The use of dispersants
in coastal areas
Romain PEDEN
Towson University, Towson, MD, USA ; Université de Bretagne Occidentale, Brest, FRANCE
Introduction
Dispersants are liquid solutions of detergent-like surfactant dissolved or suspended in a solvent. Their principal action is to allow the dilution of oil in
the water column. Dispersants have been used following the « Ixtoc I» oil spill in the Gulf of Mexico (1979) and the Exxon Valdez spill in 1989. However,
their use remains controversial because their effects on marine organisms have not been fully assessed. For that reason, in Europe, dispersants can only
be used in offshore water.
3
After the Deepwater Horizon accident, dispersants have been used in large quantity. Approximately, 4000 m have been used to treat surface water
3
and 3000 m have been injected at the well head. It is the first time that so much dispersant has been used to fight oil spill.
Advantages of dispersants
Increased
availability for degradation by microorganisms (but effects on
food web);
Protect mangrove, coastal and intertidal areas rom oil slicks;
Limit impact on tourism (oil is invisible);
Reduce impact on game fishing (!);
Lessen impact on bird (But affect feathers impermeability and
contaminate food web) ;
Lower wind_driven drift of oil slick.
Disadvantages of dispersants
Effects
on wildlife:
– Reproduction of coral
– Oil plus dispersant (Corexit) is 19 000-fold more toxic for amphipods
than oil alone;
– Multitude of effects on fishes (ontogenesis, gills, metabolism,
cardiovascular, reproduction...)
– Burn eyes and mouth, nervous system, respiratory system, immune
system, liver and kidney in mammals;
– Dispersants smell gaz (impact on tourism)
– Oil become invisible, oil can be anywhere (e.g. carbon monoxyde);
– Shellfish have a high rate of filtration, harvest can be hazardous;
Contains arsenic, chromium, detergents which can degrade lipid
membrane.
7000
6000
5000
4000
3000
Dispersants in action
Spread
Stock
2000
03/08/1
0
20/07/1
0
06/07/1
0
22/06/1
0
08/06/1
0
25/05/1
0
0
11/05/1
0
1000
27/04/1
0
Volume of dispersant (m3)
Impact of crude oil with and without dispersants in the water
(Source : ExxonMobil Research and Engineering)
The use and stock of dispersants
A C-130 Hercules spreading
dispersant in the Gulf of Mexico (Source : Deepwater Horizon Response)
Prospects - Challenges
The
The
above shows that there is no simple answer to the question of the use of dispersants in coastal waters.
use of dispersants involves a situation-specific compromise which includes water conditions (temperature, currents, salinity...), oil type (sweet/sour,
heavy/light...), inshore/offshore, presence of particular ecosystems, species, marine protected areas.
However, numerous unknowns make this compromise difficult to reach;
Among those unknowns, the long term toxicity of dispersants remains poorly studied.
In the case of the Gulf of Mexico, several issues must be considered :
– Economical aspect : fishing is a mainstay of the economy of coastal states
– Dispersion aspect : currents in the Gulf of Mexico are variable and this can affect the dilution pattern of oil by dispersants
– The Gulf of Mexico is one of the two breeding zone of bluefin tuna. The massive use of dispersants, by increasing the bioavailability of toxic molecules,
may have increased the detrimental effect of the spill upon this already fragile fish stock.
References : Susan Shaw lecture (TED.com), Dr R. Snyder lecture (CEDB, University of West Florida), CEDRE, deepwaterhorizonresponse.com,