Sunlight works against oil clean-up efforts

Oil spills, whether minor leaks or major environmental disasters, are bound to happen. Chemical dispersants are one of the tools that can help mitigate the impact of such spills, but they become less effective as oil weathers in the environment. Now, one group reports in Environmental Science and Technology Letters that sunlight has a much greater impact than previously thought on the effectiveness of these dispersants.

According to the U.S. Environmental Protection Agency, 10-25 million gallons of oil spill each year in the U.S. In large marine oil spills, chemical dispersants are often used. Dispersants break up floating oil into small droplets, which can be eaten by microbes or fall onto the soil at the bottom of the body of water, decreasing the risk of oil accumulation on shorelines.

As spilled oil floats on the water's surface, it undergoes weathering processes, such as evaporation and emulsification, in which the oil forms a temporarily stable mixture with water, making chemical dispersants less effective. These are the major processes currently taken into account in field manuals and oil spill response guides for responders.

According to these resources, photochemical oxidation, or chemical changes to the oil in response to sunlight, is a minor factor. However, in recent research, Collin P. Ward and colleagues determined that photochemical oxidation is a dominant weathering process.

Now, these researchers wanted to examine how sunlight impacts the effectiveness of dispersants.

In the laboratory, the researchers analyzed samples of oil from the Macondo well, which was the location of the Deepwater Horizon spill in 2010. They confirmed that simulated sunlight has a larger impact on the oil's properties than evaporation. In addition, dispersant effectiveness or performance decreased four-fold more when oil was exposed to sunlight than it did when the oil underwent evaporation in the absence of sunlight.

To examine effects in the field, the team combined their estimates of how fast sunlight oxidizes oil with speeds of oil floating in the Gulf of Mexico and the locations of 412 aerial applications of dispersants on the Deepwater Horizon spill.

Based on these data, the researchers say that many of the applications targeted oil that had undergone photochemical oxidation that lowered the dispersant effectiveness below 45 percent, which is below the threshold set by EPA for dispersants to be used in a spill.

The team recommends that the effects of sunlight be considered in future field manuals and oil spill response guides for responders.

Research paper