Researchers at the University of South Florida say dispersants used in the cleanup may have sent droplets of crude to the ocean floor, where it has turned up at the bottom of an undersea canyon, CNN reported Tuesday.
Plankton and other organisms at the base of the food chain showed a "strong toxic response" to the crude, and the oil could well up onto the continental shelf and resurface later, researchers say.
"The dispersant is moving the oil down out of the surface and into the deeper waters, where it can affect phytoplankton and other marine life," John Paul, a marine microbiologist at USF, said.
Efforts to seal the well permanently are on hold as engineers consider a possible new problem with pressure in the well. Retired Coast Guard Adm. Thad Allen said engineers are concerned about how to manage the risk of pressure detected in the annulus, a ring that surrounds the casing pipe at the center of the well shaft.
Scientists taking pressure measurements to gauge the effects of the mud and cement poured into the well from above during the "static kill" procedure say they believe either some of the cement breached the casing pipe and leaked into the annulus, or cement came up into the annulus from the bottom.
That may have trapped some oil between the cement and the top of the well, inside the annulus.
When it comes to giving a green light to the "bottom kill" of the well through the nearby relief well, Allen said, "nobody wants to make that declaration any more than I do."
The process "will not start until we figure out how to manage the risk of pressure in the annulus," he said.