In 1974, the oil tanker Metula ran aground near the southern tip of South America. Almost 400,000 barrels of oil spilled along the coast of Chile. The Chilean government gave up on cleaning the land, deciding instead to spend its limited cash on removing the grounded ship to stem the source of the leak. The result was a layer of oil left floating in the area’s marshes and baking on its beaches for decades—a natural laboratory for testing oil’s long-term environmental impact.
Years later, the Metula site is showing signs of life. But some parts of the coast recovered far more quickly than others, an indication of the tangle of variables, from the chemical reactions in the oil to the millions of moving parts that make up every discrete ecosystem it touches, that determine a spill’s environmental impact. As these complex interactions play out in the gulf, where a hole beneath mile-deep water off the Louisiana coast continues to spew huge amounts of crude, the Metula site’s progress, as well as information gathered at the sites of other disastrous spills, offer valuable lessons about the long-term fate of the Gulf of Mexico.
Lesson 1: The longer oil floats, the less toxic it becomes.
Some of history’s most environmentally damaging oil spills have involved oil that hit the water near the shoreline. In 1986, a leaky storage tank of refined jet fuel in Puerto Rico annihilated six hectares (more than 12 football fields) worth of mangrove forest around a single bay. Crude oil that leaked from a refinery storage tank in Panama in 1976 killed an estimated 75 hectares of mangroves.
But at least one researcher says Louisiana’s mangroves—the trees that make up the backbone of life in saltwater forests like the ones in Barataria Bay—probably won’t face oil that toxic. That’s because the most poisonous components of crude oil (benzene, kerosene, or other relatively lightweight chemicals) evaporate as they are exposed to the sun. The longer the longer the oil floats on the surface of the ocean, the more those components dissipate, leaving the heaviest part of the oil behind in the form of tar balls. That “weathered” oil can still do damage, and it can stick around—after the Panama spill researchers could still detect significant levels four years later. But forests hit by oil that has floated over longer distances and spent more time in the sun will have a better chance to recover.
“There are people who are saying that if oil touches a mangrove, the mangrove is dead. That’s the doomsday scenario, that’s absolutely not true,” said Robin Lewis, who runs an environmental consulting firm in Florida and studied the Puerto Rican spill in 1986. Even if some trees die, “mangroves recolonize areas relatively rapidly, even if there’s oil present.”
Lesson 2: It’s the soil.
Much has been written about the dire situation in the Mississippi River Delta, where oil has entered fragile wetlands in places like Barataria Bay. The slick has already had lethal consequences for turtles, seabirds, and other animals that have come in direct contact with it. But for the mangroves and grasses that bind the marshes together, the results are more complicated.
Scientists know that oil tends to leave plants unharmed when it doesn’t directly contact them. In the months following the massive 1991 oil spill in the Persian Gulf, researchers observed that submerged sea grasses had fared well, in all likelihood because the oil floated over top of them. The toxic components of the oil hadn’t seeped into the water.
One reason some observers are concerned about BP’s use of chemicals to disperse the oil in the Gulf of Mexico is that oil treated with dispersant is more likely to sink, rather than float over sea grass as it did in the Persian Gulf. As Louisiana State University scientist Irving Mendelssohn pointed out earlier this month in a long but informative Youtube video, the larger danger to plants is the risk of oil penetrating the sensitive, sandy soil where they place their roots. When roots die, the soil further erodes. And as was the case after the Amoco Cadiz oil spill off the French coast in 1978, soil erosion can delay the recovery of vegetation. In addition, humans trying to clean the marsh—and perhaps using heavy machinery to do so—might cause damage. “People tromping around pushing the oil into the sediments … makes it much worse,” Lewis said.
Like mangroves, marsh grasses can grow back as long as their roots stay untouched. But just how long that will take is difficult to predict. Five years after oil spilled from the Metula when untouched by clean-up crews in Chile, much of the marshes hadn’t grown back. Some areas, though, recovered within five months. The difference? Deader areas experienced heavier coats of oil.
Lesson 3: Get the oil off the beach, but don’t expect to remove it all.
Marshes and mangrove forests are just one ecosystem that BP’s spill will touch. Sand beaches, the other major environment where the oil will make landfall, require a much different approach.
There’s no question that it’s worth removing oil from a beach. In spills where the oil has not been removed, what remains is a layer of black, viscous tar with a cracked top layer of asphalt. Researchers returning to the site of the Metula spill in Chile two years later found what was essentially a paved roadway about two miles long and more than 500 yards wide. By 1997, 21 years after the spill, much of the asphalt had eroded, but portions remained, particularly in areas where seawater lapped up in low-energy waves.
Yet even when clean-up crews scrape oil off a beach, the impact of a spill can linger. For a study released this year, a team of Spanish researchers excavated the sand on a beach where, in 2002, the tanker Prestige had spilled oil off the coast of Spain. They found thick layers of oil-coated sand underneath the surface, sometimes three meters (about 12 feet) below ground. Oil from the 1989 Exxon Valdez spill has been found trapped underneath boulder-covered beaches in Alaska 10 years later. Scientists there have seen toxins from the oil seep into streams where salmon breed, while the populations of otters who dig on oiled beaches have recovered more slowly than populations elsewhere.
But Alaskan beaches are made mostly of gravel and boulders, not the fine grains of sand found on the gulf’s beaches. It’s known that oil can persist in finer sediments, too: researchers at a Massachusetts nonprofit found diesel fuel in the soil of marshes at West Falmouth in 2003, 30 years after a barge had spilled nearby. What happens in the gulf will be fodder for more study. Scientists there are already taking as many measurements as they can of the “before” picture of the spill. The nation nervously awaits the “after.”