National Science
Foundation
Press Release 12-005
Scientists
document how geology, biology worked together after oil disaster
http://www.nsf.gov/news/news_summ.jsp?cntn_id=122736&WT.mc_id=USNSF_51&WT.mc_ev=click
When
scientist David Valentine and colleagues published results of a study in early
2011 reporting that bacterial blooms had consumed almost all the deepwater
methane plumes after the 2010 Gulf of Mexico Deepwater Horizon oil spill,
some were skeptical.
How,
they asked the
In
new results published this week in the journal Proceedings
of the National Academy of Sciences (PNAS), Valentine; Igor Mezic, a mechanical engineer at UCSB; and coauthors report
that they used an innovative computer model to demonstrate the respective roles
of underwater topography, currents and bacteria in the Gulf of Mexico.
This
confluence led to the disappearance of methane and other chemicals that spewed
from the well after it erupted on
The
National Science Foundation (NSF) funded the research.
"As
scientists continue to peel apart the layers of the Deepwater Horizon microbial
story," said Don Rice, director of NSF's chemical oceanography program,
"we're learning a great deal about how the ocean's biogeochemical system
interacts with petroleum--every day, everywhere, twenty-four/seven. "
The
results are an extension of a 2011 study, also funded by NSF, in which Valentine
and other researchers explained the role of bacteria in consuming more
than 200,000 metric tons of dissolved methane.
"It
seemed that we were putting together a lot of pieces," Valentine said.
"We would go out, take some samples, and study what was happening in those
samples, both during and after the spill.
"There
was a transition of the microorganisms and a transition of the biodegradation,
and it became clear that we needed to incorporate the movement of the
water."
The
scientists believed that there was an important component of the physics
of the water motion--of where the water went.
Valentine
turned to Mezic, who had published results in 2011
forecasting where the oil slick would spread.
"Our
work was on the side of: here's where the oil leaked and here's where it
went," Mezic said. "We agreed that it would
be beautiful if we could put a detailed hydrodynamic model together with
a detailed bacterial model."
The
resulting computer model has data on the chemical composition of hydrocarbons
flowing into the
The
physical characteristics were based on the U.S. Navy's model of the gulf's
ocean currents and on observations of water movements immediately after the spill
and for several months after it ended.
The
scientists then sought the help of Mezic's former
colleagues--engineers at the
"We
needed somebody to build the software," Mezic
said. "It was a big task, a mad rush, but they did it.
"The
power behind this is a tour de force. A typical study of this kind would take a
year, at least. We found a way that led us to answers in three or four
months."
The
model revealed that one of the key factors in the disappearance of the
hydrocarbon plumes was the physical structure of the
"It's
the geography of the gulf," Valentine said. "It's almost like a box
canyon. As you go northward, it comes to a head.
"As
a result, it's not a river down there; it's more of a bay. And the spill
happened in a fairly enclosed area, particularly at the depths where
hydrocarbons were dissolving."
When
the hydrocarbons were released from the well, bacteria bloomed. In other
locations outside the gulf, those blooms would be swept away by prevailing
ocean currents.
But
in the Gulf of Mexico, they swirled around as if they were in a washing
machine, and often circled back over the leaking well, sometimes two or three
times.
"What
we see is that some of the water that already had been exposed to hydrocarbons
at the well and had experienced bacterial blooms, then came back over the
well," Valentine said.
"So
these waters already had a bacterial community in them, then they got a second
input of hydrocarbons."
As
the water came back over, he explained, the organisms that had already bloomed
and eaten their preferred hydrocarbons immediately attacked and went after
certain compounds.
Then
they were fed a new influx of hydrocarbons.
"When
you have these developed communities coming back over the wellhead, they
consume the hydrocarbons much more quickly," Valentine said, "and the
bacterial composition and hydrocarbon composition behaves differently. It
changes at a different rate than when the waters were first exposed."
The
model allowed the scientists to test this hypothesis and to look at some of the
factors that had been measured: oxygen deficits and microbial community
structure.
"What
we found was very good agreement between the two," Valentine said.
"We
have about a 70 percent success rate of hitting where those oxygen declines
were. It means that not only is the physics model doing a good job of moving
the water in the right place, but also that the biology and chemistry results
are doing a good job, because you need those to get the oxygen declines. It's
really a holistic view of what's going on."
There
are valuable lessons to be learned from the study, the scientists believe.
"It
tells us that the motion of the water is an important component in determining
how rapidly different hydrocarbons are broken down," Valentine said.
"It gives us concepts that we can now apply to other situations, if we
understand the physics."
Mezic said that this should be a wake-up call for
anyone thinking of drilling for oil.
"The
general perspective is that we need to pay more attention to where the currents
are flowing around the places where we have spills," he said.
"We
don't have models for most of those. Why not mandate a model?
"This
one worked--three-quarters of the predictions were correct. For almost
everything, you can build a model. You build an airplane, you have a model. But
you can drill without having a model. It's possible we can predict this.
That's what a model is for."
The
U.S. Department of Energy and the U.S. Office of Naval Research
also supported the research.
In
addition to Valentine and Mezic, co-authors of
the paper are Senka Macesic,
Nelida Crnjaric-Zic, and
Stefan Ivic, of the University of Rijeka
in Croatia; Patrick J. Hogan of the Naval Research Laboratory; Sophie Loire of
the Department of Mechanical Engineering at UCSB; and Vladimir A. Fonoberov of Aimdyn, Inc. of
Santa Barbara.
-NSF-