Hydrocarbons At Gulf Spill Surface

The giant plume of toxic oil and gas released from the mile-deep Deepwater Horizon oil well in the Gulf of Mexico during last year’s spill poses obvious hazards to marine life under the ocean. What’s not so clear are the hazards faced by cleanup workers at the ocean’s surface.

Several new studies of the air and water near the site of the Deepwater Horizon oil spill conclude that a specific collection of toxic hydrocarbons from the spill remain dissolved in the Gulf of Mexico, suggesting that workers may have escaped exposure.

However, researchers have very different takes on the significance of the data. While some tout the lack of release of these compounds into the atmosphere as good news for cleanup workers’ health, others worry that oil compounds that are less water soluble volatilized and created hazards for workers.

The compounds examined in these studies—benzene, toluene, ethylbenzene, and xylene (BTEX)—are found together in petroleum sources. These chemicals, especially benzene, are known to cause a host of human health problems, including nausea and bone marrow damage. Scientists frequently use BTEX levels as a way to estimate the environmental and health risks posed by oil spills.

But BTEX are not the only hazardous hydrocarbons that need to be considered, says David L. Valentine, microbial geochemistry professor at the University of California, Santa Barbara, and a frequent visitor to the spill site. “There were times after the spill when fumes were really bad,” he tells C&EN. Judging from the smell, he says, “there were very clearly plenty of hydrocarbons in the air—they just weren’t BTEX.”

The recent papers support the consensus that BTEX, at least, remain dissolved in the ocean. A team led by marine geochemist Christopher M. Reddy at the Woods Hole Oceanographic Institution recently tracked BTEX in samples from a 20-mile-long plume of oil and gas that was heading southwest from the Deepwater wellhead (Proc. Nat. Acad. Sci., DOI: 10.1073/pnas.1101242108). Reddy notes that since BTEX are relatively soluble in water, they remained below the surface by dissolving into the water column.

Additionally, San Francisco-based environmental consulting firm ChemRisk looked at BTEX release from a different angle (Environ. Sci. Technol., DOI: 10.1021/es200963x). Spurred by concerns of cleanup workers’ exposure to airborne pollutants from the spill, ChemRisk analyzed BTEX in the local atmosphere and found that BTEX levels were well below safety limits required by the Occupational Safety & Health Administration. They also concluded that the compounds that were detected likely originated predominantly from ship engine exhaust in the area, rather than the oil spill.

Heather J. Avens, ChemRisk health scientist and lead author of the study, tells C&EN that her group attributes their results to a very low percentage of BTEX in the Gulf spill crude oil, the likely dissolution of large fractions of BTEX in the water column prior to the oil surfacing, and the quick dispersion of volatilized BTEX into the air.

Finally, Thomas Ryerson, atmospheric chemist at the National Oceanic & Atmospheric Administration’s Earth System Research Laboratory in Boulder, Colo., and his colleagues also collected atmospheric samples above the spill. They recently reported that BTEX levels were only slightly elevated, supporting the idea that the chemicals dissolved in the ocean (Geophys. Res. Lett., DOI: 10.1029/2011GL046726).

Valentine, Reddy, and others, however, are concerned that these results may engender a false sense of safety for workers. Naphthalene and other large aromatic hydrocarbons in oil have not been well studied above the surface at the spill site. These compounds don’t readily dissolve in water, but they still are volatile and could have posed a risk to workers at the surface.

Reddy says that more extensive research is needed on environmental processes that might drive such compounds into the air.

As of yet there are no published studies focusing on the compounds that didn’t dissolve and did volatilize. These heavier molecules contain an aromatic ring attached to one or more extra carbon-containing groups, with solubility decreasing with increasing number of extra carbons. The long list of compounds includes trimethyl­benzene, ethyl methyl­benzene, propyl­benzene, and naphthalene.

And at this point, calculating the levels of those compounds that workers were exposed to last year would be “extremely difficult,” Valentine notes.

“The usual suspects, BTEX, are incarcerated in the deep ocean,” he says. “And now we need to shift our exposure concerns to other suspects.”