Toxic Dispersant-Oil Mix

The Unified Area Command used nearly 2 million gallons of Corexit 9500 dispersant on the BP oil spew. Over 1 million gallons were applied on the surface and nearly 800,000 gallons at the well leak. In June the EPA published studies testing dispersants or oil, not their combination. The EPA released updated findings showing the impact of a dispersant-oil mix. The study ratcheted up concentrations until it reached the LC 50, defined as:

The LC50 is defined as the concentration of a substance causing mortality in 50% of test organisms for a specified time interval, in this case, 48-hours for the mysid (shrimp) test and 96-hours for the silverside (fish) test.

Various officials and scientists testified before Congress on EPA's findings. The acute toxicity of dispersant-oil mix on juvenile Gulf shrimp and fish was almost a footnote in today's Congressional testimony.

The EPA's press release stated:

The results also indicate that dispersant-oil mixtures are generally no more toxic to the aquatic test species than oil alone.

Their report said:

Overall, the dispersants/LSC (oil) mixtures were classified as being highly toxic to moderately toxic depending on the test species and dispersant.

Moderate to high toxicity are defined as deadly from 0.1 to 10 parts per million (ppm). Data on Corexit 9500 shows half the shrimp killed within 48 hours at concentrations of 5.4 ppm, while 50% of the fish died within 96 hours at 7.6 ppm.

Both kill, the question is where. Dispersants move the toxic mixture into the water column, which was already known. What we don't know is the amount of devastation.

EPA did the "oil only" test on the silverside (fish), not on the mysid (shrimp), which calls into question their general conclusion or comparable toxicity. The EPA's short time frame for exposure and limited scope (species) in their studies adds more concerns. The public deserves answers from quality studies. It's not clear that's happening.