The explosion at a PetroChina factory in the northwest province of Jilin led to an outpouring of the carcinogen benzene into the 1897km-long Songhua river, the Environmental Protection Administration said on Wednesday.
"After the blast at the chemical plant the monitoring station in Jilin found that benzene went into the river and polluted the water," the EPA said in a statement on its website.
"Benzene levels were 108 times above national safety levels."
The announcement came 10 days after the November 13 explosion and followed repeated denials from government officials that there had been any major environmental impact.
In Harbin, the capital of neighbouring Heilongjiang province about 380km downstream from the blast site, water supplies were cut off at midnight on Tuesday over the pollution fears, the local government said.
The Songhua is the main source of drinking water for more than three million urban residents of Harbin, which has a total population of about nine million and is one of the biggest cities in China.
The EPA said the pollutants in the water were expected to hit Harbin around 1900 GMT on Wednesday although the contaminants would be diluted.
The polluting material index had dropped to 29 times above national safety levels when the contaminants reached the border of Jilin and Heilongjiang on Sunday, the EPA said.
However, the polluted water flowed past Songyuan and Zhaoyuan, two big cities between the blast site and Harbin that lie along the Songhua, before the government admitted to the contamination.
The spill now poses a threat to
both Chinese and Russian towns
Songyuan has a population of just under three million people and Zhaoyuan about 450,000. An official from Songyuan city government told AFP on Wednesday that its water supplies had not been cut over the past 10 days.
Russian government officials in the east of their country also said on Wednesday they were monitoring the Amur river, of which the Songhua is a tributary, for toxic substances.
The officials said the Songhua (called Sungari in Russia) was the main source of drinking water for the Russian city of Khabarovsk, home to 600,000 residents, just across the Chinese border about 600km from Harbin.
"Fairly large quantities of benzol (benzene), a potent carcinogen, have clearly been dumped into the water," the head of the Russian Far East office of conservation organisation WWF, Yuri Darman said.
"This water will reach Khabarovsk within three or four days."
Benzene is a carcinogen that can be lethal if someone is exposed to high levels, even in small doses, according to the US National Library of Medicine's website.
In Harbin, there had been scenes of panic since the weekend when rumours first surfaced that water supplies would be cut off.
With initial government denials over contaminated water being the reason for the stoppage, residents were left to speculate about a potential imminent earthquake as they hoarded water and food.
"Benzene levels were 108 times above national safety levels"
Statement of China's Environmental Protection Administration statement
The government continued to scramble on Wednesday to import bottled water from other regions. Xinhua news agency said the city's drinking needs were 18,600 tonnes a day.
Locals were also beginning to dig wells in order to tap underground water tables, Xinhua said.
Schools in Harbin were closed for a week, although hospitals remained open.
The Harbin government did not say how long the water supplies would be cut, but a Xinhua report on Wednesday evening citing a local official said the poisoned strip of water would flow out of the city on Saturday morning.
The 13 November explosion at the PetroChina petrochemical plant killed at least five people and led to the temporary evacuation of thousands of residents amid fears of the emission of toxic gases.
However, all the residents were allowed to return home shortly after the accident without any public statement from the government about the water contamination.