Mode of action
Dispersants are agents which accelerate natural dispersion by wave action, facilitating the break up of slicks on the surface into a multitude of smaller droplets dispersed in the water column. This facilitates the breakdown of the hydrocarbons by bacteria which are naturally present in the water and reduces the local toxic effect.
Limits of use
However, the use of dispersants is limited by technical factors. They must be used in precise proportions and conditions. They remain inefficient on viscous or weathered oil. The decision to use dispersants in a particular situation cannot be put on hold, as dispersion is only an option in the first few hours or at the most the first few days. The decision to disperse the oil should be anticipated during the development of the contingency plan, depending on the characteristics of the zone. Most plans distinguish zones of free use of dispersants, zones of use in certain conditions and zones where the use of dispersants is prohibited.
Dispersants remain marred by a negative image. They have been accused of being more toxic than oil and of causing oil to sink to the seafloor, forming a deadly carpet of toxins. These accusations are unfounded. Dispersants fragment oil into a multitude of droplets which spread out in the water mass, they do not drop to the bottom. Dispersing hydrocarbons causes a temporary, local increase in their toxicity, while the dispersed oil spreads and dilutes through a vast volume of water to then become harmless. This effect implies a certain limitation concerning the use of dispersants near the shoreline and sensitive areas and/or when dilution conditions are low. However, recognised, modern, concentrated dispersants generally prove less toxic than dispersed hydrocarbons.
In certain countries, dispersants undergo tests of their efficiency, toxicity and biodegradability. In France, these tests have been carried out by Cedre since 1978. The testing of a new product begins with the efficiency test, which determines whether or not the other tests are then carried out.
If a spill ignites during an incident, it will naturally reduce the amount of oil in the water. This phenomenon may occur naturally, when the accident results in an explosion, or when a spark produces a fire at the time of the spill. A form of spill response may involve controlling the fire without extinguishing it to reduce pollution. There have also occasionally been cases of slicks deliberately being set on fire when contained in fireproof booms (Exxon Valdez in 1989) and even of the vessel being set on fire (Torrey Canyon in 1967).
Deliberate burning remains, however, an exception. Technically, it can be applied to fresh oil before evaporation of the volatile parts and only in very particular conditions. The heat, combustion gases and soot released constitute other forms of pollution and tend to deter the decision to set the oil alight.
Circumstances and timeframe permitting, certain measures should be taken before the arrival of the pollution on the shoreline, in a bid to facilitate the subsequent clean-up operations and limit the impact.
Collecting solid waste and natural debris onshore before the pollution arrives helps to reduce the amount of soiled material and facilitates clean-up operations. According to the extent of breaching, this collection can be carried out either manually or using public works machinery or specialised machinery such as sand screeners. Removal must of course be as selective and methodical as possible.
The apparent protection of bays, estuaries and inlets using booms can be somewhat illusory due to strong currents sweeping across their entrances. However, more limited areas can be protected.
Protection with filtering booms should also be anticipated for creeks, marsh creeks and water intakes in shellfish parks, fish farm basins, salt pans, thalassotherapy centres and other installations using seawater. The options vary according to the size of the sites needing protecting. The main resources available are gillnets, filtering barriers or dams made of earth and other materials. Some of these methods of protection combine the retention function of the netdam and the sorption capacity of sorbents. The industry offers sorbent booms, suitable for relatively small quantities of fluid or finely divided substances. A net filled with straw can constitute a makeshift solution as long as the straw is changed often enough and correctly disposed of when soiled.