By JOHN CAREY, environmental writer
Where is all the oil? Nearly two weeks after BP finally capped the biggest oil spill in U.S. history, the oil slicks that once spread across thousands of miles of the Gulf of Mexico have largely disappeared. Nor has much oil washed up on the sandy beaches and marshes along the Louisiana coast. And the small cleanup army in the Gulf has only managed to skim up a tiny fraction of the millions of gallons of oil spilled in the 100 days since the Deepwater Horizon rig went up in flames.
So where did the oil go? "Some of the oil evaporates," explains Edward Bouwer, professor of environmental engineering at Johns Hopkins University. That’s especially true for the more toxic components of oil, which tend to be very volatile, he says. Jeffrey W. Short, a scientist with the environmental group Oceana, told the New York Times that as much as 40 percent of the oil might have evaporated when it reached the surface. High winds from two recent storms may have speeded the evaporation process.
[Photos: Latest from the Gulf oil spill]
[Related: 100 days of oil: Gulf life changed for good]
Although there were more than 4,000 boats involved in the skimming operations, those cleanup crews may have only picked up a small percentage of the oil so far. That’s not unusual; in previous oil spills, crews could only scoop up a small amount of oil. "It’s very unusual to get more than 1 or 2 percent," says Cornell University ecologist Richard Howarth, who worked on the Exxon Valdez spill. Skimming operations will continue in the Gulf for several weeks.
Some of the oil has sunk into the sediments on the ocean floor. Researchers say that’s where the spill could do the most damage. But according to a report in Wednesday’s New York Times, "federal scientists [have determined] the oil [is] primarily sitting in the water column and not on the sea floor."
Perhaps the most important cause of the oil’s disappearance, some researchers suspect, is that the oil has been devoured by microbes. The lesson from past spills is that the lion’s share of the cleanup work is done by nature in the form of oil-eating bacteria and fungi. The microbes break down the hydrocarbons in oil to use as fuel to grow and reproduce. A bit of oil in the water is like a feeding frenzy, causing microbial populations to grow exponentially.
Typically, there are enough microbes in the ocean to consume half of any oil spilled in a month or two, says Howarth. Such microbes have been found in every ocean of the world sampled, from the Arctic to Antarctica. But there are reasons to think that the process may occur more quickly in the Gulf than in other oceans.
Microbes grow faster in the warmer water of the Gulf than they do in, say, the cool waters off Alaska, where the Exxon Valdez spill occurred. Moreover, the Gulf is hardly pristine. Even before humans started drilling for oil in the Gulf — and spilling lots of it — oil naturally seeped into the water. As a result, the Gulf evolved a rich collection of petroleum-loving microbes, ready to pounce on any new spill. The microbes are clever and tough, observes Samantha Joye, microbial geochemist at the University of Georgia. Joye has shown that oxygen levels in parts of the Gulf contaminated with oil have dropped. Since microbes need oxygen to eat the petroleum, that’s evidence that the microbes are hard at work.
The controversial dispersant used to break up the oil as it gushed from the deep-sea well may have helped the microbes do their work. Microbes can more easily consume small drops of oil than big ones. And there is evidence the microbes like to munch on the dispersant as well.
It is still far too early to know how much damage the spill has done — and may still be doing — to the environment. Tar balls continue to wash up on beaches. And the risk of a leak remains, until the well is permanently capped sometime in the next few weeks.
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