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Tuesday, December 4, 2012

Bad news for cleaning up oil spills

Just when you thought the 2010 oil spill in the Gulf of Mexico couldn’t have been worse. Roberto Rico-Martínez from Universidad Autónoma de Aguascalientes and Terry Snell and Tonya Shearer from the Georgia Institute of Technology have found that the very oil dispersants used to ameliorate the problem could have made it fifty times worse.

As test subjects, the scientists chose five strains of Brachionus plicatilis. Not only are these rotifers (tiny marine organisms found in zooplankton) a critical part of the marine food chain, but they are also notoriously sensitive to environmental conditions.

The researchers exposed the rotifers to varying amounts of crude oil and/or Corexit 9500A oil dispersant (a product used in the Gulf). The LC50 (lethal concentration 50, or amount of product that will kill 50% of the organisms) for each combination was determined. Depending on the strain of B. plicatilis tested, the LC50 with crude oil alone was between 2 and 20 mg oil/liter water. Oil dispersant alone yielded LC50s ranging from 0.5 to 14 mg/L. When the two components were mixed in ratios of 1:10 or 1:50 dispersant to oil (the recommended dosages), the LC50 decreased to 0.2 to 0.3 mg/L. In other words, the combination of crude oil plus oil dispersant is far more toxic to rotifers than either chemical alone.

There was one glimmer of good news. When the researchers tested a ratio of 1:130 dispersant to oil (the conditions actually used at the Deep Water Horizon disaster), toxicity did not increase compared to using either component alone. This makes sense since the 1:50 ratio was slightly less toxic than the 1:10 ratio. Of course, you’re never going to find a ratio that’s less toxic than the dispersant alone, which is pretty darned lethal all by itself. Thus, the best you’re going to do is to not make things worse. Not exactly a glowing endorsement for a rescue technique.

Unfortunately, I think this means it’s back to the drawing board for cleaning up oil spills.

Rico-Martínez R, Snell TW, & Shearer TL (2012). Synergistic toxicity of Macondo crude oil and dispersant Corexit 9500A(®) to the Brachionus plicatilis species complex (Rotifera). Environmental pollution (Barking, Essex : 1987), 173C, 5-10 PMID: 23195520