Despite common parlance, oil is not a singular substance but rather a toxic stew of many different hydrocarbons that comes out of the ground mixed with natural gas. The oil that spewed from BP's Macondo well in the Gulf of Mexico last year was no different—and now a precise measurement of its chemical composition has been published July 18 in Proceedings of the National Academy of Sciences.

On June 21, 2010, two months and a day into the spill, researchers manipulated the robotic sub known as Millennium 42 to collect a sample directly atop the blown-out well—the only such sample gathered for scientific purposes. Keeping it and another sample taken from nearby tightly sealed and at the same pressure as on the seafloor, the scientists brought them back for precise analysis at the lab at the Woods Hole Oceanographic Institution (WHOI). Using chromatography, the scientists determined that at least 1,600 cubic feet of natural gas accompanied each barrel of oil that escaped into the water—a mix broadly similar to those previously recovered from efforts by the Q4000 drilling vessel and the Discoverer Enterprise flaring ship.

That means, in addition to the 4.1 million barrels of oil that spewed into the Gulf of Mexico, roughly 6.6 billion cubic feet of natural gas escaped. More than 80 percent of it was methane that dispersed in plumes at a depth of roughly 1,100 meters, and that therefore did not escape to the surface and into the atmosphere. All told, 1.7 trillion grams of hydrocarbons spilled, according to this analysis, which also included more than 200 water samples collected at varying depths during a research cruise on the R/V Endeavor. And neither of the wellhead samples contained briny waters from beneath the ocean floor, as sometimes accompanies flowing petroleum, which suggests only hydrocarbons flowed from the well. The mix confirms that what flowed from the Macondo well was so-called light, sweet crude oil: high in fuel content and low in sulfur, which means it literally tastes sweet.

The spilled hydrocarbons ranged from alkanes to xylenes. In fact, benzene, toluene, ethylbenze and the various xylenes were the most abundant larger hydrocarbons, at concentrations of 78 micrograms per liter, which formed from that deepwater plume at roughly 1,100 meters. All are toxic. "We don't know with certainty the adverse effects it might cause on marine life," said WHOI marine chemist Christopher Reddy, who lead the research, though his fellow scientists note that toxic levels begin at 5 milligrams per liter, roughly 100 times higher than the sampled levels.

The findings also suggest that microbes took roughly a month to gobble up the edible range of hydrocarbons in the 35-kilometer long plume, starting with the easier-to-digest natural gas and alkane portions and eating at a rate of 2 micrograms per liter per day. That is far slower than some previous estimates and suggests that marine life encountering the plume might have experienced longer-term exposure.

In addition, the researchers found some 410,00 barrels worth of so-called "polar hydrocarbons," or those that have already mixed with oxygen, nitrogen or sulfur to form slightly different compounds. These polar hydrocarbons are "not typically analyzed in field samples" and "are resistant to evaporation, biodegradation" and breaking down in sunlight, the researchers write. And that means these hydrocarbons could linger, overlooked, in the seas, swamps and sands of the Gulf long after the rest of the oil is gone.

Image: Oceaneering / NSF