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high and may exceed by far 100 ppm. Under the effect of water movements and currents, the
dispersed oil concentration falls down quickly, but the oil plume spreads in surface and in
depth. The vertical profile shows however a rapid reduction of the oil concentration as depth
increases. The seabed is the location of the most vulnerable ecosystems with a lot of seden
tary species. These ecosystems are those which will take the longer time to recover their
original functions if impacted by dispersed oil. The important point is therefore to make sure
that the concentration of dispersed oil at the seabed will remain well below 10 ppm at any
time after treatment of a slick with chemical dispersants. This leads to the definition of a min
imum water depth, in order to make sure that the seabed will not be reach by the dispersed
oil plume under the effect of its vertical expansion, but also to the definition of a minimum
distance to the coast to ensure that the plume will not reach the seabed somewhere else
under the effect of its horizontal expansion, to authorize the treatment of a slick with chemical
dispersants.
From the considerations above and from experimental measurements of dispersed oil con
centration, for each accident scenario a geographical limit has been defined in France for the
use of chemical dispersants that meet the following constraints:
• the so-called "10 ton" limit for a floating slick of 0 to 10 tons of oil:
o A water depth equal to or greater than 5 meters;
o A distance to the coast equal to or greater than or 0.5 nautical mile;
o Consideration of areas of specific interest as described below (virtual islands);
• the so-called "100 ton" limit for a floating slick of 10 to 100 tons of oil:
o A water depth equal to or greater than 10 meters;
o A distance to the coast equal to or greater than or 1 nautical mile;
o Consideration of areas of specific interest as described below (virtual islands);
• the so-called "1000 ton" limit for a floating slick of 100 to 1000 tons of oil:
o A water depth equal to or greater than 15 meters;
o A distance to the coast equal to or greater than or 2.5 nautical miles;
o Consideration of areas of specific interest as described below (virtual islands
and excluding large protected areas of dispersion for a slick over 100 tons).
For some sensitive areas more restrictive measures are taken. Protected areas are consid
ered in the above described process as "virtual islands" whose contour is taken as equivalent
to a coastline. However, for some very large areas, such as Natura 2000 areas, and in con
sultation with relevant stakeholders, it was decided to simply ban the dispersion of slicks of
more than 100 tons in these areas, dispersion of smaller slicks remaining possible, subject to
the satisfaction of the criteria limiting the dispersed oil concentration at the seabed to
10 ppm. Estuaries are also treated in a particular way. An area is calculated at the mouth of
the river where chemical dispersion is prohibited in order to be sure that under no circum
stances dispersed oil may enter the estuary under the effect of tidal currents.
During an accident, when beyond these limits, the use of chemical dispersants is considered
to be an interesting response option with a positive environmental benefit. Thus, the decision
to treat an oil slick of 50 tons of oil may be taken as soon as the slick is located beyond the
limit set for slicks of 10 to 100 tons ("100 ton" limit).
These elements are not however sufficient for making a decision. Depending on their nature,
not all hydrocarbons can be treated with chemical dispersants. Authorities shall therefore
ensure that the pollutant can be treated this way before starting operations. Another point is
to examine alternative response options. One may indeed prefer different options, such as a
mechanical recovery, in specific situations. This will be the case if the oceanic and weather
conditions are particularly favorable. In this case, the surface mechanical energy would not
