Widespread contamination prevention efforts have been underway in China and Russia since an explosion on November 13 at a PetroChina chemical factory in the northeast Chinese province of Jilin.

The accident led to the spillage of 100 tonnes of the carcinogens benzene and nitrobenzene into the Songhua River, one of China's longest waterways and a source of water for millions.

Russian authorities are particularly concerned about the potential contamination of the water supply for more than 600,000 people in Khabarovsk.

Russia has begun building a a temporary dam to protect the city, and has suggested that China do the same, according to a Chinese foreign ministry official and state media Thursday.

The benzene slick is expected to enter the Amur river bordering Russia on December 19 before continuing on towards Khabarovsk.

"These calculations have been made based on information provided by the Chinese," said Alexandr Agrilov, the head of the local meteorological and hydrological station.

The slick was being slowed by icy conditions, he added, and was currently moving at a speed of 1.3 kilometres (0.8 miles) per hour.

On Thursday night the spill would remain some 220 kilometres (136 miles) away from the area where the Songhua River flows into the Amur, Agrilov predicted.

Though the current concentration of benzene in the 130-kilometre (81-mile) slick is over 15 times authorised levels, experts believe that it will fall to just two times the normal limit by the time it reaches Khabarovsk.

A dyke has meanwhile been constructed to try to keep the pollution away from Khabarovsk's drinking water sources and authorities plan cut off all supplies from the Amur river as the pollution passes by.

However if benzene levels fall to only two times the normal limit then special active carbon filters will be used to purify the river water, experts say.

On Thursday the chemical slick arrived in the northeastern Chinese city of Jiamusi, the second largest city in Heilongjiang province with over two million people, the Chinese state news agency Xinhua said.

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