Bygone Pollutants Resurface as Glaciers Melt

Pollutants trapped in glaciers in decades past are re-entering the environment as the ice melts some 30 years later.

A new study of sediments from a lake in the Swiss Alps shows that levels of many persistent pollutants, including PCBs, dioxins and several chlorine-containing pesticides including DDT have increased since the late 1990s after declining in the 1980s due to bans on some of the chemicals and emissions controls on others.

"First we saw a peak around the 1970s, which could be attributed to use and emissions of these chemicals then," said study author Christian Bogdal of the Swiss Federal Institute of Technology Zurich. "Then the concentrations decreased because improvements were made. Since the 1990s, we see an increase again due to the release of these chemicals."

The team found the same pattern for all nine chemicals they studied. Indeed, levels today are as high in some cases as they were when the chemicals were in use in the 1970s.

One problem with this class of pollutants that they do not break down in the environment, and they remain in the fat tissues of animals and humans where they accumulate as they move up the food chain.

"The release is still going on," he added. "We have indications that there is still a reservoir of chemicals at this location."

The pollutants clearly come from the glacial melt water, the team showed. They compared levels in a nearby lake not fed by glaciers. The sediment records in the other lake showed a peak in the 1960s and 70s, but no peak today.

The team published their results in Environmental Science and Technology.

People use Alpine lakes for irrigation and harvest the lakes' fish, both of which could raise health concerns, Bogdal said. But the fact that lower altitude lakes do not show concentration peaks in modern times suggests that the release is quickly diluted by other water sources.

The problem is likely to persist in the Alps, where ice is melting rapidly. Twelve percent of glacier ice there melted between 1999 and 2008.

"One a global scale, I cannot say that we will have an enormous contamination in the future," Bogdal said. "But we must think about all the ice that's present in the polar regions, which will also melt one day if the temperature increases."

Even though polar regions are remote, research has shown that these persistent organic pollutants travel globally and are deposited in remote regions. Some pollutants are even deposited more efficiently at high elevations than at low ones.

The work shows for the first time that glaciers can delay the delivery of pollutants into the environment by decades, noted Frank Wania of the University of Toronto at Scarborough, who was not a part of the new study.

Even without global warming, the pollutants would eventually make their way back out of the ice through freezing and thawing cycles, he said. But warming accelerates the process, and would cause the pollutants to re-emerge over a shorter time frame.

"This may be why some of the compounds have higher levels now than in the 60s," said Wania.