del traffico petrolifero, maree nere, politiche di sicurezza della navigazione e implicazioni economiche, con riferimento al mondo intero e al Mediterraneo in particolare, US, Unione Petrolifera.
ITOPF, Fate of Marine Oil Spills, (2002), ITOPF Technical Information Paper, n. 2, 8 pp.