Marine Oil Pollution: The Deepwater Horizon Oil Spill

The Deepwater Horizon oil spill occurred fifty miles off the coast of Louisiana on April 20th, 2010. According to the EPA, 4 million barrels of oil (210 million gallons) flowed from the damaged Macondo well over an 87-day period, before it was finally capped on July 15, 2010. It is the largest oil spill in the history of marine oil drilling. The incident killed 11 workers and affected at least 1,300 miles of shoreline from Texas to Florida. The spill caused 17.2 billion dollars of damage to the natural resources in the Gulf of Mexico. Though it has been over 7 years since the initial incident, the effects of the spill are still evident in coastal and marine habitats.

What Went Wrong:

BP believes that the disaster was caused by eight safety system failures. Justin Mullins for New Scientist describes these eight system failures:

Dodgy cement
The cement at the bottom of the borehole did not create a seal, and oil and gas began to leak through it into the pipe leading to the surface. BP says the cement formulation seems not to have been up to the job. The day before the accident, the crew had pumped cement to the bottom of the borehole, a standard procedure intended to prevent oil leaking out. On the day of the accident, the team were conducting checks to determine that that the well had been properly sealed.

Valve failure
The bottom of the pipe to the surface was sealed in two ways. It too was filled with cement, and it also contained two mechanical valves designed to stop the flow of oil and gas. All of these failed, allowing oil and gas to travel up the pipe towards the surface.

Pressure test misinterpreted
The crew carried out various pressure tests to determine whether the well was sealed or not. The results of these tests were misinterpreted, so they thought the well was under control.

Leak not spotted soon enough
Whether a well is under control or not, the crew at the surface should be able to detect a flow of oil and gas towards the surface by looking for unexpected increases in pressure in the well. Exactly this kind of increase occurred about 50 minutes before the rig exploded, but it was not interpreted as a leak.

Valve failure no. 2
About 8 minutes before the explosion, a mixture of mud and gas began pouring onto the floor of the rig. The crew immediately attempted to close a valve in a device called the blowout preventer, which sits on the ocean floor over the top of the well borehole. It did not work properly.

Overwhelmed separator
The crew had the option of diverting the mud and gas away from the rig, venting it safely through pipes over the side. Instead, the flow was diverted to a device on board the rig designed to separate small amounts of gas from a flow of mud. The so-called mud-gas separator was quickly overwhelmed and flammable gas began to engulf the rig.
No gas alarm
The rig had an on board gas detection system that should have sounded the alarm and triggered the closure of ventilation fans to prevent the gas reaching potential causes of ignition, such as the rig’s engines. This system failed.

No battery for BOP
The explosion destroyed the control lines the crew were using to attempt to close safety valves in the blowout preventer. However, the blowout preventer has its own safety mechanism in which two separate systems should have shut the valves automatically when it lost contact with the surface. One system seems to have had a flat battery and the other a defective switch. Consequently, the blowout preventer did not close.

This short video illustrates the system failures in further detail:

Clean up:

Several clean up methods were used immediately following the spill.
  1. “In situ burns.” Workers corralled some of the oil from the slick in a fireproof boom, then ignited it.
  2. 1.8 million gallons of chemical dispersants were dumped into the Gulf. The dispersant, Corexit EC9500A, assisted natural oil-eating bacteria by breaking down the oil droplets to make them more readily consumable. The dispersant got much of the oil off of the surface but lasting effects of the spill still linger under the surface.
  3. Hundreds of people worked on the shorelines to remove tar.

For more information about oil spill clean up click here.

Marine Life:

The National Wildlife Federation published a report in 2015 that looked at the impact of the oil spill on marine life from 2010-2015. The report concluded that:

• In 2014, dolphins on the Louisiana coast were found dead at four times historic rates, and there is increasing evidence that these ongoing dolphin deaths are connected to the 2010 oil disaster.
• Between 27,000 and 65,000 Kemp’s ridley sea turtles are estimated to have died in 2010, and the annual numbers of Kemp’s ridley nests have declined in the years since the spill.
• Twelve percent of the brown pelicans and 32 percent of the laughing gulls in the northern Gulf are estimated to have died as a result of the BP oil spill.
• Oil and dispersant compounds have been found in the eggs of white pelicans nesting in three states—Minnesota, Iowa and Illinois.
• Exposure to oil has been shown to cause abnormal development in many species of fish, including mahimahi, Gulf killifish and bluefin and yellowfin tuna.
• Spotted seatrout, also known as speckled trout, spawned less frequently in 2011 in both Louisiana and Mississippi than in previous years.
• Both 2010 and 2011 had the lowest numbers of juvenile red snapper seen in the eastern Gulf fishery since 1994.
• Coral colonies in five separate locations in the Gulf—three in the deep sea and two in shallower waters—are showing significant oil damage.
• Sperm whales are spending less time foraging in the area around the wellhead.
• Oil has been found in sediments deep in the Gulf of Mexico, in a 1,200-square-mile area surrounding the wellhead.

The oil spill destroyed many of the vulnerable habitats in the Gulf. One such habitat is Cat Island, a bird-nesting ground in Louisiana:

For more information on the long term effects of the Deepwater Horizon disaster on marine life click here.

Lasting Effects:

The deployment of the dispersant Corexit EC9500A has had some unintended, long term consequences. Samantha Joye, a marine scientist at the University of Georgia, told the Associated Press, "One of the main groups of oil munchers are fat little sausage-shaped bacteria called marinobacters. They eat oil all the time and comprise about 3 percent of the bacteria in normal water. But when there’s oil, they eat and multiply like crazy until they are as much as 42 percent of the bacteria. But when the dispersant was applied, they didn’t grow. They stayed around 3 percent. Instead, a different family of bugs called colwellia multiplied more, and they don’t do nearly as good a job at munching the oil." She believes that for some reason the dispersant and marinobacters just don’t work together.

Furthermore, Joye estimates that microbes broke down 43 to 61 percent of the oil, while 2 to 15 percent sank to the sea floor. She told The Atlantic, “We really can't account for 24 to 55 percent of the oil. Where is it? I don't know and it really bothers me. I suspect that some is in marshes, some is on beaches, and probably more than we think is on the seafloor.”

In this video by UC Davis, UC Davis environmental toxicologist Ron Tjeerdema demonstrates how dispersants work and discusses why this method appeared to be a good idea at the time.


Oil accumulation on the ocean floor could be disastrous for the fragile ocean ecosystem in the Gulf. Scientists estimate the habitats on the bottom of the Gulf could take anywhere from multiple decades to hundreds of years to fully recover.

For more information about dispersants click here.

Did You Know?

Most oil that pollutes the ocean actually comes from urban runoff. According to the New York Times, about 85 percent of the 29 million gallons of marine oil pollution in North America each year comes from drivers, businesses, and boaters, not from the oil industry. More than half of the oil runoff comes from the East coast, from Virginia and Maine. Many cars drip oil onto the concrete or asphalt and that oil ends up going into storm drains which ultimately drain into the ocean. Only 8 percent of marine oil pollution comes from tanker or oil pipeline spills.

What You Can Do:

Luckily there are many ways that an individual can reduce their contribution the marine oil pollution. The EPA advises people to:

  • Dispose of household chemicals properly by following the directions on the package or by calling the local public works department for proper disposal guidelines.
  • Clean up oil spills and fix leaking automobiles.
  • Use drip pans to catch engine oil and other pollutants while repairing cars.
  • Recycle used motor oil.
  • If possible, wash your car at a commercial car wash instead of in your driveway. By law, car washes must treat their wastewater before disposing of it.