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.
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 dispersants approved for use in New Zealand are listed on the NZOSCA register.
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.
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.