The oil was lucky to last as long as it did.
As it began to rush out of the Deepwater Horizon's bent pipes and busted caps 5,000 feet beneath the sea's surface, the oil and gas unleashed last year by BP PLC's Macondo well discovered a dangerous, exotic environment. The Gulf of Mexico was full of reactive chemicals, voracious wildlife and splitting pressure. Dangers lurked at every corner.
The oil faced an epic journey after its millennial rest beneath the sea. Some would remain hidden, trapped and consumed in undersea clouds or dissolved into the water column. Bubbling upward, parts would jump into the atmosphere, or be sheared apart by passing storms and dispersants. Growing rust red and slimy, baked in the sun, the oil would reach shorelines and marshes, coating -- and taking -- life.
There is no story yet about the ultimate ecological damage last year's spill wrecked on the Gulf of Mexico. Early returns have seemed positive, but more systemic problems could appear. The spike in dolphin deaths could be tied to oil-derived chemicals. Turtle or crab growth could be stunted. Marshes may not bounce back. It's too early to know.
However, far from the spotlight and buried in technical annexes, one important narrative has started to arise. Scientists have begun to paint a portrait of how the 4.9 million barrels of oil released by the Deepwater Horizon traveled, behaved and degraded in the Gulf of Mexico.
Assembled, the studies tell oil's biography, from wellhead to beachhead. They detail, through a mix of theoretical modeling and measurement, the variable on which all future understanding of the spill depends: where the oil went. And while the results are far from definite, they make clear that the oil's story will be the Gulf's story.
That story began long before the crude reached the Gulf's surface.
The oil formed millions of years ago. Ancestral phytoplankton died, drifting down to the depths of a forgotten sea. Buried by river-borne dirt and sediment, the remains compressed and decayed, shedding oxygen. Stewed by the Earth's heat, the fossils became complex mixtures of carbon-dominated chemicals that, together, we call oil and gas.
The Macondo hydrocarbons weren't static. They needled upward through porous rock. Even down in the deep, they may have encountered bacteria, their future predators. (Heavy oil is thought to be created by microbes.) The oil rose, guided by a massive column of salt, a leftover from past evaporated seas (Greenwire, July 28, 2010). The salt punched through and stretched layers of sediment. It opened wide pores in surrounding rocks, and created traps to hold the oil.
The salt gave the oil a home.
Once BP cracked open that home, oil and gas began erupting into frigid waters. But it was not an even eruption. The oil emerged as bubbles, not a continuous flow, with varying widths. The globs likely reached up to 10 millimeters wide, with many running much smaller, according to past deep-sea experiments in the North Sea.
Bubble width might seem a minor fact, but 5,000 feet underwater, these volumes played an influential role in the oil's fate. The smaller droplets would be vagabonds, taking weeks to rise; some would take so long to surface that they would be effectively diluted. Even the larger droplets took several hours to reach the surface, according to Jerry Galt, a scientist with the National Oceanic and Atmospheric Administration.
These droplets soon found one common belief -- that oil and water don't mix -- to be slightly misleading. Crude is a complex mixture of more than 1,000 chemicals, and many of the smaller components such as benzene can dissolve, like table salt, into water. During their long rise, the Macondo bubbles began to suffer "continuous molecular extraction" of their oil by the seawater, government scientists concluded in November, a finding backed by sampling.
Of course, dissolution didn't cause the lost oil to disappear. The oil instead mixed with the water column, like sugar sweetening coffee. While possibly rendering these immediate waters toxic -- fishing bans around the wellhead were only repealed this month -- the oil was exposed. Dissolved, it would dilute into the ocean's turbulent expanse.
The dissolved oil was a few grains of sweetener; the sea, an industrial-sized coffeepot.
Under the sea
While much of the oil bubbled upward, beginning its odyssey, nearly all of the gas released by Macondo, making up more than 50 percent of the spill's volume, remained trapped.
The well was so deep that the gas's tiny droplets were unable to escape through the ocean's stratified layers, flowing instead as a diffuse cloud southwest from the wellhead. This cloud of gas was joined by minute oil droplets, formed either by the natural turbulence of their escape or chemical dispersants, which BP began applying underwater in mid-May.
At the spill's peak, in early summer, this hydrocarbon cloud stretched out over an area roughly the size of Manhattan, some 3,000 feet beneath the sea's surface. While invisible to the naked eye -- at most, the oil measured in 10 parts per million -- the cloud's oil represented at least 7 percent of the crude escaping from the well, according to scientists at the Woods Hole Oceanographic Institution, who charted its course (Greenwire, Aug. 20, 2010).
Seemingly, this oil, suspended in the water like dust in the air of an old Western film, was in a good position. At that depth, frigid temperatures prevailed, and nutrients were thought scarce. As it diluted, perhaps it could avoid the oil-eating bugs known to thrive in warm waters?
It couldn't. The Gulf had too often handled escaped oil, and soon scientists led by Terry Hazen, a microbiologist at Lawrence Berkeley National Laboratory, discovered that cold-loving microbes were making quick work of the oil (Greenwire, Aug. 24, 2010). These bugs were, in effect, oil-seeking missiles. They were highly mobile, armed with swimming flagella and protein sensors that could guide them to their oily prey, according to additional work so far unpublished by Hazen's group.
The gas had more luck. While some microbes were able to feast on complex gases like propane, an easy snack for many bacteria, most of the gas at first seemed to resist breaking down (Greenwire, Sept. 17, 2010). But the bugs would not be denied, as scientists from the University of California, Santa Barbara, and Texas A&M University found. Soon after the bacteria finished chewing through the available oil, they turned on the remaining methane gas.
The deep waters had proved to be an unfriendly avenue for the hydrocarbons. By the early fall, the only oil molecules detected in the plume were trace amounts found in bacterial scat.
The deepwater oil had left the stage.
On the sea surface
If the oil was going to find refuge, it would be through the surface.
Upon reaching the Gulf of Mexico's crowded waters, though, the crude faced immediate challenges. The oil was light, more gasoline than motor oil. Many of its shorter chemical components, if not dissolved on the way up, never had a chance once exposed to the atmosphere. They were soon lost to the air, evaporating like volatile versions of water, creating health hazards for response crews along the way.
Several laboratories studied this evaporation. Perhaps the most rigorous, by the Norwegian research institute SINTEF, studied samples of the oil in large flume chambers, simulating sea conditions. Some 50 percent of the oil was lost to evaporation within five days at sea, the researchers found. Work by S.L. Ross Environmental Research, cited in the government's final oil budget, came to similar conclusions.
Suffering from dissolution and evaporation -- parts of what's known in the oil-response world as "weathering" -- the oil fundamentally changed. Stripped of its light hydrocarbons, the crude became cruder, viscous, forming emulsions, streams of sinewy goop held together by resins and asphalts. These emulsions, under the sun's exposure, turned rusty brown and, perhaps, released more water-soluble, and toxic, components. Dispersants could still break emulsions apart, but the oil would start to resist ignition.
There was a lot of ignition going on. Responders chased oil across the waters, skimming, burning and chemically dispersing. Burns especially hounded the oil: A good at-sea burn, logistically challenging to pull off, would leave less than 5 percent of the oil's original volume as residue. As one government report put it, burns were a "highly efficient response option."
After shedding so much volume to degradation, dissolution and evaporation, emulsified oil began to congeal, hardening into tar balls. These globs are persistent, some of the spill's best survivors. The tar balls, generally less toxic than fresh oil, washed onto beaches far from the spill. It's uncertain how much oil escaped as tar balls; there are no estimates.
Stranded and shifting according to weather, the oil would soon make landfall. Marshes and beaches would be a welcome respite, for the oil was exposed at the surface to mobile bacterial predators, cousins of the deep-sea plume eaters. Dispersants increased the amount of oil available to the bugs, and so far, all signs have pointed toward a successful feeding frenzy.
Many of these signs are indirect. Scientists saw anecdotal evidence of the bugs in their sampling, and University of Houston researchers identified large shifts in bacterial populations in the water, including some known oil eaters. Microbe-consumed carbon from the oil soon made its way up the food chain, too, as scientists at the University of South Alabama detected the element in zooplankton, bacteria's natural predator.
Degradation rates for these bacteria are not available, but their existence in warm surface waters was not unexpected. Back in the late 1970s, scientists dumped oil similar to Macondo's into a North Carolina estuary. Within three weeks, more than 97 percent of the oil had vanished, suffering first from evaporation and then from bacterial feeding.
Many researchers wouldn't be surprised by similar results for the Gulf.
As long as the oil remained at sea, it faced harsh odds. To survive, it needed land.
Despite booms, berms and boats, by mid-May it found the shore, eventually staining beaches and marshes in Louisiana, Alabama, Mississippi and Florida. There was no better escape. Near or on the shore, there were multiple ways for it to linger, hidden from human and microbe, as technical federal reports detailed for Gulf Coast beaches, though not marshes, this February.
The oil splashed upon few shorelines more heavily than that of Grand Isle, La. Deeply weathered, it attacked the island's beaches on multiple fronts: from below, straight on -- even above. The crude inundated the beach repeatedly, migrating almost 40 inches down into its sand, pushing through pores similar to the sandstone that had once held it, deep underground.
Beyond its direct assault, the crude lurked at the beach's shore, mixing with sand to form oily mats hovering under the low-tide line. Periodically belching tar balls, the mats are elusive, slipping through shallow waters or broken apart by storms, only to re-form later. By early this year, several mats still remained off the island, though plans existed to clean up the stragglers.
Stormy weather provided the oil with its best chance of reprieve. Swells pushed the crude far past the beach's high-tide line, burying it under feet of sand. This oil, if undiscovered, has one of the best shots of persisting in the environment. Out of water, bacteria cannot roam freely and slowly consume oil, a rate that dwindles further when deprived of oxygen or nutrients. In such conditions, oil can persist, in decreasing amounts, for decades.
At a test site on Grand Isle, scientists uncovered buried oil that had little to no access to oxygen. The excavated sediment reeked of rotten eggs, an indication that microbes were instead using sulfates to attack the oil. Their work went slowly. According to the researchers' modeling, the most toxic oil components, polyaromatic hydrocarbons (PAHs), would be close to 90 percent of their original concentration after 10 years, barely reduced at all.
Like much of life, oil decomposition is stubbornly tied to oxygen. For instance, while the Grand Isle oil would persist, the same toxic components, buried at test sites in Alabama and Florida, would fall to about 15 to 20 percent of their previous concentration after only five years. The bugs at the Florida and Alabama sites, armed with oxygen, could do their work.
The persistence of the Grand Isle oil raised a crucial question: Would the PAHs migrate, carried by flowing groundwater? Based on wells installed at Grand Isle by the Coast Guard, two researchers at the U.S. Geological Survey and Virginia Polytechnic Institute and State University modeled their escape. The compounds, clinging to sand, wouldn't go far, no more than 7 feet, they found.
After a long migration, the beached oil had found rest.
For the surfaced oil that never reached the coastline, likely a vast percentage of the crude, its time at sea took a toll.
By August, the oil's sheen had faded. But could the bacteria really have degraded all of this oil, leaving behind so few traces? Did some of it sink, or was it pulled downward by the sediment pouring out of the Mississippi River, settling into the Gulf's murky, muddy floor?
This is one of Macondo's great unanswered questions. For it, the spill has no historic parallel.
Even the most relevant well eruption, Mexico's Ixtoc leak in 1979, sheds little light.
Ixtoc occurred in shallow water, only 165 feet deep, compared to Deepwater Horizon's 5,000 feet. Shallow waters are typically thick with sediment, and the dirt near Ixtoc pulled 25 percent of the spill back to the seafloor. Over deep waters, the sediment load lessens, shutting down escape routes to the abyss.
Even when consumed by microbes, the crude may leave traces. After eating oil, some bugs produce daughter products that can behave as endocrine disruptors, which can pose health hazards at small concentrations. And as the feeding frenzy ended, the bacteria fed upon themselves, suffering a massive die-off, evidence of which scientists discovered in sediment cores taken in the Gulf's deep: fluffy, brown layers, including algal and zooplankton detritus.
For months in the late summer and fall, government and independent scientists took core samples of sediment around the spill: in shallow waters on the continental shelf, especially near the Mississippi River's mouth, and deep waters around the wellhead. These results, shared in late December, focused on emergency response options, but remain the only published data on the oil's fate in the sediment.
At least in small amounts, it seems, degraded oil made it back to the seafloor.
Cores near the Mississippi River, where waters are thick with silt trucked down from the Rocky Mountains, showed visual indications of oil traces, the report says. However, the samples didn't exceed U.S. EPA's toxicity thresholds for marine life.
Out in deep waters, the evidence has been just as muddy. There is some evidence that sediment pulled oil down to the dirt, as 29 percent of 115 samples taken had visual evidence of small fractions of oil. Only 6 percent of these samples exceeded safety benchmarks, though, and those were within 2 miles of the well. Good evidence exists indicating that the oil in these spikes never reached the surface, instead held down by drilling mud blowouts.
When the oil's authoritative biography is written, it seems likely that some fraction, especially in shallow waters, will have migrated back into the seafloor. But that oil, degraded and diluted by the Mississippi's silt, will be a trickle, a mote in a long-running geological story. It will be compressed, condensed or destroyed, as the seafloor is constantly renewed.
It will have returned home.