The catastrophic explosion on April 20 of an offshore oil-exploration platform in the Gulf of Mexico sent oil slicks coasting toward shorelines from Louisiana to Florida. Now research vessels are tracking a more stealthy threat: huge clouds of diffuse, nearly invisible oil droplets hovering deep below the surface.
New data indicate these invisible plumes form shifting strata that fan out in a host of directions from the gushing wellhead, more than 1.5 kilometers below the water’s surface. These clouds could substantially increase estimates of the total amount of oil spilled and could poison deep-dwelling critters that provide the base of the marine food web.
BP North America — the well’s owner — had claimed for weeks that daily spill rates likely peaked at about 5,000 barrels, or 210,000 gallons. But a new oil-collection system that the company set up on June 3 was hauling in just shy of 15,000 barrels of crude per day by June 8.
Federal officials recently charged several independent research teams with figuring out precisely how much crude oil and natural gas spewed directly into the water in the days following the explosion. Preliminary estimates suggest that more than 630,000 barrels of BP oil could already be sloshing around the gulf. That means this spill, dubbed the Deepwater Horizon for the name of the platform that sank, released more than twice the volume of the next biggest spill in U.S. waters, the 1989 grounding of the Exxon Valdez.
Because the oil-collection system now in place over the wellhead can’t remove all of the spewing oil, the well will continue to foul Gulf waters until it’s capped in late summer, said Coast Guard Admiral Thad Allen at a June 7 White House briefing.
The big issue is where all of this oil is ending up, because much has still not floated to the surface. An experiment conducted in 2000 off Norway should offer Gulf analysts good clues, notes spill modeler Eric Adams of the Massachusetts Institute of Technology.
Known as DeepSpill, the Norwegian experiment — sponsored by 23 oil companies and the U.S. Minerals Management Service — released methane or a mix of methane and oil into the Atlantic during four tests, none lasting more than two hours. In tests that let off 60 cubic meters of hydrocarbons, “only between 1 and 17 cubic meters (lower and upper bound estimates)” made it to the surface, according to a 2005 analysis by Adams and Scott Socolofsky, now at Texas A&M University in College Station.
Related experiments by teams at MIT and the University of Hawaii help explain the finding. If oil droplets are very small or if enough cold, dense water is mixed in with them, “you can get a mixture that could be neutrally buoyant,” Adams says, meaning it could hover below the surface. It’s not clear what role oil dispersants may have played in fostering these hovering mixtures.
These data may explain the deep, massive clouds of oil reported on June 8 by two independent research cruises plying the Gulf. Oil concentrations were too small to stain the water and so couldn’t be easily seen, both groups said, but could be detected chemically, acoustically or on filters.
In early June, Samantha Joye of the University of Georgia in Athens and her colleagues found diffuse oil plumes up to roughly 30 kilometers southwest of BP’s leaking wellhead. The team measured clouds roughly 3 to 5 kilometers wide. “The part of the water column most impacted was generally 1,100 to 1,300 meters below the surface,” Joye notes. “So it’s a pretty big patch of water.”
The plumes’ oil and methane concentrations diminished with distance from the accident site. And when BP slapped a hood on top of the well, a plume these researchers were tracking changed trajectory. That’s “pretty convincing evidence that it was linked to the wellhead and not some natural feature, like a [seabed hydrocarbon] seep,” Joye argues.
Deep, diffuse undersea plumes “were totally independent of surface slicks,” she notes. Plume distribution is probably influenced by bottom currents, she says, driven by differences in water density (due to temperature and salinity) and by varying seafloor topography.
As plumes hit these currents, portions “will shear off and whiz around here, there and yonder,” Joye quips, explaining why different research teams may find plumes shooting in different directions at different depths.
Scientists from the University of South Florida in Tampa and the National Oceanic and Atmospheric Administration mapped oil-tainted strata northeast and southeast of the wellhead between May 23 and 26. On June 8, they reported finding oil in several undersea layers more than 70 kilometers from the spill site. Oil concentrations were low, peaking at just 500 parts per billion.
While big fish and porpoises might steer clear of oiled water, small or larval fish probably can’t, points out Wes Tunnell of the Harte Research Institute for Gulf of Mexico Studies at Texas A&M’s campus in Corpus Christi. Plankton, tiny plants and animals that just float with the currents and serve as the base of the marine food web, also can’t evade oil. If oil doesn’t kill them outright, they risk sharing the pollutant with their predators, Tunnell says.
If there is any good news in the Gulf catastrophe, it’s that the environment often encounters oil from spills and natural seeps, Tunnell says. This history means that there are already plenty of oil-degrading bacteria present, which should help the environment heal throughout the next several years.
