10 January 2012

Revisiting the Deepwater Horizon Plumes

Maybe the plumes were really clouds. I am talking about the famous plumes from the Deepwater Horizon oil spill, the event that roiled the Gulf Coast and scrambled energy politics in mid-2010. Many readers will remember reports, first carried in this newspaper, that a considerable volume of hydrocarbons released in the spill did not reach the surface of the gulf. Instead they dissolved into deep water, forming what appeared at the time to be enormous plumes of dissolved oil and gas

By JUSTIN GILLIS | The New York Times | January 9, 2012
Oil on the surface of the Gulf of Mexico in June 2010.ReutersOil on the surface of the Gulf of Mexico in June 2010.

That first report met with initial denials from the oil company BP, confusion from the government and a rush by scientists to prove or disprove the plumes’ existence. Their existence was finally confirmed by the National Oceanic and Atmospheric Administration after reports from several scientific teams and a raft of subsequent evidence bolstered the finding.

Still, some things about the plumes have never been particularly clear. For instance, several research groups found evidence of a plume spreading southwest from the Macondo well, where the blowout occurred. But other researchers found plumes drifting northeast at a different point. And chemical findings were equally puzzling: at times, for example, the hydrocarbons near the well seemed fresh, as if they had just come out of the reservoir beneath the sea floor, but at other times they appeared to be far along in decomposing, as though they had been in the water for weeks or even months.

Now an intriguing new paper appears to make sense of all this.
The paper, released on Monday afternoon by the journal Proceedings of the National Academy of Sciences, will come as a relief to many scientists who worked on the spill. It suggests that most of their measurements were valid and consistent with one another, even though that did not seem to be the case at the time. What was almost certainly wrong was the image many of us had in our heads then, of hydrocarbon plumes stretching away from the wellhead like undersea rivers.

The new work was led by David L. Valentine and Igor Mezic of the University of California, Santa Barbara. Dr. Valentine was one of the researchers who worked in the gulf during the spill. Troubled by apparent inconsistencies among the findings of various scientific teams, he enlisted Dr. Mezic, a leading expert in fluid dynamics, who in turn helped bring in a group of Croatian researchers who were prominent in that field.

They built on a computer model used by the Navy to understand ocean currents, incorporating a slew of measurements made during the spill and adding the biology needed to represent the organisms that helped break down Macondo hydrocarbons. In essence, their model reconstructed the movement and breakdown of the undersea hydrocarbons during the blowout and afterward.

A primary finding is that, instead of forming undersea rivers or plumes, the dissolved oil and gas more likely formed big, billowing clouds that drifted around the northern gulf, appearing and reappearing in different places at different times — thus confounding scientific attempts to develop a clear picture. “You could almost think of it as layered clouds, but with more of a sort of swirling motion and back and forth than you would get from a sky cloud,” Dr. Valentine said. His group has produced a video that gives a sense of how the flow worked.

The paper suggests that scientific groups that thought they were tracking a persistent plume southeast of the wellhead were actually seeing recurrent appearances of oil and gas in these drifting clouds. The hydrocarbons in a particular spot were sometimes fresh, but sometimes they were making a second or third appearance at that location and had had weeks to break down — explaining the apparently conflicting chemical measurements.

In retrospect, the basic finding should perhaps come as no great surprise. Dr. Valentine pointed out that the northern Gulf of Mexico is almost like a large enclosed bay, with land wrapping around it on three sides. No strong current dominates the deep ocean in that region, so it makes sense that the hydrocarbons essentially drifted. “It’s not a river — it’s not water moving rapidly all in one direction,” Dr. Valentine said. “It’s a lot of sloshing movement — sometimes to the southeast, sometimes to the northeast, sometimes in a swirl.”

The paper produced another intriguing finding: after a mass of water was injected with hydrocarbons the first time, bacteria capable of eating the compounds would multiply, exploiting the new food source. Then, even when the food was largely gone, the bacteria would linger for a while in abundance. If a second or third pulse of hydrocrabons flowed into the same water, the bacteria could gear up and break down those hydrocarbons far more quickly.

Dr. Valentine calls this an “autoinoculation” effect, and it could be important in future scientific work — for instance, in predicting what might happen if a large amount of methane gets injected into the ocean by global warming. (Fears were raised during the Deepwater Horizon spill that the breakdown of hydrocarbons would deplete oxygen so severely as to kill sea life, but the evidence suggests that this did not happen – probably, Dr. Valentine said, because all that sloshing kept bringing fresh oxygen-rich water into areas affected by the spill.)

In their paper, the researchers acknowledge that their model is not flawless. It cannot reproduce some of the finer features of the spill, for instance, and the reconstruction does not perfectly match the available data in every case. Moreover, I have to caution readers that most scientists have not had a chance to study the new paper in detail, so it remains to be seen whether the work will stand critical scrutiny.

Samantha B. Joye — the University of Georgia researcher who first disclosed the existence of the plumes and subsequently tilted lances with Dr. Valentine over some findings — said she could not comment on the modeling part of his study since that was beyond her expertise. But she added that the research “does reconcile things a bit.” Richard Camilli, a scientist at the Woods Hole Oceanographic Institution who ran another research cruise in the gulf, told me by e-mail that he considered the new paper “very impressive — a big step forward in understanding the spill’s subsurface transport and biodegradation dynamics.”

Thus, on its face at least, the paper seems to go a long way toward explaining what happened beneath the waves when the Macondo well blew out — and toward validating much of the observational work that was done by the scientific teams who stuck their necks out to respond to the spill.

© 2012 The New York Times Company

Read more... Sphere: Related Content

No comments: