Oil Spills Pollute Indefinitely and Invisibly

 

The Prestige oil tanker, carrying 76 million litres of fuel oil, sank off the northern coast of Spain in November 2002, releasing at least 3 million litres of oil into the waters of an extremely rich fishing region.

A report published earlier shows that in sensitive near-shore environments, the effects of an oil spill can be seen even decades later.

The findings come from a study of the aftermath of an accident that occurred in Buzzards Bay, Massachusetts, on a foggy morning in September 1969. A Boston-bound barge entering the Cape Cod Canal ran aground on rocks, spilling 700 000 litres of diesel fuel into the bay. The Prestige sank in waters that are more than 3,2 kilometres deep, about 241 kilometres off shore. Still, so far, more than 241 kilometres of beaches and coves have been fouled.

Evidence from the Buzzards Bay disaster suggests the effects of oil spills could be indefinite. Thirty years after the Massachusetts catastrophe, significant oil residues remain in local salt marsh sediments. It is clear from this study that oil spills can have a long-term impact on a coastal environment, marine chemists say.

Even after all these years, concentrations of some compounds (in at least one Buzzards Bay site) are similar to those observed immediately after the spill, and reflect the persistent nature of the oil in coastal salt marsh sediments.

Parts of Buzzards Bay were heavily contaminated by the brown, viscous oil. Fish, worms, crabs, molluscs, and other animals perished in great numbers, along with oil-smothered marsh grasses. Residents of the nearby town West Falmouth, referred to the following months as the "silent fall", referring to the absence of the usually noisy grasshoppers, waterfowl, and other animals normally in the area.

At the time of the accident, researchers assumed that oil would be naturally dispersed within a few months or years. However, surveys during the 1970's and in 1989 detected oil in marsh sediments providing strong evidence that this isn't always the case. Oil slick might disappear as far as visual sighting on the surface of the water, but petroleum hydrocarbons could still persist in sediments.

To study the consequences of the incident with Prestige oil tanker scientists collected a 36-centimetre-deep sediment core from the marshes impacted by the spill. The core was divided into small sections – less than 2 centimetres – and tested for the presence of oil.

The results confirmed that despite the otherwise pristine appearance of the marsh, oil residues remained. The team found no contamination in the first 6 centimetres of the sand and earth sample. However, the central section of the
core, retrieved from 6 to 28 centimetres 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, scientists say. 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.

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

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

The reason for the oil's persistence at this site could be due to the lack of oxygen or sulphate compounds in marsh sediments, which many oil-decomposing bacteria need to survive. Thus, the findings confirm what many scientists suspected: deeply penetrated oil in oxygen depleted marsh soil persists for long times. These results are helping understand the long-term fate and persistence of oil in these sensitive habitats

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

John Pickrell. National Geographic. 2002

 

 

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