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Oil Spill Dispersants: Top 20 Frequently Asked Questions (FAQs)
The use of chemical dispersants in maritime oil spills has always been a controversial issue and the topic of much debate between responders, scientists, the public, media and environmental interest groups.
AMSA has prepared the following answers to the top 20 most frequently asked questions about oil spill dispersants to assist the public and responders in this issue.
1. Why are chemical dispersants used on an oil spill ?
Dispersants are used to minimise the environmental impact of an oil spill.
Dispersants do not eliminate the problem of an oil spill but are intended as a means of reducing the overall environmental impact of an oil slick at sea. Oil Spill Dispersant (OSD) use accelerates the weathering and biological breakdown of oil at sea and reduces the impact of oil on sensitive foreshore environments.
Oil Spill Dispersants are also highly effective in reducing exposure of sea birds to oil as most sea birds are oiled by slicks on the surface of the sea or in near shore coastal habitats.
Undispersed slicks and residual oils are a persistent threat to foreshores, birds, mammals and intertidal communities due to the toxicity of, and contact with oil. Dispersed oil is less "sticky" than undispersed oil, therefore the adhesion and absorption onto surfaces and sediments of dispersed oil is greatly reduced compared with the original oil slick.
In a spill incident environmental trade-offs of protection and sacrifice will occur. These decisions are not taken lightly by response authorities and will be based on the best available advice and scientific data to achieve a net environmental benefit.
2. What are oil spill dispersants ?
Dispersants are chemical formulations with an active ingredient called surfactants. Surfactants are specifically designed chemicals that have both hydrophilic (water liking) and oleophilic (oil liking) groups in the chemical compound. These chemicals reduce the interfacial tension between the oil and water and helps the creation of small oil droplets, which move into the water column facilitating quicker natural biological breakdown (biodegradation) and dispersion. By decreasing the size of the oil droplets, and dispersing the droplets in the water column, the oil surface area exposed to the water increases and natural breakdown of the oil is enhanced.
Oil Spill Dispersants are composed of three main component groups;
3. Are chemical dispersants toxic to marine life ?
For the majority of laboratory tests the Australian approved Oil Spill Dispersants rate predominantly as "slightly toxic" to "practically non-toxic" by the International Maritime Organization/GESAMP classification system. All dispersants approved for use within Australian waters must pass laboratory acute toxicity testing requirements for two temperate and two tropical marine species as specified by the National Plan protocol.
The toxicity of dispersed oil is primarily due to the toxic components of the oil itself. Many laboratory studies on a range of test species have confirmed the fact that "the acute toxicity of dispersed oil generally does not reside in the dispersant but in the more toxic fractions of the oil".
The toxicity of Oil Spill Dispersants to aquatic organisms under laboratory conditions appears to relate primarily to the chemical composition of the individual dispersant.
For example:
Hence it is vital that all Oil Spill Dispersants are tested under controlled laboratory conditions to ascertain their effectiveness and toxicity rating before being authorised for use under the Australian National Plan arrangements.
Other factors that are important in Oil Spill Dispersant aquatic toxicity are:
At the application rates used in oil spill incidents, and mixing conditions of the ocean, Oil Spill Dispersants would not, in most circumstances, be toxic to marine organisms.
4. What is the difference between "generation" and "type" when describing oil spill dispersants?
There is often confusion between the use of the terms "generation" and "type" in the description of Oil Spill Dispersants. First generation Oil Spill Dispersants are not type I Oil Spill Dispersants.
The first generation dispersants are no longer used in oil spill response in Australia and were essentially "industrial cleaners", "degreasers" and "detergents" with high aquatic toxicity, and not true dispersants.
The second generation dispersants were specifically designed to treat oil spills at sea with a mixture of surfactants and solvents with much lower toxicity levels than the first generation Oil Spill Dispersants. These second generation Oil Spill Dispersants were conventional low aromatic hydrocarbon based and applied undiluted (neat) and sprayed from vessels. The third generation Oil Spill Dispersants have been designed to be applied from both aircraft and vessels as either a concentrate or diluted.
It is more useful to refer to OSDs using the "type" classification;
Type I - Conventional hydrocarbon based - used neat at sea or on foreshores.
(2nd generation)
Type II - Water diluted concentrate - diluted prior to use (up to 1:10)
with water.
(3rd generation)
Type III - Concentrate - used neat from aircraft and vessels or on foreshores.
(3rd generation)
5. Who authorises the use of dispersants during an oil spill response?
Under the Australian National Plan arrangements the On Scene Coordinator (OSC) usually has the responsibility to authorise the use of chemical dispersants in an oil spill incident. Another officer or agency may have the authority as designated by state or territory contingency plan. This decision is usually based upon the advice of the designated Commonwealth/State/Regional Environmental and Scientific Coordinator (ESC).
The advice will always consider the:
6. On what basis is the decision made to use dispersants in a spill incident?
The main basis for decision making in determining whether oil spill dispersant will be used is:
" Will the application of the chemical dispersant to the spilled oil minimise the overall environmental impact of the oil spill?"
Except for the impact on marine birds and mammals, the most damaging effect of oil spills is when the oil strands on shorelines or enters restricted shallow waters like estuaries. Oil Spill Dispersants are a prime and vital response tool to stop oil coming ashore or from entering sensitive foreshore environments especially when weather and sea conditions do not allow the use of oil containment and recovery equipment.
Oil Spill Dispersants are usually not applied to oil spills in "near shore areas" for example: where sea grass beds, oyster beds, mariculture or coral reefs are present. However, dispersant use may be authorised by the On Scene Coordinator in consultation with the Environmental and Scientific Coordinator in these circumstances when there is a possibility of an impact of oil on a more sensitive foreshore habitat, or wildlife impacts are possible. For example, when an approaching oil slick may impact sensitive fringing mangroves, or endangered species such as migratory birds.
7. What are the negative effects of dispersants on the environment ?
The acute toxicity of dispersed oil generally does not reside in the dispersant but in the more toxic fractions of the oil. Dispersing oil into the water in situations where there is little water movement or exchange, such as shallow embayments, increases exposure of subsurface, benthic organisms and fish to the toxic components of the oil.
Fish and other marine life in the larvae stage or juvenile stages are more prone to the toxicity effects of oil and dispersants. Therefore it is unlikely dispersants will be used near commercial fisheries, important breeding grounds, fish nurseries, shellfish aquaculture etc. unless it is to protect a more important environmental resource.
Seagrasses and coral reef communities are particularly sensitive to dispersed oil because instead of the oil "floating over" the reefs and submerged seagrass beds the oil/dispersant mixture in the water colour will come into direct contact with these sensitive ecosystems.
Generally there is a reluctance by spill responders to use dispersants in shallow waters less than 5 metres deep, although there may be situations where using dispersants could save foreshore impacts or wildlife.
8. What are the impacts of dispersed oil on coral reefs?
Coral reef communities are highly sensitive to both oil and oil/dispersant mixtures. For example the exposure of coral to hydrocarbons can cause:
Spill responders will avoid using Oil Spill Dispersantsin or near coral reefs, in shallow waters, sea grass beds or where poor water exchange or circulation is apparent, unless in exceptional circumstances to protect mangroves or other highly sensitive foreshores.
9. How effective are oil spill dispersants ?
Chemical dispersants aid the natural dispersion of oil by reducing the oil/water interfacial tension and, along with the natural motion of the sea, allow the break up of oil on the water into very fine droplets.
Effectiveness of oil dispersion by chemical dispersants at sea is governed by a range of conditions and include the:
10. Will dispersants work on all types of oils ?
No, dispersants will not work on all oil spills.
The first rule in combating oil spills with dispersants is that the oil must be amenable to dispersant use. It is also well understood by oil spill response agencies that dispersants are only effective on certain types of oils and the first priority is always to determine the spilled oil's physical and chemical properties in order to assess combat options.
It has been generally accepted that non-dispersable oils are;
non-spreading oils (pour point is higher than sea temperature), highly viscous oils (> 2000 Centistokes (cSt) - a measurement of the mobility of oil), a water-in-oil emulsion has formed (mousse).
A "rule of thumb" amongst spill responders as to whether or not a dispersant will work has historically been - "a dispersant may have a reasonable success rate if the oil is continuing to "flow" or spread as a fluid (not just sheening)".
Unfortunately this "rule of thumb" is only partly correct. The properties of these oils are determined by their chemical composition which vary widely. For the purposes of determining the use of dispersants at various sea temperatures the important properties are:
Pour point and viscosity of a spilt oil are the dominant factors for the determination dispersant use. An oil/sea temperature/dispersant use matrix has been prepared by AMSA to assist responders under the National Plan. Field testing of dispersants on the spilt oil may be required by the On Scene Coordinator (OSC) or the Environmental and Scientific Coordinator (ESC) before the decision to proceed with dispersant spraying operations is made.
11. How quickly do we need to apply dispersants to an oil spill ?
As quickly as possible!
There is only a limited "window of opportunity" to use chemical dispersant in an oil spill incident. This is primarily due to the changing properties of the spilt oil due to weathering of the oil, but is also governed by the location and speed of movement of the slick onto the foreshores or into estuarine environments.
This window of opportunity may be as little as only a few hours. Sometimes if the conditions are favourable, a day or two.
Therefore it is essential that the capability exists to quickly activate and deploy resources anywhere across Australia to deliver and apply oil spill dispersants at sea.
The National Plan, with the assistance of the oil industry, has in place a fixed wing aerial dispersant capability along with significant stocks of Oil Spill Dispersants around the Australian coast. This aerial dispersant capability provides "at call", commercially operated, large agricultural spray aircraft to provide the delivery of Oil Spill Dispersants during maritime incidents.
12. Won't oils naturally disperse given time ?
Yes, most oils will naturally disperse.
Some oils are naturally dispersed by the action of the sea, but it happens a lot slower than using Oil Spill Dispersants, and also depends greatly on the type of oil spilt.
Light oils, and oils that do not emulsify readily, are more easily dispersed by the action of the sea than heavy residual oils that are very persistent in the environment.
Oil Spill Dispersants speed up the process of removing oil from the water surface, before it can contaminate bird and marine life, and wash up on foreshores contaminating coastal resources.
In very calm water, dispersed oil may refloat and move as a film of dispersed oil. This slows down the natural removal processes, as its oil/water interface is much reduced compared to the situation where finely divided droplets are present in the water column in rough seas.
In these circumstances it is necessary to agitate the water surface to assist oil dispersion. This can be done using the propellers of small craft or alternatively the water jets of high pressure fire hoses.
13. At what point will oil spills not be dispersable ?
Oil Spill Dispersant effectiveness varies greatly with oil type spilt and the degree of weathering of the oil.
For example with increasing wind speed and wave action, the loss of light oil components increases. Evaporation becomes more significant, causing an increase in viscosity and density of the remaining oil, and forming emulsions with water. The oil slick becomes thicker and heavier.
It is generally accepted that for oils over 2000 cSt (Centistockes - a measurement of the mobility of oil) viscosity, the effectiveness of oil dispersants decreases significantly. An oil that was easily dispersed may change quickly by wind and wave action into an oil which is not dispersable.
The weathering on a spilled oil is determined primarily by the:
It is now possible to use computer software to assist responders in predicting the changing oil properties of oil as it weathers at sea. One such program is called "Automated Data Inquiry for Oil Spills (ADIOS)" which models the physical changes spilled oil undergoes during weathering.
This computer modeling is available from AMSA by request for state/NT agencies during oil spill incidents or exercises.
The information required to run the model includes:
Outputs of the model include time dependent graphs of:
It must be remembered that Oil Spill Dispersants are never 100% effective - they will only remove a percentage of the oil from the surface of the sea. Therefore, a visible slick may remain even after dispersant use.
14. What are the acceptance processes for dispersants in Australia ?
For the acceptance of oil spill dispersants under the National Plan each manufacturer/distributor must provide written documentation and test results to AMSA about the Oil Spill Dispersant to show that it complies with the AMSA guidelines (National Plan Guidelines for Acceptance of Oil Spill Dispersants).
At present ten oil spill dispersants have been approved under the National Plan guidelines. Their "Trade Names" are listed below:
15. What tests are carried out on dispersants to see if they are effective or safe to use in the marine environment ?
Spill incidents and emergency situations are not appropriate situations for the testing of dispersants and other oil spill treatment chemicals. Uniform and comprehensive testing is extremely difficult to plan and implement during clean-up operations and often provide inconclusive results. More importantly, these tests or trials may interfere with the smooth running of response operations.
The field testing of new dispersants and chemical agents should be the last step in a series of product evaluation, commencing with specific laboratory tests designed to answer specific questions of responders.
According to the current AMSA guidelines for Oil Spill Dispersants, the required testing data includes:
Only dispersants that pass a specified minimum level of effectiveness and a specified maximum level of acceptable toxicity to two temperate and two tropical test species are approved for use in Australian waters and purchased for National Plan stockpiles.
16. Have dispersants been used successfully in oil spill incidents here in Australia and overseas ?
Contrary to many media reports there is a well documented history of the successful use of Oil Spill Dispersants in stopping oils spills impacting foreshores around the world, the most recent example being the Sea Empressoil spill.
In the Sea Empressoil spill in Milford Haven (UK) in February 1996, 72,000 tonnes of Forties Blend crude oil was spilt into the marine environment from the grounding of the vessel, along with 370 tonnes of Heavy Fuel Oil. The use of around 445 tonnes of chemical dispersants sprayed by aircraft onto the oil slicks at sea prevented at least 36,000 tonnes of oil, which is approximately 57,000-110,000 tonnes of oil/emulsion, from the Sea Empresscoming ashore in this sensitive region of Wales.
The volume of oil that actually came ashore was around 10,000-15,000 tonnes; without dispersant spraying at sea, 5 to 10 times this amount of oil/emulsion would have impacted the foreshores. The environmental impact of this incident on sea birds, coastal waders, intertidal invertebrates, amenity areas and the cost of clean up would have been much greater. The adjoining coastline of Milford Haven has the only coastal National Park in the UK, and nearby islands are bird sanctuaries with internationally important populations of puffins, guillemots, gannets, and Manx shearwaters.
17. Can dispersants be used on oil contaminated foreshores ?
Yes. Dispersants have been used as shoreline cleaning agents in oil spill incidents.
Oil Spill Dispersants have been shown to assist in the removal of oil from contaminated shorelines and structures in a number of incidents both within Australia and overseas.
Oil Spill Disperants, applied to oil in near shore spills, have been shown to be beneficial in reducing the adherence of oil to surfaces thereby reducing the residence times of oil within a habitat or foreshore. Dispersed oil is retained less by rocks, pebbles and other surfaces and sediments, and hence tends to have less long term residence. Therefore, there is less chronic exposure to organisms and less long term toxicity in the environment.
In the Iron Baronoil spill in Tasmania in 1995, dispersants were used directly on rocky foreshores to clean oil from penguin runways and access points to foreshore rookeries. The National Plan dispersant BP-AB was found to be the most effective of those trialed, with good dissolving and dispersion capabilities on the weathered bunker fuel oil. The application by back-pack spraying and subsequent low pressure sea water flushing was found to be the most efficient and effective method, and resulted in fewer oiled penguins being contaminated at these locations. The timing of application and flushing of the dispersant was critical due to the feeding habits of the penguins.
18. How are chemical dispersants applied to an oil slick ?
Oil Spill Dispersants can be applied by fixed wing aircraft such as agricultural spray planes, helicopters utilising special spray buckets (image), or from ship based spray systems.
There are advantages and disadvantages of each method. The National Plan has equipment and resources available to spray Oil Spill Dispersants by all methods and from all different platform types.
Oil Spill Dispersants must be applied as soon as possible to the thickest parts of the oil slicks and in an optimal droplet size and rate of application. A minimum sea energy is also required before dispersants function effectively - the higher the sea the more effective the dispersant.
Training on the use and application of Oil Spill Dispersants is carried out as part of the National Plan training program across Australia.
19. What oil spill dispersant stocks are available in Australia ?
In Australia all three types of dispersants are available for use during oil spill incidents. Most dispersants in the National Plan stockpiles are type III (used neat) which can also be used as type II (diluted), and only one type I dispersant called BP-AB. Type II and type III Oil Spill Dispersants may have mixtures of both water based solvents and hydrocarbon solvents, so the presence of hydrocarbons in the formulation doesn't designate the "type" of Oil Spill Dispersant.
There are hundreds of tonnes of approved Oil Spill Dispersants purchased under the National Plan across Australia at numerous locations.
A summary of the information available on National Plan dispersants is
available along with the application techniques and suitability of use on
different oils. This information summarises data taken from Material Safety
Data Sheets and company information sheets and product application brochures.
The AMSA dispersant table [
PDF: 16KB] contains a summary of the:
20. At what water depths can dispersants be used ?
Each State/NT may designate a water depth at which dispersants are not "preapproved for use", usually between 5-10 metre contour. Inside these depth contours dispersant use may be approved by the OSC if he/she believes that the benefits of protection of the foreshore and/or wildlife outweigh the possible impacts on near shore marine habitats.
Water exchange or circulation to remove the dispersed oil from near shore habitats is the most important guide to Oil Spill Dispersant use rather than the use of a designated water depth.
It is often a much better option to disperse oil at sea or even near shore, than allowing oil to contaminate foreshores, especially low energy foreshores like mangroves and saltmarshes which may take many years, or even decades, to recover from the effects of an oil spill.
The careful and controlled use of Oil Spill Dispersants is an effective tool in combating oil spills at sea and is often fully justified if it achieves a net environmental benefit during a spill response.
For further information and comments please email Environment Protection Response
last updated: 19 October 2006