Five years on, Deepwater Horizon oil spill’s impact lingers

Anniversary of a disaster in the Gulf of Mexico shows much remains murky

Exploding Oil Rig

THE AFTERMATH  Deepwater Horizon's fifth anniversary is greeted with more questions than answers.

US Coast Guard/Wikimedia Commons

In a steamy Louisiana marsh, crickets do their best impersonation of a canary in a coal mine.

Afloat in orange cages on the coastal wetland, the featherless chirpers warn researchers of toxic fumes rising from oil. Oozing oil is a recurring yet elusive problem on the marsh in Barataria Bay, just south of New Orleans. One day, a patch of the wetland is green and lush, the next it’s drenched in thick, noxious goo. It’s a haunting vestige of North America’s largest marine oil disaster: the 2010 Deepwater Horizon spill.

At first, the possibility that the oil was still surfacing and releasing killer vapors years after the spill seemed far-fetched; everything scientists know about spills suggests that fuming oil would have vanished almost immediately after the oil was released. But the Louisiana crickets are quietly telling a different story. They’re dying.

“It’s this huge mystery,” says environmental scientist Linda Hooper-Bùi of Louisiana State University in Baton Rouge. Researchers don’t know what compound or set of compounds is wafting from the oil to kill the crickets. But if the fumes can kill insects, what can they do to people on the marsh? “It keeps me up at night,” she says.

This month marks the fifth anniversary of the explosion of the Deepwater Horizon oil rig, operated in the Gulf of Mexico by the oil and gas company BP. The April 20 blast killed 11 rig workers and started an 87-day eruption of oil and gas (SN: 7/3/10, p. 5).  Around 5 million barrels of oil and hundreds of thousands of metric tons of gas gushed from the well, known as Macondo, located 1,500 meters below sea level. The spill was so large that a swirl of oil in the Gulf could be seen from space.

During the Deepwater Horizon spill, oil washed up on the coast of Barataria Bay in south­eastern Louisiana. Charlie Riedel/Associated Press

Scientists immediately mobilized research crews, vessels and even aircraft to study the impact of such a large disaster. Since 2010, BP has provided more than $1.3 billion for research and the federal government has chipped in upwards of $30 million. With mounds of data, scientists now have a better handle on life in the Gulf. Yet plenty of unanswered questions remain. And new mysteries, like the dying crickets, continue to surface.

“It takes a very long time to sift through the impacts” of a typical oil spill, says oceanographer Ian MacDonald of Florida State University in Tallahassee. No one was ready for a spill the size of the Macondo blowout, he says. 

Of the lingering unknowns, perhaps the most striking is that scientists still don’t know exactly what happened to most of the oil. Only a quarter is neatly accounted for. The fate of up to 3.75 million barrels of the reddish-brown liquid is still murky. Researchers continue to find some of it in snotlike blobs on the Gulf’s seafloor and coastlines, including in the marshes that are home to those sentinel crickets and other creatures.

The spill was certainly dramatic, but the long-term toll on wildlife has been mixed; some species in the Gulf are struggling while others are doing fine. Instead of a dramatic collapse of life, researchers are finding subtle effects — some that only emerged three or four years after the spill — that they are still trying to sort out, says coastal ecosystem scientist R. Eugene Turner of Louisiana State.

With thousands of active oil and gas platforms still pumping in the Gulf, there is plenty of pressure to find answers and better prepare for the inevitable next spill.

Coated in questions

After birds were “degreased” and oil slicks vanished from the surf, researchers began to tally the less-obvious impacts of the oil on wildlife. They found that oil can alter development and change the swimming behavior of aquatic creatures (SN Online: 11/19/12). It can also fatally disrupt the beating of fish hearts (SN Online: 2/14/14).

More recently, Gulf researchers have found fish with sunken eyes and other eye abnormalities. It’s unclear, however, if Macondo oil is to blame, say aquatic researcher Stephen Bullard of Auburn University in Alabama and colleagues. In February, Bullard and hundreds of other researchers met in Houston to discuss their latest findings. The event was organized in part by the Gulf of Mexico Research Initiative, a BP-supported effort that funds independent research on the consequences of the Macondo blowout.

At the meeting, ecologist Selina Heppell reported a crash of the Gulf’s population of Kemp’s ridley sea turtles. The endangered turtles were rebounding between 1990 and 2010, with nest numbers expected to reach 50,000 by 2014. Instead, last year’s count was around 11,000. Because the researchers monitor only nests, it’s difficult to pinpoint when in the animals’ life span problems arise, says Heppell, of Oregon State University in Corvallis. Are turtles dying or are they just not breeding?

Bottlenose dolphins also may be experiencing long-term effects, according to a study published in PLOS ONE on February 11. Researchers led by Stephanie Venn-Watson of the National Marine Mammal Foundation in San Diego reported an unusual ongoing die-off of dolphins, whales and porpoises in the Gulf. Since 2010, more than 1,300 of the animals have stranded themselves along the coast, including 221 in 2014. More than 90 percent of the animals died.

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Awash in effects

Years after the oil slicks vanished, researchers are finding lingering changes in the Gulf of Mexico that may be linked to the 2010 Deepwater Horizon oil spill. (Tap circles on map for details.)

Credits: Map: GEOATLAS/Graphiogre, adapted by E. Otwell; Turtle, Dolphin: NOAA Gulf Spill Restoration; Mucus: Julia Sweet/UC Santa Barbara; Patty: Catherine Carmichael/Woods Hole Oceanographic Institution; Oiled marsh: Gulf Restoration Network/Flickr (CC BY 2.0); Coral: Penn State/Flickr (CC BY-NC 2.0).

Researchers are also puzzling over more subtle changes in the ocean basin. For example, sperm whales still hunt in the Gulf, lunging deep down to gulp squid and other bottom-dwelling prey. However, the whales are no longer feasting near Macondo. They avoid an approximately 4,000-square-kilometer area around the well, according to marine mammal researcher Bruce Mate and colleagues at Oregon State, who reported at the Houston meeting.

A slew of Gulf marine life, from tiny killifish to sharks, shows molecular signs of exposure to oil. How the exposure affects populations overall, their predators or rival species and the rest of the food web is difficult to determine, especially with all of the other pollution and disturbances in the Gulf. Without a lot of data about the ecosystem before the spill, scientists can’t say if what they see now is normal or a subtle effect of the spill. “We didn’t have baseline data,” Turner says with disappointment.

While some scientists struggle to understand the complexities of population and ecosystem changes, others try to tackle a question that might seem simpler: What’s in the oil?

A chemical mash-up

Petroleum that gurgles up from deep in the Earth contains a complex cocktail of chemicals, which can include toxic polycyclic aromatic hydrocarbons (sturdy conjoined rings of carbon and hydrogen) and gases such as methane and propane. Different oil wells have distinct chemical signatures — a specific set of compounds at specific ratios. Marine chemist Christopher Reddy of the Woods Hole Oceanographic Institution in Massachusetts and others are still working on the exact chemical fingerprint of Macondo oil. They need that fingerprint to accurately trace the oil’s fate.

The researchers are closing in on a consensus fingerprint of pure Macondo oil, but they’ll need more than that. As oil basks in the sun or mingles with other chemicals in the environment, it mutates, forming a more complex — and potentially more toxic —  set of chemicals. Much of the oil still surfacing on the coasts, as in the Louisiana marshes, isn’t fresh Macondo oil — it’s weathered. And weathered oil is trickier to analyze.

In 2010, Reddy and colleagues sailed out to the Chandeleur Islands, thin strips of land east of New Orleans, to scoop up some of that weathered oil. Tagging along with a cleanup crew, they collected asphaltlike patties of oil dotting the beaches. Back in the lab, they got their first glimpses of weathered oil.

Many Macondo chemicals are missing from the weathered samples, Reddy says. They probably evaporated in the summer heat or broke down in the sun. “The majority of what we’re seeing was not originally in there,” he says. Some of the chemicals that remain have sucked in oxygen and morphed into big, undefined compounds. Reddy jokes that they could call them “spilsphaltenes,” a mash-up of spilled asphalt-looking chemicals.

The signature of those weathered chemicals will help researchers track the oil in years to come and calculate just how much is still sloshing around in the Gulf. This is especially important since the amount of missing oil is unresolved, except in court.

On January 15, 2015, U.S. District Judge Carl Barbier of the Eastern District of Louisiana in New Orleans ruled that 4 million barrels of oil exited the reservoir, with 3.19 million barrels of that released into the Gulf during the 2010 spill. The count was a compromise between BP’s numbers and estimates by independent scientists.

Slippery tallies

BP has consistently given the lowest estimates of how much oil gushed into the Gulf. In the days after the rig exploded, BP reported that 1,000 to 5,000 barrels of oil was escaping each day. In court, the company argued that 3.26 million barrels in total (more than 37,000 barrels a day) were released during the 87 days. Outside scientists were quick to dispute BP’s count and settled on a tally of around 5 million barrels (nearly 58,000 barrels a day) plus about 500,000 metric tons of gas.

Independent researchers used satellite images, water sampling, atmospheric samples and video analysis to dispute BP’s estimates. One fast and cheap method came from atmospheric chemist Thomas Ryerson of the National Oceanic and Atmospheric Administration in Boulder, Colo., and colleagues. During the spill, Ryerson was in California with a WP-3D airplane bedecked with analytical chemistry equipment for assessing air quality. By early June, he and his team were flying over the Gulf, measuring methane, aromatic hydrocarbons and other compounds in the plume of vapors downwind of the spill.

To come up with an oil estimate, Ryerson likens the situation to pouring buckets of chicken soup into a swimming pool and then trying to figure out how much soup is in there. You don’t know how many buckets of soup went in, but you do have a recipe for chicken soup. “If you know that the recipe says that it’s one carrot per gallon of soup and you find 10,000 carrots, you know that you have at least 10,000 gallons of soup in your pool,” he says. Using hydrocarbon measurements as their carrots and other soup ingredients, Ryerson and his colleagues estimated that up to 47,700 barrels of oil made a mark on the atmosphere each day.

The next step was to figure out where else the oil and gas went. Aboard research vessels floating around the Gulf through the fall of 2010, researchers tracked underwater plumes of gas — mostly methane plus some propane and ethane — trapped in deep layers of the ocean. The researchers saw oxygen levels in the plume sink, suggesting that microbes, such as species of the bacteria named Methylosinus, were sucking in oxygen as they gobbled up spilled propane, ethane and methane (SN: 1/29/11, p. 11). Some scientists estimate that most of the gases, if not all, were consumed, though not all researchers agree (SN Online: 5/13/14).

Of the estimated 5 million barrels of oil, roughly 25 percent is accounted for: Responders collected 17 percent as it spewed out of the well, burned 5 percent and skimmed 3 percent from the surface, according to government estimates. The remaining 75 percent is missing. Some probably evaporated or dissolved into deep layers of the ocean where it scattered in currents or succumbed to oil-eating microbes. Recent calculations put a substantial percentage of the spilled oil on the floor of the Gulf.

“We’re not going to be able to track down each and every drop of oil,” says biogeochemist David Valentine of the University of California, Santa Barbara. But he and others are working to piece together a reasonable picture.

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Gone goo

Most researchers estimate that nearly 5 million barrels of oil gushed out of the Macondo well. Only 25 percent of the oil is neatly accounted for. Researchers have only rough estimates for the rest.

Source:  Federal Interagency Solutions Group, “Oil Budget Calculator: Deepwater Horizon.” November 2010.

To get at how much of the oil might have sunk, Valentine and colleagues analyzed sediment from the seafloor collected in 2010, 2011 and 2012. The group traced a component of the oil called hopane and measured how much of it is in Macondo oil. In a study published last November in the Proceedings of the National Academy of Sciences, the researchers back-calculated that from 1.8 to 14.4 percent of the total oil released ended up coating the Gulf floor. A study reported January 20 in Environmental Science & Technology reached similar conclusions. Led by chemical oceanographer Jeff Chanton of Florida State University in Tallahassee, the researchers traced the carbon isotope signature of the oil in sea sediments and found that 0.5 to 9.1 percent of the oil is on the Gulf floor.

“I think I’ve lowballed it,” Chanton says. Though he’s comfortable giving a conservative estimate rather than an overestimate, he suspects more oil will be found in the Gulf in the coming years.

Now that scientists know that oil is lying at the bottom of the Gulf, they want to know how likely it is to reemerge.

A snotty undertow

Some of the oil on the seafloor arrived in mysterious mucuslike blobs. Researchers suspect that the blobs formed when oil compounds clumped with phytoplankton or with slime from oil-degrading microbes, such as Cycloclasticus bacteria. The blobs also could have formed around charred particles from the burned oil, or around chemicals called dispersants that were sprayed during the spill to help break up oil slicks (see sidebar, below).

“When we were out on the water in May of 2010, early on in the Macondo blowout, we observed a tremendous amount of this … ‘sea snot’ on the surface,” says marine scientist Samantha Joye of the University of Georgia in Athens. “After six or seven weeks,” she says, “it was all gone.”

Joye and her team tracked the blobs as they sank about 300 to 400 meters per day (the mass sinking of the blobs is sometimes referred to as the “dirty blizzard” or “marine snow”). The researchers returned to the Gulf in September and December of 2010, and every year since, to collect hundreds of blob-covered sediment cores from the seafloor around Macondo. 

With each survey, the researchers noticed something odd. The spots that were oiled seemed to change from year to year. After some chemical analysis, the researchers figured out what was happening: “These layers are still there, except they’re moving,” Joye reported at the meeting in Houston in February.

Whether the oily globs will keep moving is a key question, Joye says. She and her team think that the knolls and trenches on the seafloor around Macondo create sharp currents that stir up the snot. The finding opens the potential for oil to be hurled onto marshes and beaches during storms or heavy winds, or sloshed into new seafloor communities. Many other researchers at the meeting reported that deep-sea corals around the Gulf are still dying, losing branches and crumbling in the years after the spill, possibly due to this movement of the oil.

It’s also possible, says Joye, that the oil globs could eventually settle into a low spot in the Gulf and do no more harm. 

Asphalt marshes

Back in the Louisiana marshes, Linda Hooper-Bùi isn’t sure where the oil she’s still seeing is coming from. It could be stuck in the marsh and resurfacing in storms or it could be snot balls washing ashore from the deep. Regardless, when it shows up it sometimes forms a crust on the surface. “It looks almost like the marsh is paved,” she says. Some grasses grow through the asphaltlike layer, so overall the marsh might look fine. But, in the heat, the fake pavement cracks and reddish-brown oil oozes up. That ooze is what Hooper-Bùi thinks is the source of the killer fumes.

Hooper-Bùi and colleagues reported in the October 15 Marine Pollution Bulletin that levels of oil chemicals in Louisiana’s coastal wetlands were 33 times as high in 2013 as they had been in May 2010. 

With those kinds of numbers and so many questions remaining five years after the spill, some researchers are pessimistic that the Gulf community is any better prepared for another Macondo-sized disaster. But researchers like Hooper-Bùi are hopeful that the new data on the Gulf will at least give them a better starting point for the next spill. “We really didn’t know anything about the ecosystem,” she says. “Now we have some idea.” The new data provide the baseline that ecologist Eugene Turner and other researchers wished they’d had before the Deepwater Horizon explosion.

“Oil is a natural component of the Gulf of Mexico ecosystem,” says environmental chemist Edward Overton of Louisiana State, who works with Hooper-Bùi. “Unfortunately, there’s going to be another spill sooner or later.” Researchers need to continue to collect data and track long-term effects, he says.

Meanwhile, oil and gas production marches on in the Gulf. An American company, LLOG Exploration, has begun pumping oil just 900 meters northeast of Macondo.

No clear verdict on oil dispersants

Amid the torrent of questions lingering about the Deepwater Horizon oil spill, the most contentious by far concerns the use of oil dispersants. These chemicals are a mixture of hydrocarbons and soaplike compounds that break the oil into small droplets that sink. When dispersants are sprayed, long slicks and bobbing balls of oil vanish from sight, becoming less likely to coat coastal creatures or roll onto beaches.

A Basler BT-67 aircraft releases dispersant over oil discharged from the Macondo well on May 5, 2010. Petty Officer 3rd Class Stephen Lehmann/U.S. Coast Guard District 8

The downside is that the tiny, mostly invisible droplets of oil are left to freely float in the depths of the ocean, their movements and final destination difficult to track.

During the Macondo blowout, first responders dumped, sprayed and injected nearly 7 million liters of dispersants into the Gulf. Scientists are still torn on whether that was the right thing to do.

“It’s a worse idea not to disperse,” says Louisiana State’s Edward Overton. He and others argue that the chemicals kept some of the oil at bay, sparing birds, turtles and other creatures. Coastal communities, such as the Louisiana marshes, are arguably the most vulnerable to the effects of toxic oil. Marshes “act like chemical sponges,” says Christopher Reddy of Woods Hole Oceanographic Institution in Massachusetts. Also potentially spared were coastal economies that rely on tourists, which may have suffered if more shoreline had become coated.

But the chemical dispersants are toxic, causing skin and eye irritation and liver and kidney damage in humans. Joining forces with oil, the dispersants can become more toxic, some researchers say, though the data are mixed.

Dispersants may even get in the way of natural cleanup mechanisms. Samantha Joye of the University of Georgia and her colleagues have found evidence that dispersants may discourage the best oil-gobbling microbes, slowing natural degradation.

Researchers are still arguing over the decision to use dispersants, Overton says, “and probably will be forever.” 

This article appeared in the April 18, 2015, issue of Science News under the headline, “An oil spill’s aftermath.”

Editor’s note: This article was updated on April 8, 2015, to clarify  the amount of oil that U.S. District Judge Carl Barbier determined was released into the Gulf of Mexico in the 2010 spill.  

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