By Tyra Dempster

DUJIANGYAN, China, May 29 (Reuters) - Five thousand tonnes of dangerous chemicals is adding to the mix of threats downstream from one of China's "quake lakes" in danger of bursting their banks, a newspaper said on Thursday.

About 5,000 tonnes of chemicals, including sulphuric and hydrochloric acid, were trapped downstream from the Tangjiashan lake and had to be moved to safe ground, the Beijing News said, citing local environmental authorities.

China has evacuated more than 150,000 people living below the swollen Tangjiashan lake formed by this month's devastating earthquake amid fears it could burst and trigger massive flooding.

The chemicals, adding pollution to the threat of flooding, were stranded in different work sites downstream, the newspaper said.

But radioactive materials had been largely recovered, the newspaper cited Ma Ning, director of the Southwest China Environmental Protection Supervision Centre, as saying. The disaster area is home to China's chief nuclear weapons research lab in Mianyang, as well as several secretive atomic sites, but no nuclear power stations.

The Tangjiashan lake was created when landslides blocked the Jianjiang river above the town and county of Beichuan in mountainous Sichuan province, near the epicentre of China's most destructive earthquake in decades.

SUDDEN BREAK

Alexander Densmore, a seismologist at Durham University in Britain, said any break in a quake lake would likely be sudden.

"It's a very real problem," said Densmore by telephone. "These landslide dams pose a really significant risk in these mountain regions, and in these narrow valleys it doesn't take much material to create a complete blockage."

Once a breach occurred, there could be an accelerating process leading to a sudden rush of water downstream.

"Once that process starts, it's virtually impossible to do anything to decrease the water... When they fail, they tend to fail catastrophically," he said of the quake dams.

Given the topography of Sichuan, with the western mountain country giving way to plains around Mianyang, a major rush of water could spill downstream and possibly affect lower-lying areas of cities such as Mianyang, he said.

The region along the faultline is densely packed with dams, raising concerns that if either the quake lakes or the weakened dams burst, the rush of water could cause others to fail.

The official death toll from the 7.9 magnitude quake is already more than 68,000 and is certain to rise further, with nearly 20,000 listed as missing. Aftershocks on Tuesday toppled 420,000 houses, many already uninhabitable.

A massive relief effort, which involves providing food, tents and clothing for millions and the reconstruction of housing and infrastructure, is expected to take up to three years.

Thousands of injured have been transferred to other provinces and the capital for treatment to ease the pressure on local hospitals.

Song Liangwei, 9, had always dreamed of visiting Beijing, but not as a quake victim.

"I wanted to go to Tiananmen Square, to climb the Great Wall, and to watch the Olympic Games," he said. (Writing by Nick Macfie; Additional reporting by Lindsay Beck, Chris Buckley and Phyllis Xu in Beijing; Editing by Bill Tarrant)

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