Draft final

Discussion Note 10.1 ASSESSING DISPERANT EFFECTIVENESS

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  Dispersant applications must be monitored to confirm whether or not dispersant use is effective, and to determine the fate and transport of treated oil.
  
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  Dispersant applications should not be delayed simply because monitoring is not in place.
  
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  Visual observation is the minimum level of monitoring. Observations teams may use the forms in Attachment III.
  
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  There will be very few instances where a dispersant application is possible but visual monitoring is not.
  
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  Because dispersed oil plumes are often highly irregular in shape and thickness, it can be difficult to accurately estimate dispersant efficiency.
  
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  The appropriate dispersant application dose depends on the oil thickness (see Attachments III.a and III.b for common dose rates based on oil thickness). Slicks are generally not of uniform thickness, and it is not always possible to distinguish among thicker and thinner portions of the same slick. It is therefore possible to apply too much or too little dispersant to some parts of a slick. Because over- and under-dosing can lead to variations in effectiveness, these variations should be noted.
  
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  On-site monitoring of oil dispersed in the water column should support visual monitoring whenever possible. See Attachment III for additional information and forms.
  
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  Decisions to terminate operations due to poor effectiveness should ideally be based on on-site monitoring results.
  
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  A visible coffee-colored cloud in the water column indicates the dispersant is working.
  
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  A milky-white plume in the water column can indicate excessive dispersant application.
  
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  When dispersant is working, oil remaining on the water surface may also change color.
  
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  A difference in the appearance of treated and untreated slicks indicates dispersion is likely.
  
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  Absence of a visible cloud in the water column makes it difficult to determine whether the dispersant is working. When the water is turbid, you may not be able to see a plume. Oil remaining at the surface and sheens can also obscure an ability to see oil dispersing under the slick.
  
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  Successful dispersion can occur with no visible indication of dispersion.
  
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  A subsurface plume may not form instantly once dispersant has been applied. In some cases (e.g., emulsified oil) it can take several hours for a plume to form. In other cases, a visible plume may not form, and you may wish to use sampling to learn whether dispersion has occurred.
  
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  Boat wakes may physically part oil, falsely indicating successful dispersion. Mechanically dispersed oil will re-coalesce and float to the surface.
  
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  Dispersants sometimes have a herding effect on oil after initial applications, making a slick appear to be shrinking when, in fact, the dispersant is “pushing” the oil together. The effect results from the surfactants in the dispersant, which causes a horizontal spreading of thin oil films. This can cause parts of a slick to seem to disappear from the sea surface for a short time.
  

From Cawthron 2000 and NOAA Oil Spill Job Aids

Discussion Note 10.2 WHEN DISPERSANT IS NOT EFFECTIVE

If monitoring shows dispersion does not appear effective, review all aspects of the application and monitoring for possible reasons why. Aspects to consider include:

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  Dispersant formulation
  
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  Application ratios (increase or decrease oil: dispersant ratio)
  
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  Application methods
  
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  Monitoring methods
  
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  Interpretation of monitoring results
  
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  Oil weathering
  
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  Weather conditions
  

From Cawthron, 2000

BOX 12 DO NOT USE DISPERSANT

Pre-approval to use dispersants does not apply if any of the following occur:

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  The spill cannot be chemically dispersed with an approved and available agent (see Box 2 and);Oceanographic and weather conditions are not potentially conducive to dispersant use (see Box 3 and DISPERSANT ASSESSMENT WORKSHEET);
  

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  The spilled oil is closer than 3 nautical miles from shore, within the boundaries of a National Marine Sanctuary (see Box 4), within 3 miles of the CA/Mexico borders, will be applied surface water oil spill for more than 5 days, or is a subsea use. Approval to use dispersants outside the Pre-Approval Zone, and/or for uses that have not been pre-approved in any zone (surface use for more than 5 days, subsea use) do not fall within the Pre-Approval guidelines, and will instead need to be considered under the RRT Expedited Approval Process (see Box 4a and Attachment 4910b Section II);
  
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  The dispersant will not have a net environmental benefit (see Box 7a);
  
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  The dispersant cannot be applied safely (see Box 5), with suitable weather (Box 5a) or available resources (Box 5b);
  
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  The dispersant is not significantly more effective than natural dispersion or other response options (see Box 9).
  

IF DISPERSANT USE IS CONSIDERED INAPPROPRATE, CONSIDER OTHER RESPONSE OPTIONS.
Go to Box 1a.

CONTINUE TO MONITOR APPLICATION PARAMETERS AND Run

additional dispersant sorties as necessary

More than one dispersant sortie (run) may be necessary to effectively treat the oil spill. Continue to monitor information on the spill extent, dispersant effectiveness, continued availability of suitable weather “windows” and dispersant application equipment and personnel, and perform addition applications as necessary.

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  Record information from each sortie on the Dispersant Decision Summary.
  
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  Inform RRT when all runs are completed (fax Dispersant Decision Summary form to RRT contacts in Attachment IX).
  

There will be a point when dispersants ARE no longer effective.