Oil Dispersants - A Primer

Natural dispersion of an oil slick occurs when waves and other turbulence at the sea surface cause all or part of the slick to break up into droplets and enter into the water column. The addition of dispersants is intended to accelerate this natural process.

Dispersants have two main components: a surfactant and a solvent. Surfactants molecules are made up of two parts: an oleophilic part (with an attraction to oil) and a hydrophilic part (with an attraction to water). When dispersants are sprayed onto an oil slick, the solvent will transport and distribute the surfactants through the oil slick to the oil/water interface where they arrange themselves so that the oleophilic part of the molecule is in the oil and the hydrophilic part is in the water. This creates a reduction in the surface tension at the oil/water interface and small oil droplets will break away from the oil slick with the help of wave energy. These droplets will be of varying sizes and although the larger ones may rise back to the surface some will remain in suspension and will drift apart and become degraded by naturally occurring bacteria. If dispersion is successful, a characteristic brown plume will spread slowly down from the water surface a few minutes after treatment.

A federal official monitoring the environmental impact of the massive oil spill in the Gulf of Mexico said Tuesday his agency is closely watching what he called BP's "unique idea" to break up the spill with dispersant chemicals underwater.

In addition to the traditional method of using dispersants on the surface, BP has attempted to inject chemicals near the source of the leak deep underwater. Oil has been gushing out of the seabed since at least April 22, when the Deepwater Horizon sank two days after an explosion and fire.

Charlie Henry, scientific support coordinator with the National Oceanic and Atmospheric Administration, told reporters Tuesday that the agency is evaluating the pros and cons of the effort.

Oil spill dispersants do not actually reduce the total amount of oil entering the environment. Rather, they change the inherent chemical and physical properties of oil, thereby changing the oil’s transport, fate, and potential effects. Small amounts of spilled oil naturally disperse into the water column, through the action of waves and other environmental processes.

The objective of dispersant use is to enhance the amount of oil that physically mixes into the water column, reducing the potential that a surface slick will contaminate shoreline habitats or come into contact with birds, marine mammals, or other organisms that exist on the water surface or shoreline. Conversely, by promoting dispersion of oil into the water column, dispersants increase the potential exposure of water-column and benthic biota to spilled oil. Dispersant application thus represents a conscious decision to increase the hydrocarbon load (resulting from a spill) on one component of the ecosystem (e.g., the water column) while reducing the load on another (e.g., coastal wetland).

Decisions to use dispersants, therefore, involve trade-offs between decreasing the risk to water surface and shoreline habitats while increasing the potential risk to organisms in the water column and on the seafloor. This trade-off reflects the complex interplay of many variables, including the type of oil spilled, the volume of the spill, sea state and weather, water depth, degree of turbulence (thus mixing and dilution of the oil), and relative abundance and life stages of resident organisms.