It has been almost ten years since the day the waters of the Gulf of Mexico turned black, devastated by the worst environmental disaster in American history: the Deepwater Horizon oil rig accident. On 20 April 2010, the platform, owned by British company BP, suffered a catastrophic failure when a well blew out at a depth of 1,500 metres, beginning to spew oil into the sea at a rate of a thousand barrels a day for 87 consecutive days.

Since then, the Gulf's environment and wildlife have been paying an enormous price: approximately 12 per cent of the Gulf of Mexico's brown pelican (Pelecanus occidentalis) population died as a result of the accident, and bottlenose dolphins and sea turtles are dying at an unprecedented rate. The effects could be even more long-lasting than previously thought, as oil buried in the beaches along the Gulf coast could take decades to biodegrade.

A buried threat

This is the conclusion of a new study conducted by a group of ecologists at Florida State University and published in the journal Scientific Reports. According to Markus Huettel, the study's lead author and professor of oceanography at the American university, the Gulf's beaches are littered with golf-ball-sized clumps of oil and sand that take at least 30 years to decompose. "This oil contains substances that are harmful to the environment and to humans," said Huettel, "and can persist for long periods of time."

Read also

• The Deepwater Horizon disaster. What happened, the causes and those responsible
• Oil is to blame for the dolphin calf die-off in the Gulf of Mexico
• A handful of dollars for an environmental disaster that will never end

Poisoned coastlines

The oil that spilled from the Deepwater Horizon contaminated approximately 965 kilometres of sandy beaches along the Gulf of Mexico coastline. Some of the oil that washed ashore was rapidly removed, thanks both to the tireless work of microbes and microorganisms living in the sand and to the numerous clean-up operations carried out. However, these efforts were not sufficient, the Florida State University researchers found, as some of the crude oil clumps are buried up to 70 centimetres deep in the sand.

The results of the experiment

To quantify the scale of the problem, Huettel and his team conducted a three-year experiment on a beach in Pensacola, in northern Florida. The aim was to measure how long it takes for oil clumps buried within 50 centimetres of sand to degrade. The researchers found that these buried lumps of oil and sediment, which typically measure less than 10 centimetres in diameter, take an average of 32 years to decompose completely. Larger clumps, though fewer in number, persist in the environment for even longer. "We also found sediment clumps the size of an office printer, and even larger ones," Huettel explained. "If buried under the sand, these residues could last far longer than our golf-ball-sized samples."

The breath of the beach

Thirty years is undoubtedly a very long time; however, the study showed that this timeframe could extend even further, exceeding a hundred years, without the unique ecological properties of a sandy beach that are needed to promote the decomposition of organic material. The sand on beaches, rich in microbial colonies and microorganisms, is capable of removing degradable material in a relatively short time, also thanks to the relentless motion of the waves.

Read also

• Oil from the Deepwater Horizon disaster has entered the food chain
• With the film Deepwater Horizon, Hollywood steps onto the oil rig of the disaster
• Deepwater Horizon. Don't trust BP and the other companies responsible ever again

"The beach, breathing with the rhythm of the tides, can therefore be likened to a large organism that aerobically digests organic matter, including oil, inhaling oxygen and exhaling carbon dioxide," said Huettel. Without this natural process, which is at risk in many coastal areas due to the decline of marine ecosystems, clumps of oil and other toxic substances would accumulate on the beach, compromising the ecology of the area and water quality.