Look at a Magnum Photos gallery on the BP oil spill.
The Deepwater Horizon oil spill in the Gulf of Mexico was, by far, the biggest environmental story of 2010. We're turning our minds and diaries toward 2011, but nature doesn't abide by the Gregorian calendar. What's going on with the 207 million gallons (PDF) of oil? Will the Gulf ever be oil-free again?
No, but the vast majority the oil from the Deepwater Horizon spill is already gone. Trying to remove all oil from the Gulf would be a Sisyphean task. Every year, oil tankers, drilling platforms, and boats spill more than 310,000 gallons of oil into the Gulf. But even if we halted human activity in the Gulf, natural seeps would still send 42 million gallons of oil coursing from the sea floor into Gulf waters each year.
These natural oil seeps actually prevented the BP spill from becoming an even worse disaster. The Gulf has more natural seeps than any other body of water in or around North America. Because of this constant supply of hydrocarbons, there is always a healthy population of bacteria floating around the Gulf looking for more food. When BP's Macondo well began gushing, the steady drip of nutrients turned into a feeding frenzy. (The lack of natural seeps, and oil-eating bacteria, is one of the main reasons Prince William Sound has been slow to bounce back from the Exxon Valdez disaster.) Bacteria are far more reproductively agile (so to speak) than mammals, which take years to adjust their population size to changes in food supply. Within 24 hours of the spill, the number of oil-eating bacteria around the wellhead had grown tenfold.
Under the right conditions, bacteria can degrade spilled oil remarkably quickly. Fortunately, the Gulf conditions are quite friendly to oil-gobbling bacteria, and the overwhelming majority of the Macondo oil was recaptured, burned off, evaporated, flushed out by ocean currents, or eaten by bacteria by the end of July, just two weeks after BP managed to seal off the well. Because the bacteria and cleanup crews worked so quickly and the spill is now so diluted, it's impossible to say exactly what percentage of the original oil is still around, but it's a small fraction.
When will every drop of Macondo oil be gone? It's hard to say. A gallon of oil contains a wide variety of compounds. Even hydrocarbons, the basic building blocks of oil, come in thousands of different varieties, because the two elements can combine in lots of different ways. Different types of bacteria feed on different kinds of hydrocarbons, and some do their work faster than others. In addition, a single bacterial cell doesn't take in a drop of hydrocarbon, fully digest it, and excrete carbon dioxide and water. It's more of a team effort, with several intermediate compounds being passed between hungry microbes.
Like nuclear physicists, bioremediation experts instead talk about half-life—the amount of time it takes for the bacteria to break down half of the oil. If you can keep cutting the amounts in half, you'll never get to zero, but you'll quickly get to inconsequential concentrations. Those last few molecules of hydrocarbons—the chemical compounds in oil that cause most of its health risks—aren't particularly important when they're diluted in 643 quadrillion gallons of sea water.
Researchers have used a bunch of fascinating strategies to measure biodegradation rates, both in the open water and in the laboratory. For example, they have set so-called bug traps—saturated beads of oil that attract bacteria—in the Gulf. They watch which bacteria make their way to the oil, and how long it takes them to break it down. By most estimations, the right colony of bacteria can break down oil so quickly that within six days the hydrocarbons are undetectable using the best available instruments.
Between July and August, researchers at Lawrence Berkeley National Laboratory took 170 samples of Gulf water, starting at the wellhead and following the oil's likely path for several hundred kilometers. They couldn't find any Macondo oil, which means the concentration was less than two parts per billion. (These researchers and others have developed techniques to "fingerprint" oil, so we can tell fairly accurately whether the oil we find in the Gulf came from the from Macondo well or elsewhere.) Another team of researchers from the Woods Hole Oceanographic Institution reported that underwater plumes of oil—that is, oil that hasn't reached the Gulf's surface—may be breaking down at a slower rate. But these researchers haven't found many of these plumes during the last six months, suggesting that they probably aren't having a significant ecological impact at this point.
It's too soon to declare a happy, green ending to this story. We don't really know how the chemicals that bacteria release after eating oil will affect human and animal health. Some of them are known to mimic testosterone and estrogen. It's possible, although somewhat unlikely, that these compounds hamper endocrine functions at exceedingly low concentrations.
Another concern is oil that's been trapped in sediment. Bacteria in open water work much more efficiently than their mud-based cousins, because the latter have less access to oxygen and nutrients. That's one of the many reasons the spill response was so focused on preventing the muck from making landfall. The bacteria in marshlands and other coastal areas are still breaking down Macondo oil. Recovery workers are experimenting with techniques to help them along, like using aeration tubes to pump in oxygen and nutrients.