Mapping and Characterizing an Underwater Plume

Deepwater Horizon, 2010
Principle Investigator: Rich Camilli

As the Deepwater Horizon spill progressed, it became increasingly important to know where all the oil and gas was ending up. There was a great deal of speculation and anecdotal evidence of vast underwater “plumes” of oil spreading beneath the surface, but no one had yet documented the phenomenon.

For the past decade, scientists and engineers at WHOI have been using underwater sensors to search for the chemical signature of hydrothermal vents deep on the seafloor. Recently, they had also had success mounting a highly sensitive and extremely compact instrument known as a mass spectrometer to an autonomous underwater vehicle (AUV) to hunt for faint signs hydrocarbons in the water signifying the presence of natural “cold seeps” on the seafloor off the coast of southern California.

In June, with funding from the National Science Foundation’s Rapid Response program, a team took the AUV Sentry fitted with the TETHYS mass spectrometer down to the Gulf. After detailed reconnaissance around the well with traditional oceanographic instruments, the researchers programmed Sentry to look for signs of ten different hydrocarbons, like a bloodhound following its nose in pursuit of a chemical trail.

Sentry found and tracked a continuous plume of hydrocarbons stretching southwest from the well 3,600 feet (1,100 meters) below the surface and at least 22 miles (35 kilometers) long. The scientists were then able to deploy rosette samplers into the plume and bring water up for more detailed analysis. With these, they were able to determine that the plume at the time consisted primarily of BTEX (benzene, toluene, ethylbenzene, and total xylenes) in concentrations far above what could be accounted for by natural seep activity in the Gulf.