The cleanup technique used by the Environmental Protection Agency (EPA) following the 2010 Deepwater Horizon oil spill in the Gulf of Mexico actually made the spilled oil more than 50 times more toxic than doing nothing, according to a study conducted by researchers from the Georgia Institute of Technology and the Universidad Autonoma de Aguascalientes (UAA), Mexico. The findings were published in the journal Environmental Pollution.
Over the course of three months, 4.9 million barrels of oil spilled into the Gulf of Mexico (although some scientists believe that the spill is likely ongoing to this day). To aid in the cleanup, the EPA ordered another two million gallons of a chemical "dispersant" known as Corexit poured into the Gulf.
"There is a synergistic interaction between crude oil and the dispersant that makes it more toxic," study co-author Terry Snell said.
The idea behind dispersants such as Corexit is that they break apart large masses of oil into tiny droplets, that then spread out over a wider area and have less of an impact in any one place. However, oil actually becomes more toxic in smaller droplets, because it is more easily absorbed into cells (bioavailable).
"Dispersants are preapproved to help clean up oil spills and are widely used during disasters," said lead researcher Roberto-Rico Martinez. "But we have a poor understanding of their toxicity. Our study indicates the increase in toxicity may have been greatly underestimated following the Macondo well explosion."
Ecosystem-wide effectThe researchers performed laboratory tests in which they exposed five different strains of microscopic animals known as rotifers to either oil from the Deepwater Horizon spill, or to a mix of oil from the spill with Corexit. They found that the mixture was 52 times more toxic than the oil alone.
The mixture was not just toxic to adult rotifers; in concentrations as low as 2.6 percent, the mix also reduced hatching success among rotifer eggs by 50 percent.
"The effect is specifically a toxic synergy - the sum is worse than the parts," said Ian MacDonald of Florida State University, who was not involved in the study.
The study was conducted on rotifers in part because they respond quickly and strongly to toxic chemicals, but also because they form the base of the Gulf food chain. They are eaten by shrimp, crabs, and baby fish, and a crash in their population could easily radiate all the way up to the ocean's large predators. The effect on egg-laying success is particularly troubling, the researchers noted, as it suggests longer-term impacts.
"This is a cautionary tale," Snell said.
"In this case, the Corexit is simply there to make the oil disperse and go out of sight. But out of sight doesn't mean it's safe in regard to the food web."
In the future, Snell said, it would probably be less damaging to let oil spills disperse naturally.
"It's hard to sit by and not do something," Snell said. "But in this case, doing something actually made it more toxic."
