In 2010, the Deepwater Horizon drilling rig spilled up to 4.6 million barrels of fossil petroleum into the Gulf of Mexico. But after all the clean up attempts, various government and BP crews still haven’t located all 200 million gallons. Now, some of that “missing” oil has finally turned up—or down, rather, buried in the sediment on the Gulf floor. The oil caused particles in the open water to clump together and sink.
To find out where these might have settled, Jeffrey Chanton of Florida State University and colleagues used radioactive isotope carbon-14 as a tracer. Since oil doesn’t have carbon-14, any sediment containing oil would be conspicuous. They took 62 sediment cores from an area encompassing 24,000 square kilometers (9,266 square miles) around the spill site, Live Science reports, avoiding areas with natural oil seeps. Then, using geographic information system (GIS) mapping, the team was able to chart the distribution of the oiled sediment.
According to their calculations, some 6 million to 10 million gallons are buried about a hundred kilometers southeast of the Mississippi Delta. This likely represents between 3 and 4.9 percent of the petrocarbon that was released (though some estimates put it at 9.1 percent). "This is the first time we've ever really demonstrated that this is happening,” Chanton tells the Tallahassee Democrat. “There was anecdotal information this was happening. This really quantifies it."
Sinking to the bottom of the ocean might have been good in the short-term, since it clears the water. But in the long run, he explains, it’s a problem. Because there’s less oxygen on the ocean floor compared to the water column, the oiled particles are more likely to become hypoxic—or oxygen deficient. That makes it harder for microbes to help decompose the oil and it’ll linger longer. “This is going to affect the Gulf for years to come,” Chanton says in a news release. “Fish will likely ingest contaminants because worms ingest the sediment, and fish eat the worms. It’s a conduit for contamination into the food web.”
The findings were published in the American Chemical Society’s Environmental Science & Technology last month.
Last year, Chanton’s team showed that methane-derived carbon from the spill has already entered the food web. Methane-eating bacteria were very efficient in converting the natural gas into biomass, and their populations bloomed when the spill occurred.