Dispersants: A Guided Tour - Part Four

What Constraints Affect Dispersant Use?

The effectiveness of a given dispersant depends on the type of oil spilled (it is easier to disperse lighter oils), weather conditions, and how quickly dispersants can be applied once oil has spilled. Dispersants must mix with the oil and water, so some energy is required for effective dispersion. But too much wind and wave energy may result in conditions that are dangerous for flying aircraft and that make it difficult to target the oil and properly apply the right amount of dispersant. Heavier oils or highly emulsified oils (oils that have mixed with water to form a heavy froth) are less amenable to successful dispersion, although research on new types of dispersants is attempting to address this problem.

The graphic above contrasts the kinds of conditions under which dispersants are likely to be more effective (shown on the left) with the kinds of conditions under which it's harder to disperse oil (shown on the right).

What Happens to Dispersed Oil?

Initially, dispersed oil moves down into the water column to depths ranging from 1 to 10 meters (about 3 to 30 feet). To avoid contaminating the sea floor, most dispersant use to date has been restricted to waters deeper than 10 meters (about 30 feet). The diagram above shows concentrations of dispersed oil at different depths (estimated from field studies), during the first few hours after dispersants have been applied. These concentrations drop within hours as currents and waves disperse the oil further.

Eventually, dispersed oil droplets degrade into naturally occurring substances. There is evidence that dispersed oil degrades more quickly than oil that has not been dispersed. The diagram above illustrates how the oil may be processed in the marine ecosystem. First, the droplets of oil and dispersant are colonized by bacteria that then begin to degrade them. Next, protozoans and nematodes (small worms) join the colonies. Eventually, the oil may be further broken down and incorporated into the food web.