BP applied nearly 2 million gallons of a chemical dispersant to the oil as it spewed from the broken underwater well. The aim was to break apart the oil into tiny droplets so huge slicks wouldn't tarnish shorelines and coat marine animals, and to make the oil degrade more rapidly.
A report released Wednesday by the National Oceanic and Atmospheric Administration shows that 9.6 percent of the estimated 172 million gallons of oil released into the Gulf was dispersed by the chemicals.
Sen. Sheldon Whitehouse, a Democrat, called use of the chemicals a "grand experiment." He said it was unclear whether it would limit damage from the spill, or cause greater harm.
Paul Anastas, the assistant administrator for the Office of Research and Development at the U.S. Environmental Protection Agency, said that while the effects of such a large quantity of dispersants are unknown, tests so far have not found dispersants near coasts or wetlands. Laboratory tests conducted by the EPA comparing the chemicals to oil alone and to mixtures of oil and dispersants also show that they are not more toxic.
"When you look at all of the tools to combat this tragedy ... dispersants have shown to be one important tool in that toolbox," Anastas told lawmakers.
The chemical Corexit 9500 was on a federal list of preapproved dispersants, but in May the EPA directed BP to use less of the toxic chemical because its long-term effects were unknown.
While Corexit was used in previous oil spills, BP for the first time applied the chemical beneath the water surface, where the oil was coming out of the well. Typically, dispersant is applied to oil pooled on the surface. And never before had such a large amount of the dispersant been used.