By David Brown
Washington Post Staff Writer
Wednesday, June 9, 2010; A04
There are many uncertainties about the Deepwater Horizon spill in the Gulf of Mexico. But the ability of scientists to identify oil from that event is not one of them.
Oil "fingerprinting" is sufficiently reliable that researchers should be able to say with confidence -- both now and a year from now -- whether a blob of oil floating in the water or washed up on shore came from the Deepwater Horizon blowout.
"It is our statistical contention that each oil is unique," said Wayne Gronlund, a onetime chemistry professor who runs the Coast Guard Marine Safety Laboratory in New London, Conn., which is analyzing dozens of samples from the gulf, now that the spill is in its eighth week.
"You never want to say something is 100 percent, but we can pretty robustly argue that two oils are from the same source, or not," said Christopher M. Reddy, a marine chemist at the Woods Hole Oceanographic Institution in Massachusetts.
Petroleum consists of carbon atoms strung in chains, branches and rings, with many hydrogen atoms attached. It contains thousands of distinct chemical compounds. They range from simple ones that evaporate easily because they contain only a handful of carbon atoms, to 40-carbon behemoths that aren't broken down by weather, sunlight and microbes and end up as baseball-heavy tar balls.
Chemists can identify both the presence and quantity of hundreds of compounds using procedures called gas chromatography and mass spectroscopy. The ratios of one compound to another (with many compounds compared) is often enough to distinguish one oil sample from another.
If the effects of time and the elements have changed those ratios in a way that makes a water-floating sample very different from a fresh-out-of-the-pipe one, scientists can turn to rare "biomarkers" for help. Those are remnants of compounds that existed in the plants and animals that heat, pressure and time turned into oil. Many have a structure containing four carbon rings and are descended from cholesterol-like substances in cell walls.
"The good thing is that these molecular fossils are pretty tough. They won't be affected by the weathering and the environment," Reddy said.
Oils from different fields -- say, Saudi Arabia and Alaska -- are radically different. But even oil from the same geographic area is distinguishable by source.
After the 1989 Exxon Valdez oil spill in Alaska, scientists detected oil in the sediments of Prince William Sound that were clearly different from what poured out of the grounded tanker. It turned out to be from natural petroleum seeps in the eastern Gulf of Alaska that currents had carried into the sound. In a paper published in 1996, a team of researchers led by Bowdoin College chemist David S. Page reported that "only about 15 percent" of the oil in bottom sediments near oiled shorelines came from the spill.
The Coast Guard lab investigates about 200 spills a year, some involving up to a hundred samples. Its gulf samples go back to four days after the blowout.
An unusual aspect of this spill is that samples on the surface have passed through 5,000 feet of water before they are collected. That upward journey may extract many compounds from the crude (despite the truism that oil and water don't mix). Scientists are interested in comparing samples collected at the pipe mouth, in underwater plumes, and on the surface.
"Hopefully we're going to learn a lot from this spill. This is a grand experiment," Reddy said.