15 August 2010

Issue: The use of dispersants in the Deepwater Horizon oil spill.

Writing, as we have been, about science and the environment, we knew we'd be doing some posting about the deepwater-horizon oil spill in the Gulf of Mexico. We didn't want to try tackling the whole damn thing, so I'm grabbing a smaller part of the problem about which you might not have had so much information thrown at you. (For an overall summary of the spill and clean-up efforts, go here)

How dispersants work. (NYtimes)
When the spill began, focus immediately went to stopping the gushing oil. When we realized that the spill was not going to stop quickly, focus shifted towards the clean-up. With this came the usage of dispersants. 1.8 million gallons of the stuff, which is more than has ever been used in one location. The primary dispersants used were "Corexit" brand dispersants, though the exact reasons why this brand was used isn't really known. BP claims that these dispersants were the only one available in the quantities needed, but there is much deliberation about that, and the company's connection to Nalco, the producer of Corexit.

Dispersants work on oil exactly the same way in which detergent cuts grease off of dirty dishes, with the use of surfactants. These are molecules with two ends, one is a water-seeking end that finds and attaches to water molecules and the other is an oil-seeking end that grabs onto an oil molecule. This negates the water-repelling properties inherent in oil, resulting in small droplets of oil mixing into the water column, rather than a large sheet of oil spread out on top of the water.

Un-dispersed oil can be collected with booms. (AMSA)
Dispersants are used in a spill situation for a few reasons. The primary one is to increase the surface area available for naturally-occurring bacteria to begin breaking it down, as natural oil is biodegradable. They also keep oil from washing up on shore where it coats plant material and soil. With the delicate wetlands being abundant at the Louisiana coastline, this is the most convincing reason for the high usage of dispersants. Some say that it is likely that BP used the dispersants' properties in order to hide the magnitude of the spill, as less oil is visible, as well as lowering clean-up costs, since dispersed oil is near impossible to clean up with human technology. However, there are some real benefits that come with the use of dispersants, so rushing to judge BP's motivation is a poor way of addressing the situation. But, as with any complex situation such as this, for every benefit, there is also a negative, and all must be considered.

There is, of course, the natural toxicity of dispersants. There are many claims stating that dispersants are too toxic to use in water so close to humans and that the combination of oil and dispersants are much more toxic than either alone, but convincing research is pretty thin. After the use of dispersants in the Gulf, the EPA began doing several tests to discover the true toxicity of these dispersants. Their research can only be described as conflicting. One test done on quite a few different dispersants found Corexit brand dispersants to be more toxic and less effective than other brands, but a more recent test states that Corexit dispersants were no more toxic than other brands of dispersant (to two species of marine life). Overall, it's not really disputed that dispersants are toxic in high concentrations, they are made of hazardous chemicals after all, it's just not really known how toxic they are to sea life and to humans in the long-term. The biggest controversy is over the massive use of chemicals with effects that were and are still largely unknown.

Toxicity aside, the choice of using or not using dispersants comes down to which part of the ecosystem is to be put at risk. If the dispersants aren't used, the oil primarily stays on the surface of the water. This makes it easier to clean using boats and booms, but increases the exposure to shore-lines and coastal wetlands, and could be a huge problem when hurricane season hits. However, when dispersants are used, the oil is spread throughout the water column, allowing it to coat delicate corral systems, plant matter suspended and attached to the bottom, and affect fish and other marine life that would not otherwise encounter the oil. Fish, squid, krill, and other animals swim through the dispersed oil, which coats their gills as they breathe the water, but it does not seem to accumulate in edible portions of the animal. When the Gulf spill occurred, BP chose to protect the coastline. The scientific criticism is that BP did not show evidence proving this was the best course of action for the gulf ecosystem.

The main problem is that there is little evidence for or against dispersants to determine how the situation to should be handled. Dispersants help with some parts of the problem and exacerbate other parts. We can, of course, criticize the decisions the BP made with the use of the dispersants, especially making the decision to use a chemical with largely unknown effects on human health, but it's difficult to say what the best course of action should have been.

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