January 27, 2011

Fate Of Dispersants In Deepwater Horizon Oil Spill

A new study released Wednesday finds that dispersants placed deep in the Gulf of Mexico in the aftermath of the BP oil spill seemed to keep some oil from contaminating the water's surface.  However, the chemicals in the dispersant lingered underwater, adding to growing concerns about the long-term consequences for the region.

The study, which appears in the journal Environmental Science & Technology, was the first peer-reviewed research published on the fate of oil dispersants added to underwater ocean environments. 

According to the report, key chemical components of the 770,000 gallons of oil dispersants pumped to the damaged well head a mile below the Gulf surface did mix with the spewing oil and gas, and remained deep in the ocean for two months or longer without degrading.  

However, it was not possible to determine if the first deep ocean use of oil dispersants worked as intended in breaking up and dissipating the oil.  The scientists also noted ongoing concern about the environmental fate of the 1.4 million gallons of dispersant applied to the ocean's surface.

Previous studies have shown that dispersants added to surface oil spills prevent them from coating and harming sensitive coastal environments.  However, there have been no large-scale applications of dispersants in deep water prior to the BP spill, so no data exists on the environmental fate of dispersants in deep water.

The scientists collected and analyzed seawater samples from the Gulf for the presence of a key dispersant ingredient known as DOSS, or dioctyl sodium sulfosuccinate, during the active oil flow and again after the flow had stopped.

They discovered that the DOSS became concentrated in the deepwater plumes of suspended oil and gas at depths of up to three-quarters of a mile, and did not mix with the surface applications of dispersant.

They also detected the dispersant ingredient nearly 200 miles away from the well two months after deepwater dispersant applications had ceased, indicating the dispersant was not rapidly biodegraded.

Although the study alone is insufficient to determine whether the dispersant was effective in dispersing the oil coming out of the wellhead, the scientists said the persistence of the dispersant over such long distances and time periods justifies further examination of the effects of chemical dispersant and oil mixture exposure.

Meanwhile, members of the Senate plan to introduce new legislation in the coming weeks designed to avert future spills, the AFP news agency reported on Wednesday, citing a key lawmaker.

A comparable bill that would have revised offshore drilling rules had previously been considered, but never went before the Senate for a full vote.

"We must ensure that we have systems in place in our government and in the industry so that this cannot happen again," the AFP quoted Senator Jeff Bingaman, chairman of Senate Committee on Energy and Natural Resources, as saying.

"Beyond that, we should lead the world in development of these systems and technology and not settle for standards that are less rigorous than those of other nations."

The presidential commission created to investigate the BP spill called for updating industry practices and creating a safety watchdog group to avoid future spills.

"This is a complex and challenging matter. This committee unanimously reported legislation in the 111th Congress (last session) that would take many of the necessary steps," Bingaman added.

"Since then, the Department of the Interior has taken a number of important actions to address these issues. Nevertheless, I continue to believe that legislative change is necessary to fully ensure safe operations going forward, and intend to introduce legislation again in this Congress."

The explosion in April on the Deepwater Horizon rig off the coast of Louisiana killed 11 workers, and spewed 4.9 million barrels of toxic crude into the Gulf of Mexico before the Macondo well head was capped three months later.

The region is still feeling the impact from the disaster, with authorities having closed down large areas of water to commercial fishing and shrimping as oil washes up on beaches, tainting fragile marshlands.  The region's tourism industry has also been hard hit.


Image 2: A fresh oil slick from the Deepwater Horizon spill, during June 2010.  Note that one drop of detergent was added to the oil slick, forming the cleared circle. (Photo by David L. Valentine, University of California Santa Barbara)

Image 3: When oil and gas mixtures are ejected from a deep wellhead, liquid oil droplets of many different sizes form and rise toward the ocean surface. Because the smaller droplets become as dense as the surrounding water deep below the surface--in this case at about 1,100 meters"”they are swept away laterally by prevailing ocean currents (left panel). When a dispersant is added at the depth of the wellhead, a component called a surfactant breaks up the oil into small droplets (middle panel). If the dispersant works perfectly, virtually all the liquid oil is in these "neutrally buoyant" droplets and is carried away before ever reaching the surface and the droplets become small enough to be consumed, or "biodegraded," by bacteria. In the Deepwater Horizon spill (right panel), scientists found evidence that the dispersant mixed with the small droplets in the deep-water hydrocarbon plume but also discovered the oil/dispersant mix had not yet biodegraded several months after the spill. The study could not distinguish between oil droplets coated with surfactant (which would suggest the dispersant worked as planned) and surfactant floating freely on its own (suggesting the substance did not attach to the oil, as intended). (Illustration by Jack Cook, Woods Hole Oceanographic Institution)


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