OIL AND RELATED CHEMICALS - Marine Pollution: What Everyone Needs to Know - page 70

animals. To protect marshes from incoming oil, booms were
set around islands and shorelines, and two million gallons of
the dispersant Corexit ® were applied on and beneath the sur-
face of the sea to break up the oil. Dispersants are complex
mixtures of chemicals that have surfactant (wetting) proper-
ties, which allows them to act as emulsifiers, essentially letting
the oil and the water mix. After extensive use, oil was no lon-
ger visible on the surface of the water, and some claimed that
it was gone and degraded by microbes.
Why was the use of dispersants so controversial?
By enhancing the amount of oil that physically mixes into
the water column, dispersants reduce the amount of oil that
reaches the shoreline. Dispersants also stimulate the natural
process of aerobic biodegradation by breaking oil up into tiny
droplets that are so diluted that the natural levels of available
nitrogen, phosphorus, and oxygen are sufficient for microbial
growth to degrade the oil. On the other hand, once the oil is
dispersed in deep water, it cannot be recovered. When com-
bined with dispersants in the water, oil may be more toxic than
either the oil or the dispersant alone. Most studies found that
the combination of oil and dispersant increased toxicity. Two
dispersants, Corexit ® 9500 and 9527A, were used. Although
they are EPA-approved, they are ranked by EPA as more toxic
and less effective than other dispersants.
What happened to the oil and dispersants?
The well blowout occurred in deep water, where a turbulent
discharge of hot pressurized oil and gas mixed with seawa-
ter and dispersed by itself into droplets, emulsions and gas
hydrates without the use of chemicals. This naturally dispersed
mixture did not rise to float on the surface as oil typically
does but stayed in a subsurface plume. Amid reports of the
oil being nearly “all gone,” a plume of hydrocarbons about 22