Gulf of Mexico oil spillDeep-water oil spills do most of the damage deep down

Published 19 May 2010

Oil spills like the one in the Gulf do most of their damage in the deep; the oil visible on the surface accounts for only 2 percent of the oil spilling into the Gulf; most of the oil remains submerged in the form of droplets that only slowly make their way to the surface

Surface slicks may account for as little as 2 percent of the oil now spilling into the Gulf of Mexico, according to a study of a controlled deep-water spill conducted in 2000 by the U.S. Minerals Management Service (MMS) and a consortium of oil companies, including BP.

Phil McKenna writes that the study challenges the estimate by federal officials, based on the amount of oil on the sea surface, that around 5,000 barrels (800 cubic meters) of oil per day are pouring into the sea from the site where the BP-operated drilling rig Deepwater Horizon was destroyed by fire last month. It also adds weight to reports of massive underwater oil plumes that government officials are now downplaying.

In June 2000 Project Deep Spill released hydrocarbons into the sea off the coast of Norway at a depth of about 800 meters. The tests included releases of 60 cubic meters of crude oil and 60 cubic meters of diesel fuel over separate 1-hour periods.

Researchers were unable to calculate the amount of crude oil that surfaced because it emulsifies or mixes with water. They did, however, determine that only between 2 and 28 percent of the diesel fuel that was released rose to the surface. The average was 8.7 percent.

Under a controlled, well-monitored experiment, you could not find it all, says environmental engineer Eric Adams of the Massachusetts Institute of Technology. “Now you’ve gone deeper by a factor of 2, with a more violent release; it’s not surprising that you might not see it all.”

The large percentage of diesel fuel that went missing in the 2000 study was put down to evaporation and natural dispersion. In a 2005 review of the experiment, however, Adams suggests that much of the diesel fuel and crude oil remained submerged in the form of droplets that only slowly made their way to the surface.

In the Deepwater Horizon spill, the chances of oil remaining below the surface are even greater, Adams says. If oil mixes with water at depth, the high density of this water can balance out the hydrocarbons’ natural buoyancy. “It can reach a point where the aggregate density of water and oil is neutral to its surrounding environment.”

The result could be large quantities of oil remaining suspended in the water column in droplet form. The recent addition of chemical dispersants injected into the plume at depth is likely to encourage this.

McKenna writes massive subsurface oil plumes have been reported from the Deepwater Horizon spill by the research vessel Pelican, including one up to 90 meters thick that extends for 16 kilometers by 4.8 kilometers. Researchers aboard the Pelican also reported a 30 percent reduction in oxygen levels in waters near some of the plumes.

Note that the National Oceanic and Atmospheric Administration, which funds the Pelican, has called the reports misleading and premature. NOAA’s Charlie Henry says it is “totally untrue” to call the plumes — samples of which reveal them to be transparent — layers of oil. NOAA also says the reduction in oxygen is not marked enough to be of concern.


Carys Mitchelmore of the University of Maryland’s Chesapeake Biological Laboratory says a 30 percent reduction in oxygen would be “highly significant” to some species. Toxicologists, she says, need detailed information on the chemical make-up, concentration, movement and longevity of the suspended hydrocarbons. So far, federal agencies have not been forthcoming, Mitchelmore says. “We don’t even know how much oil is being released.”