Richard Denison, Ph.D., is a Senior Scientist.
I had reported in an earlier post – based on data provided to EPA by Nalco, the maker of Corexit® 9500, the main dispersant being used in the Gulf – that the dispersant appeared to increase the acute aquatic toxicity of oil.
At a press conference today, EPA released data from the second round of its own testing on Corexit 9500 (and seven other dispersants), and concluded that the acute toxicity of the dispersant-oil mixture is about the same as the oil by itself.
What explains the discrepancy? To put it most simply: It's not that the dispersant-oil mixture was less toxic in the EPA tests, it's that the oil EPA used – which is the actual oil that has been leaking into the Gulf – was more toxic than the fuel oil Nalco had used. Here's a cartoon illustrating what I'm saying (the arrow shows the biggest change):
So the good news is that the dispersant doesn't appear to be increasing the acute aquatic toxicity of the oil released into the Gulf. The bad news is that the oil is pretty toxic, and the dispersant certainly doesn't help directly with that. And of course, the bigger questions about longer-term effects of dispersants and dispersed oil are not addressed by the new data.
Here's more detail on the new data EPA released today and how they compare to the earlier data submitted by Nalco.
The biggest reason for the difference in the two sets of tests is that Louisiana sweet crude oil is quite a bit more acutely toxic by itself than is the No. 2 fuel oil used in the standardized test Nalco used: 4- to 6-fold more toxic.
The toxicity of the dispersant-oil mixture, in contrast, was much more similar in the two sets of tests, differing by only a factor of 2 to 3. (EPA regards values differing by a factor of 3 or less not to be significantly different.)
The first tests conducted by Nalco using No. 2 fuel oil found that the dispersant-oil mixture was 4- to 5-fold more toxic than the oil by itself.
The new tests conducted by EPA using Louisiana sweet crude oil (obviously a more relevant oil to use to mimic the conditions of the BP oil disaster) found that the dispersant-oil mixture was only 2- to 3-fold less toxic than the oil by itself. (Again, EPA regards values differing by a factor of 3 or less not to be significantly different.)
|LC50* values in parts per million (ppm)|
|Menidia (inland silverside fish)|
|EPA test of Corexit® EC9500A using Louisiana sweet crude oil (LSC)||Dispersant alone||130||42|
|LSC alone||>2.9||Mixture is <2.6 x less toxic than LSC alone||2.7||Mixture is 2.6 x less toxic than LSC alone|
|Dispersant : LSC (1 : 10)||7.6||5.4|
|Nalco test of Corexit® EC9500A using No. 2 fuel oil (per NCP Product Schedule protocol)||Dispersant alone||25.2||14.6|
|No. 2 fuel oil alone||10.7||Mixture is 4.1 x more toxic than No. 2 alone||16.1||Mixture is 4.7 x more toxic than No. 2 alone|
|Dispersant : oil (1 : 10)||2.6||3.4|
|Ratio of EPA to Nalco data||Dispersant alone||5.2 x lower toxicity||2.9 x lower toxicity|
|Oilalone||<3.7 x higher toxicity||6.0 x higher toxicity|
|Dispersant : oil (1:10)||2.9 x lower toxicity||1.6 x lower toxicity|
|*LC50 = concentration at which 50% of the test organisms are killed within the duration of the test. NOTE: higher value = less toxic; values differing by <3-fold are not considered significantly different|