Toxic Tradeoff: An Oil-Dispersant Story

12 Jul

I’ve been interested in the discussions on oil dispersants being used in the BP gulf disaster recently, specifically from the microbial break-down perspective.  Theoretically, dispersing the oil should give it greater surface area, allowing more rapid chemical and biological degradation rates. Speedy biodegradation seems like a good thing.   However, I just discovered an article published this spring in the International Journal of Environmental Research   which gave me pause to think on the whole issue in a way I hadn’t before. 

The article’s author (Dr. A. Otitoloju – a research scientist in the department of Zoology at the University of Lagos, Nigeria ) evaluated crude oil break-down under natural, versus dispersant-controlled settings and the findings seem especially meaningful in light on the current situation in the gulf. First, I should mention that this subject area is much more toxicology than microbiology and therefore my knowledge base is limited.  So it was particularly interesting for me to find out from this article that, “most of the dispersants that have been initially introduced into the market were found to either to be very toxic on their own or enhance the toxicity of the spilled oil on the receiving habitat or exposed organism when deployed to control oil spills.”  Of course this isn’t news to everybody and BP’s use of the toxic COREXIT ® has been the subject of some heated debate already  mostly due to the fact that there are  other dispersants tested by the EPA  found to be less toxic and more effective.

Perhaps it isn’t surprising then that this research article by Otitoloju found increased toxicity of the oil-dispersant mixture to the test organisms, African catfish.  But what is interesting is that over the 28 days of the study, the level of toxicity did not decrease but actually increased over time.  Overall, the dispersant mixture was about twice as deadly to the fish as was the uncontrolled oil, mostly likely due to increased exposure (if the oil is in dense clods or balls, or floating on the surface of the water, the fish can’t get as easily coated with and absorb the oil  as it can when the oil has been chemically dispersed thoughout the water column).

However, it was clear that microbial growth was enhanced (more than 7-fold) and breakdown of the oil was more rapid when the dispersant was used.  So while the dispersant-oil mixture was clearly toxic to the fish (and likely other vertebrates), it enhanced degradation of the oil by microbial populations.  Worth noting, though, are the facts that:

1.) Even without dispersant added, the oil did begin to break down over the course of the 28-day study and gradually decreased in overall toxicity to the fish; and,

2.) Inreased microbial breakdown may be short-lived, according to an article in the ScienceInsider which discusses dispersant interference with natural microbial processes in marine systems.

Now, in thinking about how this pertains to the Gulf of Mexico, where almost 2 million gallons of dispersant have been applied already (and more every day), how will this play out in the long-run?  The dispersant makes the oil less obvious floating on the surface of the water and reduces the appearances of “tar-balls” on beaches, but increases bioavailability and exposure of the wildlife significantly, resulting in higher levels of toxicity, despite [temporarily]  increased microbial degradation rates.

So what exactly is the trade-off? 

Dispersant use equates more dead fish, coral, and so on, in the short-term, but with more rapid disappearance of the oil from the waters and coastline in the long-run (see also the article by M. Torrice cited below).

Degradation rates might be even more critical when we consider hurricane season could bring substantial quantities of oil inland. Maybe a dispersant is a necessary evil.  But in that case it seems only right to use the least toxic, most effective dispersant available, which certainly isn’t COREXIT ®.

(For more toxicity info, see the EPA’s Toxicity Testing of Dispersants for the Gulf, last updated June 30th)

________________________________________________________________________________________________________________________________________________________________________

Otitoloju, A.A. (2010) Evaluation of crude oil degradation under a no-control and dispersant-control settings, based on biological and physical techniques International Journal of Enivronmental Research, 4(2): 353-360.

Torrice, M. (2010) Cleaning up the Gulf oil spill.  Chemical & Engineering News, 88(20): 36-37.

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