Saturday, August 22, 2009

The Australian Maritime Safety Authority is a largely self-funded government agency with the charter of enhancing efficiency in the delivery of safety and other services to the Australian maritime industry. The following information was obtained from

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:
oil type (persistent or evaporative),
location of the slick,
spread of the oil,
movement of the slick,
proximity of the oil to sensitive environments,
wildlife priorities,
safety concerns,
water depth,
water exchange in the area,
and whether the oil is amenable to dispersant application, etc.

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.

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.

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:

loss of symbiotic zooxanthellae (tiny algae living in coral),
reduced metabolism,
cellular atrophy,
decreased reproductive success,
impaired tissue development, and
death of the coral.
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.

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:
type and chemistry of the oil,
degree of weathering of the oil,
the thickness of the oil slick,
type of dispersant,
droplet size and application ratio,
prevailing sea conditions (wave mixing energy), and
sea temperature and salinity.

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:

the specific gravity (or API gravity),
pour point, and

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.

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.

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