Scientists are worried about the increasing amounts of oil dispersants creeping onshore, and its impact on the ecosystem.
Initially, BP and the federal agencies involved in the spill response made the decision to use dispersant offshore to limit the amount of viscous oil washing up on beaches and into the wetlands, said Ed Overton, an environmental chemist at Louisiana State University.
"Corexit contains a petroleum solvent, we're actually putting petroleum solvent on top of a petroleum spill. So it's increasing the hydrocarbon in the water column," Nature quoted Susan Shaw, a marine toxicologist and director of the Marine Environmental Research Institute in Blue Hill, Maine, as saying.
"There's just as good a chance that this dispersant is killing off a critical portion of the microbial community as it is that it's stimulating the breakdown of oil," said Samantha Joye, a biogeochemist at the University of Georgia in Athens.
"The real issue we've got in this spill is the massive amount of oil that just keeps coming," said Ed Overton, an environmental chemist at Louisiana State University.
"We've gone past any normal use of dispersants."
So far, more than 6.6 million litres of dispersant have been applied, more than 4 million litres offshore and more than 2.5 million litres at the site of the leak.
Still, Overton is not convinced that dispersed oil is making its way to shallow waters.
"The whole idea of dispersing only offshore is to do it only where there's so much water that it gets dispersed below dangerous levels very quickly," he said.
"The complication here is the trade-offs," said Robert Twilley, a coastal scientist from Louisiana State University in Baton Rouge.
"There are certain things with dispersants that are of benefit, and there are negatives, and we're having problems evaluating those trade-offs."