Not all oils behave the same way when spilled into the marine environment.
In many ship-source oil spills the type of oil spilled and the weather and ocean conditions are the most influential factors in determining how oil will behave in the marine environment.
How does the type of oil spilled influence its behaviour?
Oils fall into two broad categories: heavy and light. In terms of how these oils behave in the environment, these categories equate to “persistent oils” (heavy) and “non-persistent oils” (light).
Persistent Oils:
In general, persistent oils dissipate slowly in the marine environment. Because these oils stay in the environment for longer periods, they can have prolonged impacts which are more easily observed. Persistent oils can include:
- Bitumens;
- Asphaltenes;
- Resins; and
- Waxes.
Non-Persistent Oils:
Non-persistent oils tend to dissipate rapidly in the marine environment, so while their impacts may be acute, they can be more difficult to recognize than persistent oils. Non-persistent oils can include:
- Gasoline;
- Jet Fuels
- Diesel; and
- Benzene.
Persistent oil (left) versus non-persistent oil (right)
(ITOPF, 2018)
What impact do weather and ocean conditions have on how oil types behave?
When oil is spilled in the marine environment it undergoes many chemical and physical changes caused by the weather and ocean conditions at the time of the spill which determines how the oil will behave in the environment and ultimately what will happen to it.
What is oil weathering?
There are eight main chemical and physical changes that oil may undergo when spilled in the marine environment. The type of oil spilled and the length of time that the oil has been in the marine environment can influence what chemical and physical changes oil undergoes. Collectively, these changes are known as “weathering”.
Weathering changes common in the early stage of a spill include:
Spreading:
Spreading over the sea surface begins as soon as oil is spilled, creating a slick. After a period of time the slick will begin to break up due to wind and wave action. The speed at which oil spreads depends on the type of oil spilled. Non-persistent oils usually spread faster than persistent oils.
Evaporation:
The rate of evaporation depends on the type of oil spilled. Non-persistent oils evaporate more readily than persistent oils. Rough seas and high winds as well as high temperatures tend to increase the rate of evaporation of both types of oil.
Dispersion:
Waves and turbulence at the sea surface can cause oil slicks to separate into droplets. Dispersion encourages other natural processes to help break down the oil. The speed at which an oil spill is dispersed depends on the type of oil spilled, with non-persistent oils dispersing more rapidly than persistent oils.
Emulsification:
An emulsion is formed when two liquids combine, with one ending up suspended in the other (typically marine water suspended in the oil). Emulsion is more typical with persistent oils and delays further weathering processes.
Dissolution:
Water soluble compounds in the spilled oil can dissolve into surrounding water. Non-persistent oils are most susceptible to dissolution if they have not already evaporated.
Weathering changes common in the later stage of a spill:
Oxidation:
Oils react chemically with oxygen either breaking down into soluble products or forming persistent compounds called tars. This process is promoted by sunlight but is very slow. Oxidation is more common with persistent oils.
Sedimentation:
Oil mixing with and attaching itself to sediment is referred to as sedimentation and can result in the creation of tarballs. Few oils sink in the marine environment, so sedimentation most commonly occurs when floating oil reaches the shore and mixes with sand and other sediments. If this mixture then washes off the beach back into the sea, it may sink as it has been made heavier by the sediment.
Biodegradation:
Sea water contains a wide range of microbes that can use oil as a source of energy and can partially or completely reduce oil to water soluble compounds and eventually to carbon dioxide and water. Biodegradation is the main natural process through which the majority of oil spilled into the marine environment, which cannot be recovered, is eventually broken down and eliminated.
Today, many oils transported by ship as cargo or as fuel contain a mixture of both persistent and non-persistent oils, which is one variable making predictions regarding the behaviour of ship-source oil spills complex and challenging. Learn more about predicting oil spill behaviour through models in a blog post coming soon.
Learn More:
Persistent Vs Non-Persistent Oils: What You Need to Know
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