The puzzle over what happened to the oil and gas released during the Deepwater Horizon oil spill in the Gulf of Mexico last year has been partially solved.
Oil is composed of many thousands of different chemicals but the plume that stretched through the Gulf contained relatively few. Now chemists have worked out what happened to the rest.
Christopher Reddy, an environmental chemist at the Woods Hole Oceanographic Institution in Massachusetts, and colleagues, used a remotely operated submarine to collect samples directly from the leaking well in June 2010 and compared these with samples taken from elsewhere in the oil plume.
Reddy likens the oil and gas molecules gushing out of the wellhead to passengers on an elevator. “We wanted to know which compounds got off the elevator instead of going up,” he says.
The team found that water-soluble compounds dissolved in neutrally buoyant seawater about 400 metres above the wellhead. These included benzene, toluene, ethylbenzene and xylene – a toxic suite collectively referred to as BTEX. And in this layer they stayed. By contrast, the compounds that reached the surface were mainly insoluble.
Reddy’s work helps to answer one of the major questions from the oil spill – what happened to all that oil and gas, says David Valentine, a microbial geochemist at the University of California, Santa Barbara.
The results show how deep oil spills differ from surface spills, where many toxic compounds quickly evaporate rather than contaminating the water.
The team’s measurements also show that BTEX concentrations reached up to 78 micrograms per litre. That level is several orders of magnitude higher than known toxicity levels for marine organisms, according to Judith McDowell, a zoologist also at Woods Hole.
Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1101242108