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)|
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)|
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.