Oil Spills Pollute Indefinitely and Invisibly, Study Says

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At the time of the accident, researchers assumed that oil would be naturally dispersed within a few months or years, said Reddy. However, surveys during the 1970s and in 1989 detected oil in marsh sediments providing strong evidence that this isn't always the case.

The original study carried out by Woods Hole researchers in the early 1970s, is "one of the classic oil spill studies, that informed public policy about how to consider the fate and effects of spilled oil," said John Farrington, a Woods Hole researcher who studied Buzzards Bay in the late 1980s.

That study was the first to show that "an oil slick might disappear as far as visual sighting on the surface of the water, but petroleum hydrocarbons could still persist...in sediments," he said.

The current study uses chemical testing techniques that were not available in earlier studies.

Reddy's team, working with colleagues from the U.S. Coast Guard Academy in New London, Connecticut, collected a 14-inch-deep (36-centimeter-deep) sediment core from the marshes impacted by the spill. The core was divided into small sections—less than an inch (2 centimeters)—and tested for the presence of oil.

Hidden Danger

The results confirmed that despite the otherwise pristine appearance of the marsh, oil residues remained. The team found no contamination in the first 2 inches (6 centimeters) of the sand and earth sample. However, the central section of the core, retrieved from 2 to 11 inches (6 to 28 centimeters) below the surface, contained diesel oil compounds.

Oil that has decomposed in the environment should show a different mixture of petroleum compounds to fresh oil, said Reddy. However, many typical diesel oil compounds were observed in the core sample. This suggests that the oil degraded very little over time.

"Bacteria and Mother Nature have not significantly weathered the oil," said Reddy.

In addition, some of the chemicals in the sample core were at the same high concentrations found directly following the 1969 accident, he said.

"At the time of the spill, I doubt many people would have been able to predict the oil was still present after 30 years," said Reddy. "This study shows that oil can last for a long time, and is important when assessing the fate and clean-up of future spills."

Long-Term Effects

The reason for the oil's persistence at this site could be due to the lack of oxygen or sulfate compounds in marsh sediments, said Reddy, which many oil-decomposing bacteria need to survive.

"These results are helping us understand the long-term fate and persistence of oil in these sensitive habitats," said oil spill expert Jacqueline Michel, president of Research Planning, Inc., an environmental research company in Columbia, South Carolina.

The findings confirm what many scientists suspected, she said. "Deeply penetrated oil in [oxygen depleted] marsh soil persists for long times."

The next question, said Michel, is what are the ecological consequences of this long-lasting contamination?

Despite the fact that oil persists in high concentrations in some marsh sediments, it's important to note that many of the less heavily contaminated areas of Buzzards Bay showed little trace of oil after ten years, said Farrington.

"If a critical habitat happens to be the one with the longest lasting of the spilled oil then there will be long-lasting effects. If not, then the long-lasting effects may be less severe," he said.

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