Scientists funded by the National Science Foundation (NSF) and affiliated with the Woods Hole Oceanographic Institution (WHOI) have detected a plume of hydrocarbons at least 22 miles long and more than 3,000 feet below the surface of the Gulf of Mexico, a residue of the BP Deepwater Horizon oil spill.

The 1.2-mile-wide, 650-foot-high plume of trapped hydrocarbons was detected during a 10-day subsurface sampling effort from June 19-28, 2010 near the wellhead. The results provide a snapshot of where the oil has gone as surface slicks shrink and disappear.

The study — which was enabled by three rapid response grants from NSF”s chemical oceanography program, with additional funding from the U.S. Coast Guard and NOAA through the Natural Resource Damage Assessment Program — confirms once again that a continuous plume was found “at petroleum hydrocarbon levels that are noteworthy and detectable,” according to researchers working on the study.

The researchers measured petroleum hydrocarbons in the plume and, using them as an investigative tool, determined that the source of the plume could not have been natural oil seeps but had to have come from the Deepwater Horizon blowout at the Macondo well.

They reported that deep-sea microbes were degrading the plume relatively slowly, and that it was possible that the plume had and could persist for some time if the rate of microbial degradation or the dilution of the plume does not accelerate.

NSF has so far issued a total of 90 RAPID grant awards to investigators; the grants to date are worth $10.2 million for study of the spill. NSF has invested an additional $3 million in ship-related operating costs.

The research team working on this study based its findings on some 57,000 discrete chemical analyses measured in real time during a June 19-28, 2010, scientific cruise aboard the R/V Endeavor, which is owned by NSF and operated by the University of Rhode Island.

The scientists accomplished the feat using two advanced technologies: the autonomous underwater vehicle (AUV) Sentry and a type of underwater mass spectrometer known as TETHYS (Tethered Yearlong Spectrometer).
The researchers began tracking the plume about three miles from the well head and out to about 22 miles (35 kilometers) — until the approach of Hurricane Alex forced them away from the study area.

Whether the plume”s existence poses a significant threat to the Gulf is not yet clear, the researchers say.

The mass spectrometer used in this study was developed by researchers working in close industrial partnership with Monitor Instruments Co. in Cheswick, Pa., through a grant from the National Ocean Partnership Program.

The TETHYS — which is small enough to fit within a shoebox — is capable of identifying minute quantities of petroleum and other chemical compounds in seawater instantly.

Sentry, funded by NSF and developed and operated by WHOI, is capable of exploring the ocean down to 14,764 feet (4,500 meters) depth. Equipped with its advanced analytical systems, it was able to criss-cross plume boundaries continuously 19 times to help determine the trapped plume”s size, shape, and composition.

This knowledge of the plume structure guided the team in collecting physical samples for further laboratory analyses using a traditional oceanographic tool, a cable-lowered water sampling system that measures conductivity, temperature, and depth (CTD). This CTD, however, was instrumented with a TETHYS. In each case, the mass spectrometer was used to positively identify areas containing petroleum hydrocarbons.

In previous research, investigators relied mostly on a conventional technique called vertical profiling. Researchers working on this study scanned large areas horizontally, which enabled them to target the vertical profiles more effectively. The methods provided much better information about the size and shape of the plume.

The researchers detected a class of petroleum hydrocarbons at concentrations of more than 50 micrograms per liter. The water samples collected at these depths had no odor of oil and were clear. But researchers say this is not to say the plume isn”t harmful to the environment.

Contrary to previous predictions by other scientists, researchers found no “dead zones,” regions of significant oxygen depletion within the plume where almost no fish or other marine animals could survive. They attributed the discrepancy to a potential problem with more modern measuring devices that can give artificially low oxygen readings when coated by oil.

The team on Endeavor used an established chemical test developed in the 1880s to check the concentration of dissolved oxygen in water samples, called a Winkler titration. Of the dozens of samples analyzed for oxygen only a few from the plume layer were below expected levels, and even these samples were only slightly depleted.

Gas chromatographic analyses of plume samples confirm the existence of benzene, toluene, ethybenzene, and total xylenes, together called BTEX, at concentrations in excess of 50 micrograms per liter. While the plume is not pure oil, researchers say it does contain oil compounds.