The oil can end up anywhere, depending on factors such as wind speed, wind direction, sea conditions and/or the volume of oil spilled.
When the oil is spilled, it spreads over the sea surface and as oil has a lower density than water it stays afloat. The speed at which it spreads depends on how viscous the oil is. If it has a high viscosity, it will spread more slowly than if it has a low viscosity. As time passes the oil slick starts to break up and form narrow strips due to wind, water turbulence and waves. Lighter components of the oil will evaporate into the atmosphere, an oil with a high percentage of light, volatile compounds will evaporate more than one with a larger proportion of heavy compounds. Evaporation increases with high temperatures, high wind speeds and when the surface area increases. Waves and water turbulence can cause the oil to break up into little droplets (this is called dispersion), the smaller droplets will stay suspended in the sea water, whereas the larger tend to rise to the surface where they either spread out into a thin film of oil or where they rejoin with other droplets to form another slick.
These are the main processes involved in the early stages of an oil slick.
Oils tend to react chemically with the oxygen in the water and air, this is called oxidation. The rate of oxidation increases as the light intensity increases. Tars form as the thick layers of oil oxidise, producing tarballs which often wash up on seashores. The density of crude heavy oils can exceed 1g/cm3, so such would sink in fresh water. However, very few crude oils have a density greater than that of sea water (1.025 g/cm3) so rarely sink in marine environments. Sinking may occur if the oil mixes with sand from the shore and is then washed back into the sea. Sedimentation may also occur if the oil catches fire - the residues can have a higher density than sea water and will sink. Another fate of oil is biodegradation. This is the process by which micro-organisms and microbes feed off the oil, partially degrading it initially into water soluble compounds and eventually into carbon dioxide and water. However, not all compounds can be degraded as some are very resistant to micro-organisms and microbes. The level of phosphorus (P), nitrogen (N) and oxygen (O) in the water must be high enough for the microbes and micro-organims to live, because their nutrients are P and N and they need O to respire.
These processes are more important later on and determine the ultimate fate of the oil.
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