It's the first Earth Day since the BP oil spill. People the world over are still angry, Gulf Coast fishing has not quite recovered, and yet BP might actually have banner profits this year. Of course, as any Earth Day activist would tell you, the Gulf Coast will be coping with the BP oil spill for many Earth Days to come. But how long will those effects last?

The closest precedent is the Exxon Valdez spill off the coast of Alaska in 1989. Though the BP oil spill spewed over 200 million gallons of oil, far larger than the 11-32 million gallons of oil from the Exxon Valdez, the latter was still the biggest spill on record until last year. (Ironically, the Valdez spill occurred on March 23rd, just about one month before Earth Day, and many credited the spill with strong attendance at that year's Earth Day festivities.) In a 2008 study in Environmental Science and Technology, researchers from Bowdoin College, University of Louisville, and environmental consulting firms took hundreds of sediment samples from many of the sites most heavily oiled in 1989. All the samples were analyzed for how much oil had survived after nearly two decades in the water.

Fortunately, the study found that "over 90% of the samples from all sites contained light or no [subsurface oil] at all."  Most of the oil appears to have weathered--i.e. disappeared via evaporation,  biodegradation, or some other natural process--especially in higher elevations along so-called intertidal zones.  The researchers conclude that “most of the Exxon Valdez oil in the Prince William Sound has been eliminated due to natural weathering."   Some isolated oil residues remain sequestered beneath large boulders and rocks, but those patches are widely separated from areas frequented by wildlife.  So, take comfort Gulf Coasters—20 years after a major disaster a fraction the size of the BP spill, the water's nearly fine!