Zhanfei Liu from the University of Texas at Austin and his colleagues have been studying how the chemistry of the spill is changing over time. Their findings so far indicate that the warm and well-mixed waters of the Gulf of Mexico have aided the breakdown of the oil, helping to minimize long-term environmental damage.

Just weeks after the disaster, Liu and his co-workers participated in cruises to collect samples of oil from the sea surface at locations 85 and 130 km away from the wellhead. In addition they collected a sample from the salt marshes at Marsh Point, Mississippi, 186 km away, in July 2010. One year after the accident they returned and collected two sediment samples located within 6 km of the wellhead.

Not surprisingly, the researchers found that the majority of low-molecular-weight components of the oil were already lost from samples taken at the sea surface. "We see a high rate of weathering at the sea surface, where there is lots of evaporation, a certain degree of biodegradation or dissolution due to wave action, and warm temperatures," said Liu. By contrast, sedimentary oil samples were much slower to degrade. "In this case we think that biodegradation was slow due to the low temperatures, 4 °C, and limited availability of dissolved oxygen in this deepwater sediment," explained Liu.

The degree of weathering seen in the sea-surface samples reflected one possible movement of the oil spill in the weeks following the disaster – first westward (the sample 130 km from the wellhead), then north-eastwards (the sample 85 km from the wellhead), and finally over to the salt marshes. "The good news is that most of the highly toxic polycyclic aromatic hydrocarbons were gone, probably due to their higher solubility and rates of evaporation and biodegradation," said Liu. However, trace metals such as aluminium, iron, manganese, nickel, copper and lead were found to increase in concentration along the route, perhaps being scavenged from ocean waters. The findings are published in Environmental Research Letters (ERL) as part of the ERL Focus issue on the Deepwater Horizon Oil Spill.

One of the big outstanding questions is whether the two-million gallons of dispersant, applied to the Gulf waters in response to the spill, were justified. Liu and his colleagues have no clear answers as yet, but their results indicate that nature has done a pretty good job of cleaning up the Deepwater spill unaided. "Compared to the Exxon Valdez spill in Alaska in 1989 this is a very different environment with warm temperatures and dynamic waters, where nature can work fast," said Liu.

Liu and the team are now analysing samples collected from subsequent cruises, to assess the rate of weathering over the longer term.