Speakeasy Science

Discussing Dispersants

In the past week, a lot of writers – and, yes, that includes me – have focused on the chemical dispersants being sprayed into the Gulf of Mexico to help manage the ever-expanding oil spill from BP’s deepwater drilling rig.

For instance, I recently pointed out while dispersants do help break apart a slick into smaller and more biodegradable fragments, dispersed oil is a great deal more poisonous than crude oil. Further, that post, A Lethal Concentration, generated a great discussion, and one that taught me a lot.

I hadn’t considered that the thriving bacterial colonies of warm water environments like the Gulf may work to our advantage, breaking down oil far more rapidly than, say, in the Exxon Valdez disaster in icy Alaskan waters. And readers actually found better toxicity numbers than I had. Some of these strengthened my conclusion that BP’s chosen dispersant, Corexit, rather alarmingly increases the toxicity of crude oil. For instance, Corexit alone has an LC50 for silver fish of 25.2 parts-per-million. But the EPA’s dispersant data shows that Corexit plus fuel oil has an LC50 of 2.61, almost ten times as toxic.

This is not what you might think of as a happy synergy – I definitely don’t – but there are some trade offs here. Dispersants do help keep some of the oil from oozing its way on shore, where it does horrific damage. And – there’s some somewhat positive research cited in the comments – which reminds us that we don’t really know how rapidly these compounds are diluted in deep waters, how quickly bacteria may chew them up, or what chemical changes may occur as they filter down the water column.

Tomorrow, I want to take up the chemistry of crude oil itself, which is actually poisonous enough without help from dispersants. But today, I just want to say thanks to those who wrote in on the subject. I wish the people in charge knew as much as you do.


  1. #1 mousedude
    May 26, 2010

    I suspect the people in charge (at least at BP) know all this. My suspicion is that the main reason they used these dispersants was to hide the oil from view. Since their public estimate of the spill volume and flow rate (which they continue to officially use despite the fact that it has been thoroughly debunked)is based mostly on surface observations. Anything that will keep the oil out of site below the surface allows them a certain measure of plausible deniability regarding their knowledge of the spills true magnitude. It also makes it much more difficult for anyone else to directly estimate the volume of the spill, And this provides legal cover down the road, if they get fined according to the volume spilled. Their goal is to “muddy the waters” as it were, and create as much ambiguity as possible about the severity of the spill

  2. #2 GoatRider
    May 26, 2010

    That makes sense mousedude, especially when you consider that they’re now spreading the dispersant directly into the plume underwater. That way the oil won’t even reach the surface.

  3. #3 daedalus2u
    May 27, 2010

    If you are going to use dispersant, the earlier you use it the better. Injecting it into the oil plume is the best place to inject it. Because this is a high methane crude oil, that will also have a large (and largely unknown) effect, and that effect will depend a lot on pressure with dispersant working better at higher pressures, but less well at lower temperatures.

    I think there is a big effort on the part of BP to minimize the aesthetic and visual impact of the spill. Unfortunately, the “damage” to the environment isn’t going to be determined by science or a rational analysis of harms to wildlife, fisheries and marshes and costs to mitigate that damage, it is going to be determined by how non-scientists feel about it, by politics. How people feel about it is largely determined by their previous mind set. Rand Paul thinks “accidents happen”, no big deal and others think “off with their heads”.

  4. #4 rszasz
    May 27, 2010

    I know that I saw this in a comment here at scienceblogs but cant find the exact one. http://www.youtube.com/watch?v=i6ZN6r5-1QE skip to 1:50 for the most useful bits Assuming this is correct, and from the small but non-zero amount of experience I have one the water it passes the smell test, I’m sorely tested to launch into an unending stream of profanities while I get out my stomping boots. To use the language of the people in the field, you wondered why the fucking booms weren’t fucking working? Watch the video, it will open your eyes to where the fubar is really happening (hint: its the boom deployment).

  5. #5 Betsy
    May 27, 2010

    First- great science blog!
    Second- I watched the above linked you tube clip on “proper booming”.
    …Just when I thought it couldn’t get more incompetent….

  6. #6 PH Lippel
    May 28, 2010

    As Sylvia Earle pointed out in Congressional testimony last week, the major effect of the dispersants is to affect which species are killed. If the oil is dispersed, less will get to the fish and shrimp, more to the smaller critters further down the food chain which live in the deeper water. It’s an ugly bargain, but I’m not quite as cynical as some of the other commenters who attribute it all to aesthetics for the sake of PR. In trying to do some meaningful disaster managment, it’s not unreasonable to place a higher short-term value on the species which provide jobs to the coastal inhabitants and food to the rest of us. Of course it still makes sense to choose lower toxicity dispersants from among those available.

  7. #7 Deborah Blum
    May 28, 2010

    When I was in grad school, I took a class in natural resources managment and our professor had us read the economist Lester Thurow’s book,The Zero-Sum Society. His argument was that Thurow’s macroeconomic argument – which I’m going to oversimplify as in a world of finite financial resources, for every winner there is a loser – also applied to the decisions we make about environmental resources. I think you make an excellent point here about the trade offs involved. I also suspect that there’s some validity to the cosmetic nature of the decisions. And I too wish that in choosing a dispersant, the decision had been for another on the EPA’s list. A number of those that were less toxic were also far more effective in breaking Louisiana crude. Even in a zero-sum game, it pays to make the smartest possible choice.

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