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Le Lann - Chemical dispersion as an oil pollution response - French approach 
Chemical dispersants which have passed the above three tests are identified in a list of quali 
fied dispersants which is available online on Cedre's website. Some of these dispersants 
were purchased by French authorities to build up a national emergency stockpile which is 
immediately available in case of oil spill following a maritime accident. Efficiency tests are 
regularly carried out on the dispersants in stockpiles to make sure that their performance 
does not degrade over time. 
Decision to spray chemical dispersants is based on a cost / benefit analysis, the NE- 
BA 
The choice whether to use chemical dispersants on a drifting oil slick is guided by a cost- 
benefit analysis. The procedure is known and documented as the Net Environmental Benefit 
Analysis (NEBA). The principle is to compare the potential impact of non-dispersed oil slick 
with that of the oil plume after using chemical dispersants. The analysis is based both on the 
knowledge of the pollutant (the quantity, the toxicity, ...), on an atlas of environmental and 
economic sensitivity of areas potentially impacted by the pollution, on the particular sensitivi 
ty of elements identified in the atlas to the dispersed and non-dispersed oil, and finally the 
prediction of the oil drifts with or without dispersant use. The difficulty of the exercise is that 
in most cases such information is not readily available during the accident, or, if available, its 
accuracy may be low. In addition, the decision must be made quickly because the time win 
dow during which the oil can be treated with chemical dispersants is relatively short, a few 
hours after the spill to, at most, a few days. 
The consequence is that, while the full analysis can be performed on small areas during the 
preparation of emergency response plans, it is much more complicated to do so in an acci 
dental context of which we know neither the place nor the time it will happen. This led France 
to set up a decision-making process based on some simple criteria that are applied to all 
areas under national jurisdiction to enable a very quick decision to implement chemical dis 
persion as a pollution response action, while ensuring that the environmental benefit of this 
action will be positive. 
A simpler approach for a quick decision 
The decision process implemented in France is based on three scenarios of accident leading 
to the presence at the surface of the sea of oil slicks with respective masses of 10, 100 and 
1,000 tons. No scenario beyond a 1,000 tons release was considered because the likelihood 
of having a single drifting slick of more than 1,000 tons of oil is extremely low. Similarly, off 
shore exploration and production of hydrocarbons are undeveloped in maritime areas under 
French jurisdiction, and therefore the decision process does not include the use of disper 
sants from underwater leaks. The decision process has been prepared solely for decisions 
related to the treatment of surface slicks. A new analysis would be necessary for the defini 
tion of decision criteria related to the use of chemical dispersant underwater, by which to 
ensure that the concentration of dispersed oil in the water column would remain permanently 
within acceptable limits. 
Studies on the toxicity of dispersed oil carried out in the past have found that concentrations 
below 10 ppm do not have a significant and long-lasting toxic effect on marine species. This 
led to a study to determine under which specific conditions it is ensured, for the three scenar 
ios considered, that the oil concentration in the water column remains below the toxicity limit. 
Existing models are not able to represent in a reliable way dispersed oil concentration in the 
water column after treatment of a slick. Information on concentrations of oil below dispersant- 
treated slicks comes therefore mainly from field experiments in open waters or from meas 
urements made during real accidents where dispersants were used. Immediately after appli 
cation of the chemical dispersant on a slick, the concentration of oil in water just under it is