Paul Voosen, E&E reporter
Published: Tuesday, January 10, 2012
It’s the most tangible intangible disaster of the past decade.
Following the undersea blowout of the Deepwater Horizon two years
ago, a majority of the oil and gas escaping from the rig’s out-of-
control well never surfaced. Instead, it flowed in a diffuse layer to
the southwest, thousands of feet below sea level. Largely invisible,
this snaking “plume” nevertheless entered the imaginations of
millions of people — at least until its demise to the Gulf’s vast
size and a host of hungry microbes.
A compelling image — but it never happened. At least, not the way
scientists imagined.
Rather than flowing in a tidy path to the southwest, pulled along by
a steady current, the Deepwater Horizon plume was a mess of swirl and
slosh. Virgin water exposed to the spill, rather than whisking away
permanently, would return after weeks, carrying with it microbes
already primed to chew hydrocarbons, according to a study published
yesterday in the Proceedings of the National Academy of Sciences.
The study presents a unified theory of the plume, its model results
matching many of the often contradictory observations made by
scientists during the first months of the spill. Understanding the
bathtub circulation of the Gulf of Mexico suddenly sorted these
findings into a comprehensible whole, said Dave Valentine, the
paper’s lead author and a microbial geochemist at the University of
California, Santa Barbara.
Click here to watch Dave Valentine’s model of the Deepwater Horizon
plume, matched against observations recorded during the accident.”
“There’s no perfect way of explaining something as amorphous and
ever-changing as this was,” he said. It is almost irreducibly
complex, he added. “It’s almost like an enclosed bay. It’s not a
simple current where things move from point A to point B.”
The Valentine study comes at a crucial time for Gulf research. A
government study published last month confirmed that nearly half of
the oil — and almost all of the gas — released from the BP well
likely remained trapped in deep waters. In all, some 33,000 barrels
of oil a day remained in the deep, the study found, an estimate in
line with a chemical study of the oil’s fate also released yesterday.
Folding these mature estimates of the released oil, along with
evidence of microbial degradation, into a plausible theoretical
framework is essential to the government’s ongoing investigation of
the spill’s environmental damage, according to NOAA Administrator
Jane Lubchenco, who found time from her high-profile job to edit
Valentine’s study.
“These results may help us better understand the variability in the
rapid rates of hydrocarbon consumption by bacteria in the plume, as
observed by several groups of researchers,” she said in a release to
Greenwire, “while our scientists continue to examine the impacts of
the Deepwater Horizon spill on the Gulf ecosystem.”
This is not Valentine’s first foray into the plume. Previously, his
work uncovered the large amount of gas that remained trapped
underwater during the spill (Greenwire, Sept. 17, 2010). Valentine
also found that, much to his amazement, the recalcitrant methane had
vanished, degraded by bacteria, during a follow-up cruise in the
early fall (Greenwire, Jan. 7, 2011).
Scientists who studied the plume found Valentine’s model convincing.
While it did not match every observation perfectly, and its
resolution was somewhat coarse, those are simply improvements that
can be made on what seems like a foundational step.
“Their approach is holistic and does an excellent job of explaining
large-scale patterns observed in the Gulf of Mexico following the
spill,” said John Kessler, a chemical oceanographer at Texas A&M
University and one of the plume’s chief researchers.
“This is probably a slam-dunk understanding of how the plume worked,”
added Chris Reddy, a chemist at the Woods Hole Oceanographic
Institution. “The plume activity is a lot more complicated than we
really thought.”
Shaping perceptions
Reddy was part of the Woods Hole team that, early on, helped shape
perception of the plume with a report published in Science the month
after BP’s well was capped (Greenwire, Aug. 20, 2010). They described
what seemed like a diffuse cloud of hydrocarbons — on average, the
plume had a concentration of 1 part hydrocarbon to every million
parts water — lurking underwater, stretching over an area the size
of Manhattan.
Their view then, and really until Valentine’s study, was that the oil
came out of the well “and took a right-hand turn,” Reddy said. It was
a simplistic idea, in retrospect, he said.
The model helps explain several confounding findings, added Rich
Camilli, the lead author of the Woods Hole study. Their cruise
arrived at the Deepwater Horizon site just before warnings of an
incipient hurricane. And while they saw signs of hydrocarbons to the
well’s northeast, the lead was not strong enough.
“We had limited time on site, limited resources and a hurricane
coming at us,” Camilli said with some regret. “We had to focus our
energies. And we focused on the southwest because it seemed to be a
bigger signal.”
The most important finding from Valentine’s model was its discovery
of microbial priming, several scientists said. For deep waters not
previously exposed to the spill, the carbon-hungry bugs followed a
predictable pattern, one species after another blooming to consume
its favored hydrocarbon, said Terry Hazen, the microbiologist who
gained fame by identifying an oil-eating bug feasting on the plume
(Greenwire, Aug. 24, 2010).
In unexposed water, the easiest-to-digest hydrocarbons would go
first, Hazen said. To put it in human terms: “The candy went away
first,” he said. “Then we got into the meat and potatoes. And then we
got into the gristle.”
This pattern changed once water previously exposed to the spill,
after sloshing in deep spirals that could stretch for 50 miles,
returned to the wellhead. Their bibs already on, the host of microbes
began eating the candy (propane), meat (alkanes) and gristle
(aromatic hydrocarbons) all at the same time. It was a smorgasbord.
Valentine suspects this priming dynamic happens all the time in
waters home to oil and gas seeps. But no one has been able to find
it, he said, “largely because we’ve never the controlled release
[necessary] — or in this case, an uncontrolled release.”
The layering of old and new water also explains observation
differences recorded by the Woods Hole group, Valentine and Samantha
Joye, a biochemical oceanographer at the University of Georgia. Both
the Woods Hole group and Joye had found similar ratios of propane to
methane in their samples, while Valentine had contradictory data.
“And Dave is not a hack,” Reddy said. “We were going, ‘How can we
have this discrepancy? Our data was solid.’ We used a lot of brain
power trying to figure out why Dave’s data was different.”
Reddy even began giving presentations about the differences between
Valentine and Joye’s data to study confusion about the plume. Then,
finally, Reddy and Valentine were sitting together on another ocean
research cruise, and Reddy remarked, “Dave, how do we figure out this
propane shit?” In 30 seconds, Valentine sketched out his new model,
where consumption rates would vary with old and new water.
“He really unified theories about the plume,” Reddy said.
Navy models play key role
That unification could not have happened without some elaborate
modeling, however, including heavy mathematical lifting by one of
Valentine’s co-authors, Igor Mezic, an engineer at Santa Barbara.
Mezic adapted the Gulf models used by the Navy to keep their gliders
from running into the seafloor, adding mixing diagnostics he had
previously applied to describing the oil’s movement on the surface.
Combined with the huge amount of data recorded during the spill — 10
times the normal amount — a model that is typically used for short-
term predictions becomes far more rigorous.
“It’s an approach that really showed where the action was,” Valentine
said.
The group then seeded the physical model with both the hydrocarbons
erupting from the well and 52 theoretical bacteria types, each tuned
to a different feedstock. Tracking the movement of these bugs, which
included exemplars of the microbes previously discovered in the plume
by Hazen and Valentine, revealed how important the microbial “memory”
of the plume became after the spill’s first few weeks.
“It seems like in the early stages, the first week and first month,
there were more dramatic swings and blooms of variability, then
things stabilize a bit,” Valentine said, thanks to the layered
presence of multiple primed bacteria.
While microbial degradation was an important part of the plume’s
demise, that does not mean all of the hydrocarbons were consumed,
Valentine added. Oil contains a host of complex chemicals like
polycyclic aromatic hydrocarbons, which many bacteria find difficult
to break down. It is quite possible those plume components vanished
due to the dilution over the Gulf of Mexico’s vast expanse, rather
than any bacterial work.
“We don’t really know what happened to a lot of that stuff,”
Valentine said.
Special thanks to Richard Charter