In salt marshes along the Gulf of Mexico, everything's still coming up oil.
Four years after an explosion on a BP-operated Deepwater Horizon drilling rig spewed 210 million gallons of oil into the water, the substance continues to wash ashore in the form of tar balls, or oil-soaked sand patties.
How do we know this oil is from this particular spill? Scientists at the Woods Hole Oceanographic Institution and Bigelow Laboratory for Ocean Sciences in Maine have developed a technique that can detect oil from specific spills years after they happen and reveal how that oil has changed over time in the sea. Every oil reservoir has its own specific amounts of biomarkers, which are molecular fossils that scientists can use for identifying the source of spills, like human fingerprints.
Researchers were able to connect the biomarkers found in oil-filled sand clumps, collected over a 28-month period, that washed up on shore from the Gulf of Mexico near the Deepwater Horizon spill in 2010. Some had degraded since then, which occurs after prolonged exposure to the natural environment.
"These biomarkers are not as resilient as once thought, and they may provide a future window into determining how much, and how quickly, these oil components may linger in the environment when exposed to air, sunlight, and the elements," said Chris Reddy, a scientist at the Woods Hole Oceanographic Institution and a coauthor of the sand-patty study, published in the journal Environmental Science & Technology on Thursday.
This kind of technique gives scientists an idea of what happens to oil once it hits the high seas—and stays there. Much of the oil from the 2010 spill has either degraded, evaporated, or been removed by humans, and when it reaches shore these days, it doesn't come in its usual liquid form. "Although some Macondo [well] material does continue to be identified, the amounts and frequencies are much lower than before, which is why it is important to distinguish it from oily material from other sources, including the many natural seeps in the Gulf," BP spokesman Jason Ryan told National Journal.
However, experts say that at least 60 percent of oil from the spill remains unaccounted for in the ocean.
This story has been updated.