Guidelines for the use of dispersants for combating oil pollution at sea in the Mediterranean region
Part II: Basic information on dispersants and their application
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concentrations (total amount of dispersant or oil divided by the total volume of water in the
experimental chamber) rather than measured concentrations of materials to which organisms
are actually exposed. Last but not the least, testing conditions may lead to overestimate the oil
toxicity. Indeed oil toxicity tests are often performed on fresh oil while, at sea, the oil would
have been partly weathered, for few hours, losing its more toxic compounds (see paragraph
below “Toxicity of oil”).
Figure 10: Toxicity tests with shrimps conducted on dispersed oil in UK
Intrinsic toxicity of dispersant
Lethal concentrations of dispersants have been the main concern and most toxicity tests aim at
determining these. However, certain sub-lethal effects including changes in reproduction,
behaviour, growth, metabolism and respiration may also occur when organisms are exposed to
levels well below lethal thresholds.
It is to be emphasized that these responses have been noted in laboratory experiments where
the duration of exposure is 1 to 4 days, much longer than those expected in most dispersant
use situations in open water. Besides exposure concentrations of reported sub-lethal effects
normally are 1 or 2 orders of magnitude above highest anticipated concentrations in field use.
Few reports of measurements of concentrations following the use of dispersants in the field
exist, however, these suggest that even initial concentrations in the water column are typically
below estimated lethal and sub lethal concentrations derived from experiments.
In conclusion, results of studies investigating the effects of dispersants suggest that major
effects should not occur in the near-surface waters due to a dispersant alone, provided
properly screened dispersants are used at recommended application rates.
Toxicity of oil
Oils of different types contain a small proportion of chemical compounds that are toxic to many
marine organisms. Some of the more acutely toxic lower molecular weight compounds
(benzene, toluene, ethyl benzene and xylenes, often referred to as BTEX compounds) are also
volatile and water-soluble to some degree. Freshly spilled crude oils are much more acutely
toxic than modern oil spill dispersants.
Higher molecular weight compounds that are present in low concentrations in many oils that
often cause concern over toxicity are the PAHs (Polycyclic Aromatic Hydrocarbons). PAHs are
known to be carcinogenic and can cause other effects by chronic exposure.
Guidelines for the use of dispersants for combating oil pollution at sea in the Mediterranean region
Part II: Basic information on dispersants and their application
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Page 15
Toxicity of oil affected by the use of dispersants
Dispersing spilled oil converts the oil from a surface slick to a plume or „cloud‟ of very small oil
droplets dispersed in the water column. These oil droplets might be ingested by filter feeding
organisms, such as copepods, oysters, scallops and clams.
The widening of the oil surface area increases the rate at which partially water-soluble
chemical compounds in the oil are transferred into the sea. The localised concentration of
these potentially toxic Water Accommodated Fraction (WAF) compounds will rise before they
are diluted. This is the justification for the argument that dispersants can never be a valid oil
spill response because its use, if they are effective, will inevitably cause an increase in the
dispersed oil concentration in the water column, leading to toxic effects on marine life.
However, it is important to distinguish between:
(i)
the increased potential for toxic effects to occur; and,
(ii)
the possibility of toxic effects actually occurring.
Dispersed oil concentrations will certainly be higher if dispersants are used, than if they are not.
This does not mean that the dispersed oil concentrations will be high enough, or persist for
long enough, to cause actual toxic effects. Most spilled oils will naturally disperse to some
degree in the initial stages of an oil spill, before the oil becomes emulsified. The successful use
of dispersants will obviously increase the concentration of dispersed oil in the sea. However,
this is a matter of degree rather than an absolute difference; some spilled oil is likely to
naturally dissolve and/or disperse even if dispersants are not used.
By dispersing the oil in the water column the exposure of the organisms living in the upper
layer of the water column increases. If the dilution of the plume of dispersed oil in the water
column is rapid the exposure will be low: experience from both experimental field trials and
dispersant offshore operations at real spills have shown that dispersed oil will quickly be diluted
into the sea. The concentration of oil in water rapidly drops from a maximum of 30-50 ppm just
below the spill short time after treatment, to concentrations under 1-10 ppm of oil in the top 10-
20 meters after a few hours.
Because oil will disseminate in the environment by natural dispersion which is a process that
proceeds quite rapidly in rough seas with low viscosity oils, exposure of some marine
organisms to dispersed oil at some concentration will occur even when dispersants are not
used.
Table 6: Data from the Sea Empress incident
During the “Sea Empress” incident, (Wales 1996), which led to the largest dispersant treatment
operation (440 tons of dispersant where applied on fresh crude at sea), oil concentrations were
monitored in the upper water column as follows:
Time after dispersant application
Oil concentration in the upper water column (ppm)
Just after treatment
10
2 days
1
1 week
0.5
1 month
0.2
3 months
Background level