Read about dispersants and how they are used for marine spill response.

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Using dispersants for marine pollution response

Dispersants are chemicals that help remove oil from the sea surface by breaking oil slicks into small droplets. The small droplets are then dispersed and diluted into the underlying seawater by wave action, where they are broken down by bacteria.

Dispersants are the most commonly known group of New Zealand Oil Spill Control Agents (NZOSCAs). Any NZOSCA require approval before being used in a spill response in New Zealand waters.

More about NZOSCAs

Maritime New Zealand (MNZ) holds stocks of five different dispersants approved in New Zealand, which may be considered for use during an oil spill response. Some are more effective on heavy oils (fuel and bunker oils), while some are better suited to lighter oils, such as crude oil.

The five approved dispersants held in stock in New Zealand are:

  • Corexit 9500
  • Corexit 9527
  • Shell Dispersant VDC
  • Slickgone NS
  • Gamlen Oil Dispersant LT.

The decision to use any dispersant during a spill response is only made after full consideration of a wide range of factors, including the type of oil and the conditions and circumstances at the time of the incident. This includes consideration of the principles of NEBA (Net Environmental Benefit Analysis). These principles, when applied to dispersant use, focus on the toxicity of dispersed oil in the water column compared to potential impacts that might occur if the oil persists on the water or strands on the shoreline. The primary aim of this assessment, and the use of dispersants, is to minimise the environmental harm caused by the oil spill.

Any decision is based on the MNZ Guidelines for the Use of Oil Spill Dispersants, which reflect internationally recognised guidelines. Guidelines for the Use of Oil Spill Dispersants[PDF: 423kB, 36 pages]

Toxicity of dispersants

Modern dispersants have been formulated to have low toxicity and all dispersants used in New Zealand are tested against established international criteria to assess their effectiveness and suitability for use.

While it would not normally be desirable to put any chemical into the sea, dispersants are only used when the environmental risk posed by an oil spill is greater than the risk posed by the dispersant.

Oil from a spill floats on the surface of water and when in a slick, its toxicity is very high as its concentration is near 100% oil. The aim, when adding dispersant to a floating layer of oil, is to break the oil into small particles to reduce the concentration and accelerate the natural process by which it is broken down by bacteria.

When a slick is broken down by dispersants, the small particles no longer float on the surface but are spread through the water column. At the start of this process the concentration of droplets just below the water surface is very high. This will initially increase toxicity. But as the oil dilutes through the water column, the toxicity levels quickly drop below that of the original slick. The process also gives naturally occurring bacteria much greater access to the oil so it breaks down more quickly.

Use of Corexit in New Zealand

During the Rena response in 2011, MNZ undertook a limited on-water trial of the dispersant Corexit 9500 and of a very small amount of Corexit 9527 on heavy fuel oil spilled in the early days of the response.

The trial was conducted over a total of three days but was then stopped, as the results showed the dispersant was not effective in the prevailing conditions.

The dispersants used in the trial were used according to the New Zealand Guidelines for the Use of Oil Spill Dispersants, under controlled conditions in deep water around 20km offshore from the mainland.

About 3 cubic metres of dispersant was used in the Bay of Plenty, which has an estimated 5.4 trillion cubic metres of water.

MNZ worked closely with Bay of Plenty Public Health authorities throughout the Rena response to ensure any public health concerns were addressed appropriately. MNZ has received no reports of any cases of ill health linked to either the oil or dispersant.

MNZ has not received any peer-reviewed scientific data on the health effects of Corexit used in the Gulf of Mexico oil spill response. Nor has MNZ received any scientific data that confirms the statements made in a recent 60 Minutes programme on Corexit use in the Gulf of Mexico. The Gulf of Mexico spill released 780,000 cubic metres of oil, the toxicity of which is not in doubt.

MNZ regularly reviews new information and studies on dispersant use and its effects. This ensures lessons from peer-reviewed scientific information, for example resulting from studies into Corexit use in the Gulf of Mexico, will help inform MNZ’s dispersant use. To date, MNZ has not received any scientific data, or advice from overseas oil response agencies, that would indicate a specific review of Corexit, outside of the ongoing work, is necessary.

Use of Corexit in other countries

To MNZ's knowledge, no country has “banned” the use of Corexit. There may be a range of reasons why Corexit products may not be on a particular country’s list of approved dispersants – including how effective a particular dispersant is proven to be on the oils that are most likely to be spilt in a country's waters.

There are a number of dispersants not on New Zealand’s list of approved dispersants that are approved for use in other countries. This does not mean they are “banned” in New Zealand – it simply means they have not been through the testing and approval process for use in our waters.

The Australian Maritime Safety Authority (AMSA) has advised MNZ that these products remain on the Australian Oil Spill Control Agent (OSCA) Registry list as acceptable for use in Australia, under a “grandfathering” policy. However, AMSA has declared its stock of Corexit 9500A obsolete.

The criteria for obsolescence include effectiveness testing, stock age, and whether or not the product is likely to be available for purchase in the near future.

As such, AMSA decided in early 2013 to remove from active service and arrange for disposal and replacement of a number of dispersants stocks, including Corexit 9527.

Because Nalco, the manufacturer of Corexit products, has indicated it is not intending to make an application for its Corexit products to be recognised under the updated 2012 Australian OSCA policy, they will not be available for purchase in Australia in the near future.

However, as other Australian oil spill response agencies still hold stocks of these dispersants, products remain on the OSCA Registry list until used up or disposed of.

Alternatives to using dispersants

Dispersant is only one of a range of options for oil spill response. When considering what action to take in dealing with an oil spill, a number of factors are considered, including but not limited to:

  • Water conditions – for example, rough seas will break up an oil spill more quickly, but will also mean many response options, such as booms and skimmers, will not work.
  • The oil trajectory – if a spill is heading towards an area that is easy to clean, or has very low environmental sensitivity, it may be best to allow the spill to reach the shoreline, and then conduct a clean-up. However, if the spill is moving towards a more sensitive area, for example an estuary or wildlife habitat, the response team will need to prevent as much oil as possible from reaching the shoreline.
  • The type of oil – some oil will break up quickly in the natural environment and require little intervention. Other oils will persist for a long time if not dispersed or collected.

Dispersant is only used when careful analysis shows its use will provide a net environmental benefit.

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