Dec. 17, 2008 -- Next year will mark the 20th anniversary of the Exxon Valdez oil spill - perhaps the most notorious human-caused environmental disaster in history.
But, according to the latest survey of Prince William Sound in Alaska - where the oil tanker foundered in 1989 - very little oil remains and most of what does is not in a form or location that can harm animals, plants or humans.
Although scientists funded by Exxon and others working for Greenpeace agree on these facts, they are still at odds over whether the area can be given a clean bill of health.
Paul Boehm of Exponent International, a scientific consultancy that specialises in chemical contamination, led the survey together with colleagues from two other private companies and two US research universities. The study received funding from the Exxon Mobil Corporation.
Boehm and colleagues collected over 700 samples from 25 sites throughout the sound that were known to have been heavily contaminated by the 1989 spill. Two of the sites are now actively foraged by sea otters, they say.
"We found that the remnants from the spill today are found in small patches at very few beaches," says Boehm. The survey showed that the oil which does remain is deep in cracks between boulders and pebbles, and much of it is degraded.
The team concludes that while some oil does remain in the sound, most is not "bio-available" - that is, not in a chemical form that can be assimilated by animals, nor in locations where animals are likely to come into contact with it.
Olof Linden of the World Maritime University in Sweden and former scientific advisor the UN environment programme says the research is sound. "I am surprised that they even find any oil at all after 18 years," he says. "Biologically it is of practically no significance."
However, David Santillo of the Greenpeace Research Laboratories at the University of Exeter in the UK is reluctant to give the area a clean bill of health.
"The jury is still out over whether the levels of exposure are harmful to fish and mammals," he says. "In the long term, we don't know what the effects will be on the species' reproduction."
The debate is not whether the spill has any lasting consequences - some groups say local killer whale populations have still not recovered from losing pod members to the spill. Rather, it is whether the oil residues that remain are still causing harm.
Boehm says he has carried out multiple biological investigations over the last decade - which have been confirmed by government scientists. "All of these government and Exxon-funded studies consistently show that the animals that wildlife use for prey are as clean as any in un-oiled locations in Prince William Sound," he says.
After a spill, the oil which is not cleaned up manually is dispersed and degraded by the elements. How fast this happens depends on the local temperatures and conditions such as how still the water is.
Residues that seep deep into sediment and end up in places that are low in oxygen degrade more slowly and can therefore be detected decades later.
ExxonMobil says that since 1989 over $1 billion "largely underwritten by ExxonMobil settlement funds" has been spent investigating the question of ecological damage following the Valdez oil spill.
"The environment in Prince William Sound is healthy, robust and thriving," said an Exxon spokesperson.