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The central question to be answered in assessing Net Environmental Benefit is

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The central question to be answered in assessing Net Environmental Benefit is:

Will dispersant use significantly reduce the impact of the spilled oil?

  • Rapid decisions on use are essential as dispersant must be applied quickly to be effective.

  • Decision-makers must consider the various environmental, social, economic, political and cultural

factors unique to each spill.

  • Tradeoffs will be necessary, as no response is likely to satisfy all parties and protect all resources.

The ecological impacts of oil are generally longer-lasting and more persistent than most other impacts.

  • Ecological effects will be due primarily to the spilled oil. Dispersant applied at recommended rates is unlikely to cause significant adverse effects, even in multiple applications.

  • Oil dispersed into water depths greater than 30 feet (~10 meters) will quickly dilute to levels where acute toxic effects are unlikely.

  • The California policy is that dispersants will not be used over water shallower than 60 feet deep.

  • Few acute toxic effects have been reported for crude oil dispersed into less than 30’ (10m) water depth if the water is well-flushed.

  • Small spills seldom require dispersants use.

  • Dispersant use is not recommended for spills of light fuels such as gasoline, jet fuel, kerosene, diesel

and light fuel oils.

  • Dispersants are not recommended for use on oil sheens.


At the time of an actual oil spill or a decision to use chemical dispersants on the oil, marine species are expected to be on the water surface or in the upper water column. Before using chemical dispersants, the FOSC will have decided that there may be a net environmental benefit from dispersant use. Information on regional sensitive species and habitat information from the Net Environmental Benefit Analyses (NEBA), summarized for each region in Attachment I, can help the FOSC determine which species might actually be in the area and scouted for by the aerial observers (Box 1b and Box 7c). This additional information can provide further validation and justification to a FOSC that impacts of chemical dispersant application will be minimized wherever possible, and net environmental benefit maximized.


The FOSC can take additional information and advantage from the Aerial Wildlife Observers if they have been deployed (Box 1b), or information from the Wildlife Aerial Survey Form (Attachment IV.c) available from other aerial spotters, or information from wildlife spotters (Attachment IV) available to the FOSC from other data collection forms or notes used by those spotters. Any of these resources will provide real-time or near real-time information on marine seabird and mammal presence, and can guide the FOSC on dispersant application parameters that may minimize impacts to those resources.


  • Use the information on estimated oil spill volume from the DISPERSANT ASSESSMENT WORKSHEET and Discussion Note 8.1 below to:

  • Determine the dispersant application ratio (usually 1:20), and

  • Calculate the volume of dispersant required (Attachments III.a and III.b).

    • Record the details on the Dispersant Application Summary Form (Attachment III.d);

    • Mobilize application team;

    • If not already done, mobilize SMART. Some blank SMART forms are included in Attachment III for use by other trained professionals, if appropriate and when approved by the FOSC.

  • Inform RRT (see Attachment XI for contact information).

Decision: Dispersants applied?

  • Yes Go to Box 9

  • No Explain.

Make a note of the decision in the Dispersant Decision Summary box on the Dispersant Use Flowchart.
Reassess as necessary and appropriate.


The FOSC can approve the use of dispersants within the 3 miles zone of the California/Oregon border. Once a dispersant use decision is made, the FOSC should contact the RRT X Liaison of the decision as soon as possible and should also endeavor to fax the Dispersant Record of Decision as well. The CG Coordinator to the RRT IX, or the NOAA SSC (Attachment XI) can assist with RRT X contact information.


  • The FOSC has final responsibility for operational aspects of dispersant applications.

  • Dispersant must only be applied by experienced spray applicators and in accordance with manufacturer instructions.

  • The persons applying dispersant are responsible for the calibration and operation of the spraying system, and the safety and maintenance of the application platform.

  • Droplet size is the key variable influencing dispersant effectiveness. Undersized droplets (e.g., fog or mist) will be lost through drift and evaporation. Oversized droplets will punch through the oil and be lost in the water column.

  • Dispersants pre-diluted in water are less effective than undiluted dispersant.

  • Only undiluted concentrate dispersant is applied from aircraft. Dispersant should, where possible, be applied into the wind and parallel with the slick.

  • Dispersant should be applied in a methodical and continuous manner to ensure the entire target area is treated.

  • Spraying effort should concentrate on the thickest sections, and/or the leading edges, of oil that threaten sensitive areas.

  • Thick portions of the slick may require several applications.

  • Oil sheen should not be sprayed with dispersant.

Regarding the relationship between Dispersant-to-Oil Ratio (DOR) and the concentration of oil being treated:

  • Regardless of DOR ratios suggested by dispersant manufacturers, there are many factors that influence dispersibility (e.g., oil characteristics, degree of weathering, water salinity, sea state) that may make it very difficult to select an appropriate DOR for the conditions faced on the day of a specific spill

  • The variability of slick thickness (or oil concentration) is such that one can never really characterize the actual oil concentration for more than a few seconds within the speed and swath constraints of a particular application system.

  • With most application systems, one is usually overdosing and under-dosing as the system moves through light, heavy and sometimes “no” oil on the water surface.

  • The best estimate of the average oil thickness (or average volume of oil per unit are) must be used.

  • Given that precise spray parameters are extremely difficult to achieve, dispersant applicators generally use about 5 gallons of dispersant per acre on their first run.

  • Area, volume and thickness can be related with the following expression:

104 x Area (hectare) x Thickness (mm) = Volume (liters)


Volume (liters/Area (hectares) = 104 x Thickness (mm)

► To convert liters/hectare to gallons/acre, multiply by 0.107. To convert liters/hectare to gallons/square kilometer,

multiply by 26.42.

► These values (in any units) multiplied by the DOR (as a fraction, e.g., 1:5 = 1/5 or .2) will then yield the desired dosage (in those units) for that value of DOR.

► Refer to Attachment III.a for some pre-calculated values.

From Cawthron, 2000 and Al Allen (Spilltec), 2003 personal communication

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