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Study finds combination of dispersant with Macondo oil increases toxicity up to 52-fold over oil alone

In toxicity tests in the lab, a study by a team from the Georgia Institute of Technology and Universidad Autonoma de Aguascalientes (UAA), Mexico found that mixing the dispersant Corexit 9500A—used in the 2010 Deepwater Horizon disaster—with oil from the Macondo well increased toxicity of the mixture up to 52-fold over the oil alone. (In their testing, the team also found that Corexit 9500A and oil are similar in their toxicity.) The findings are published online by the journal Environmental Pollution and will appear in the February 2013 print edition.

The team used a standardized 24-hour acute toxicity test for the marine environment using the rotifer Brachionus plicatilis to estimate the toxicity of Corexit 9500A, propylene glycol, and Macondo oil.

Using the marine rotifer Brachionus plicatilis acute toxicity tests, we estimated the Ratios of 1:10, 1:50 and 1:130 for Corexit 9500A®:Macondo oil mixture represent: maximum exposure concentrations, recommended ratios for deploying Corexit (1:10–1:50), 1:130 the actual dispersant:oil ratio used in the Deep Water Horizon spill. Corexit 9500A® and oil are similar in their toxicity.

However, when Corexit 9500A® and oil are mixed, toxicity to B. manjavacas increases up to 52-fold. Extrapolating these results to the oil released by the Macondo well, suggests underestimation of increased toxicity from Corexit application. We found small differences in sensitivity among species of the B. plicatilis species complex, likely reflecting phylogenetic similarity. Just 2.6% of the water-accommodated fraction of oil inhibited rotifer cyst hatching by 50%, an ecologically significant result because rotifer cyst in sediments are critical resources for the recolonization of populations each Spring.

—Rico-Martínez et al.

Use of the Corexit dispersant was pre-authorized by the US Environmental Protection Agency (EPA) for cleaning up the crude flowing from the Macondo well during the disaster. EPA requires toxicology tests and reports for all dispersants that are approved on the National Contingency Plan (NCP) Product Schedule, the authorized list of dispersants. All determinations regarding the specific application or use of a dispersant are made by the Federal On-Scene Coordinator in charge of the response to a given event. According to the EPA, “Dispersants are generally less toxic than oil.”

Rotifers have long been used by ecotoxicologists to assess toxicity in marine waters because of their fast response time, ease of use in tests and sensitivity to toxicants. Inhibition of rotifer egg hatching from the sediments as noted above is important because these eggs hatch into rotifers each spring, reproduce in the water column, and provide food for baby fish, shrimp and crabs in estuaries.

Dispersants are pre-approved to help clean up oil spills and are widely used during disasters. But we have a poor understanding of their toxicity. Our study indicates the increase in toxicity may have been greatly underestimated following the Macondo well explosion.

—Roberto-Rico Martinez, study leader

Martinez performed the research while he was a Fulbright Fellow at Georgia Tech in the lab of School of Biology Professor Terry Snell. The team hopes that the study will encourage more scientists to investigate how oil and dispersants impact marine food webs and lead to improved management of future oil spills.

What remains to be determined is whether the benefits of dispersing the oil by using Corexit are outweighed by the substantial increase in toxicity of the mixture. Perhaps we should allow the oil to naturally disperse. It might take longer, but it would have less toxic impact on marine ecosystems.

—Terry Snell


  • Roberto Rico-Martínez, Terry W. Snell, Tonya L. Shearer (2013) Synergistic toxicity of Macondo crude oil and dispersant Corexit 9500A® to the Brachionus plicatilis species complex (Rotifera), Environmental Pollution, Volume 173 Pages 5-10 doi: 10.1016/j.envpol.2012.09.024



Not very good news for marine life? Wasn't that known before?


This is probably not unexpected.

They say; "What remains to be determined is whether the benefits of dispersing ... are outweighed by the substantial increase in toxicity of the mixture."

I assume the dispersal is very important vs. letting the oil remain concentrated.

Rapid "billion -to-one" dispersal into some fraction of the 643 quadrillion (6.43*10^17) gallons of water in the GOM may overshadow the toxicity.

This should be pre-established by the EPA as the article implies was done before Macondo.


Ya the goal is to make the oil soo dillute it cant kill anything anymore as rapidly as possible. It doesnt matter thats its 52x as bad per unit as long as you only get 1/1000th as much in you... and that is easy to get in the ocean rather quick.. heck likely even 100k dilluation is easy with dispersants.


The other side of that is that the dispersants allow decomposing bacteria to attack and consume the oil more quickly.  If the toxic oil is mixed and consumed by the time the rotifers are ready to hatch, there may be less of a problem than if the oil remains as a coating.


Has anybody tested whether or not this toxic mess is bioaccumulative like DDT or heavy metals?


No, the dispersants are essentially detergents.  They biodegrade like the oil does.

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