The dispersant was released in the subsurface and to oil slicks on the surface to prevent the vast quantities of oil from fouling beaches and marshes.

It was thought that the dispersant would break down easily and rapidly in the environment. However, a team of researchers from Haverford College in Pennsylvania and Woods Hole Oceanographic Institution have found traces of the product in the environment four years later.

A study conducted by the team of scientists examined samples collected from deep-sea corals and surrounding sediments, as well as oil-soaked clumps of sand referred to as sand patties.

"We found DOSS persisted in variable quantities in deep sea coral communities six months after the spill and on Gulf of Mexico beaches 26 to 45 months after the spill," Haverford assistant chemistry professor and lead author Helen White said.

"These results indicate that the dispersant, which was thought to undergo rapid degradation in the water column, remains associated with oil in the environment and can persist for around four years."

Earlier work by associate scientist at WHOI Elizabeth Kujawinski found that DOSS was slow to degrade in deep ocean environments.

"The deep sea is cold and dark and the degradation of dispersant components happens much more slowly under these conditions," Kujawinski said.

"The interesting thing is that the sand patties we're finding on beaches four years after the spill have DOSS in them. That was somewhat unexpected.

"The amounts we detected were quite small, but we're finding this compound in locations where we expected the dispersants to disappear, either by dissolving in water or by being degraded by bacteria."

The research team is not yet sure of the effects of the dispersant on health or the environment.

"It's hard to say, because we don't know how toxic it might be," White said

"The EPA has determined what concentrations of DOSS may be harmful to marine life in the water, but the toxicity of DOSS in solid [non-aqueous] forms like sediments or sand patties is not known.

"We know that if you measure ‘x' amount of this compound in ‘y' amount of water, that's toxic. But you can't compare those numbers to what we've found in the sand patties because we're looking at this compound in a mixture of sand and oil."

The team now aims to work out how the dispersant has survived in the environment for so long.