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  • On 5 October 2011, the cargo vessel Rena ran aground on the Astrolabe Reef. The ship was carrying 1700 tonnes of heavy fuel oil and about 200 tonnes of diesel in its tanks. Over 360 tonnes of thick fuel oil was spilled into the sea. This threatened the seabird wildlife and the coastline in the Bay of Plenty.

    The immediate response to the Rena oil spill was to use oil dispersants. Dispersant application using Corexit 9500 began on 6 October but was suspended a week later when results proved inconclusive. Environmental experts warned that dispersants might do more harm than good to marine life.

    Corexit 9500

    Maritime New Zealand (MNZ) had previously tested a range of oil dispersants and identified Corexit 9500 as the most effective. Although initial field trials showed Corexit 9500 dispersed fresh oil rapidly, application to the oil at sea was not as effective as anticipated. In particular, it wasn’t effective on weathered oil (in the ocean for more than 3–4 hours).

    What are dispersants and how do they work?

    Oil dispersants work in the same way as dishwashing liquid. Both are made up mainly of surfactant molecules – these are molecules that have a hydrophilic (water-loving) head that is attracted to water molecules and a hydrophobic (water-hating) tail that repels water and simultaneously attaches itself to oil and grease. The result of these opposing forces is that the oil is broken up into tiny droplets suspended in water.

    Breaking the oil at sea into oil droplets allows them to be dispersed into the underlying sea by wave action. It is hoped that naturally occurring bacteria would then consume the droplets and the dispersant would rapidly biodegrade.

    Dispersants are usually applied to oil on water by spraying from surface vehicles or by applying from small fixed-wing aircraft or helicopters.

    Problems with dispersants

    International scientists argue that, although modern dispersants are less toxic than ones in the past, dispersants help to spread oil more widely into the environment. Dispersants do help to stop large amounts of oil fouling beaches and coating surface-dwelling animals, but animals on the seafloor will experience more pollution. Oil droplets reaching the seafloor tend to be the same size as food particles for filter-feeding organisms. Environmental scientists say it’s a poor trade-off – just trading one species for another.

    Many environmental scientists are also concerned about the inherent toxicity of dispersants. Some argue that dispersants themselves can be toxic – some at least as toxic as dishwashing liquid – and could be more harmful to the environment than the oil itself. There is potential for dispersants to bioaccumulate in seafood. The general consensus, however, is that there is a dearth of studies about the impacts of dispersants, so long-term effects are really unknown.

    Other methods for oil clean-up

    Besides chemical dispersants, there are other key methods for cleaning up ocean oil spills.

    • Booms and skimmers: Booms contain the oil so that skimmers can collect it. Booms are floating barriers placed around the oil or whatever is leaking the oil. Skimmers can be boats, vacuum machines, sponges or oil-absorbent ropes that skim spilled oil from the water’s surface within the booms.
    • Leave the oil alone: Some scientists argue that oil spills should be left to disperse naturally. Oil spills are dispersed by natural physical processes in high-energy environments – where strong winds, currents and wave action help to break up the oil. However, oil that reaches low-energy environments gets buried in sediments and may persist for several years.
    • In situ burning: Freshly spilled oil is ignited while it’s still floating on the water.

    Dispersants and booms and skimmers are the most frequently used methods to clean up ocean oil spills. All methods have advantages and disadvantages. The effectiveness depends on the situation – the amount and type of oil, the ocean currents and tides and the weather. Some methods can be harmful to the environment. Oil clean-up agencies have to make decisions about the safety of chemicals used in water – such as the dispersant Corexit 9500.

    Nature of science

    In science, there are often discrepancies between experiments carried out in the laboratory and an application in a real-life situation. This is because of the many variables involved. For example, Corexit 9500 worked extremely well as an oil dispersant when it was trialled. However, when it was applied to the Rena oil spill, it was not effective under the prevailing conditions.


    The last reported sighting of Rena-related oil was in March 2014. No oil residue that could be attributed to the disaster was found in coastal beach sediment cores in the 12 months that followed.

    Related content

    Efforts by the science and local communities to clean up and restore the area and the effects it had on these communities are explained in these articles:

    Activity ideas

    These two activities are associated with the clean-up effort, why try them with your students:

    • Responding to Rena uses video resources to help students consider short-term and long-term responses to an environmental disaster.
    • Cleaning up oil in water experiments with sorbents, dispersants and cleaning oil from bird feathers.

    Follow up with the activity Water pollutants on trial – students research the effect of common pollutants on our waterways and hold a mock trial to determine the worst pollutant in the country.

      Published 11 January 2012, Updated 8 August 2018 Referencing Hub articles
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