Deep Water Disaster

The Deepwater Horizon oil rig in the Gulf of Mexico was a real record-setter. In September 2009 it drilled the deepest oil well in history, the Tiber field well, On April 22, 2010, while drilling in another location, the Macondo prospect well, the rig exploded and then collapsed, setting another record as the largest offshore oil spill in U.S. history as it relentlessly unleashed millions of barrels of oil and gas into the waters of the Gulf. Deepwater Horizon also has the dubious honor of being the world’s first deep-water gusher; all other oil spills have occurred on or near the sea surface. This brought in its wake a slew of unknowns: What happens to the oil and gas as it blasts upwards through the water from 1500 meters down? Where do these chemicals go, and in what form? What are the implications for marine life in the Gulf of Mexico and even farther afield? EPFL Professor Samuel Arey, a specialist in aquatic chemistry, and colleagues from the Woods Hole Oceanographic Institute were immediately on the front lines, working to answer these questions.

In June 2010, Woods Hole sent a remotely operated vehicle down to the sea floor to collect samples of the oil and gas as they exited the well, and then brought the samples to the surface for analysis, maintained under hydrostatic pressure during transport. They also collected hundreds of water samples between the well and the sea surface. Using water sample analysis and in-situ mass spectrometry, the group studied a giant chemical plume of dissolved hydrocarbons at a depth of about 1,000 m, stretching in a southwesterly direction over tens of kilometers. The deep water plume contained methane, light aromatic hydrocarbons (benzene, toluene, ethylbenzene, and total xylenes – collectively referred to as BTEX), and many other compounds. Based on this intensive sampling campaign, the scientists hoped to better understand the physical, chemical, and biological processes that transform and transport oil and gas in the deep sea.

By comparing the composition of the original oil and gas to that of the deep sea plume, Arey and his colleagues developed and tested a model to explain the fractionation of petroleum components. The results will be published in an article in the Proceedings of the National Academy of Sciences. “Very few studies have quantified chemical dissolution into the water column during an oil spill, and never in the deep sea,” explains Arey. Insoluble liquid oil components formed both buoyant droplets that rose to the surface and heavier blobs that sank to the sea floor. Most of the gas fraction – which represents a significant portion of the carbon release – quickly dissolved as it rose through the water. But many semi-soluble hydrocarbons – including the nasty BTEX hydrocarbons – had an uncertain fate. During conventional oil spills near the sea surface, volatile hydrocarbons compounds quickly evaporate and escape into the atmosphere. But at the Deepwater Horizon site, many of these compounds dissolved instead and were trapped in the deep sea. “Hydrocarbons dissolved at a depth of 1,000 m may become trapped there and never make it to the sea surface,” says Arey. “Most of these compounds will eventually be degraded by organisms. But the ecological consequences will be quite different from conventional oil spills, which always occur at the sea surface.” Arey and his colleagues were able to demonstrate what fraction of these substances dissolved on their way up through the water column.

The fate of dissolved hydrocarbons is critical to the deep water ecosystems of the Gulf. “Before they eventually biodegrade, dissolved hydrocarbons could affect aquatic wildlife over a large area,” explains Arey. “So it’s important to understand how deep sea plumes are formed.” The trends observed in the current study can be used by marine biologists and other specialists to analyze the ecological consequences of the spill.