Harbin residents shop for bottled water yesterday.-Xinhua

An 80-kilometre swathe of polluted water in the Songhua River was expected to reach the water-sourcing area of the city at about 5 am today, officials said yesterday in the first detailed explanation for taps running dry.

The worst stretch of contaminated water will pass the capital of Northeast China's Heilongjiang Province on Saturday morning, an official with the Provincial Environment Protection Bureau said yesterday.

The State Environment Protection Administration confirmed yesterday that pollutants containing benzene and nitrobenzene contaminated the river after a chemical-plant blast at the upper reaches, leaving a trail of dead fish.

The explosion on November 13 in Jilin city in neighbouring Jilin Province forced Harbin, a city of 3.8 million which relies largely on the Songhua for its water and is situated on the middle reaches of the river, to suspend supply for four days starting on Tuesday midnight.

On November 20, a water-checking station in Zhaoyuan County on the provincial border found that the level for the two chemicals was much higher than the State standard, said Li Ping, the Heilongjiang environmental bureau spokesman.

At one time, the nitrobenzene level was 103.6 times higher than normal; and the contaminated belt of water still considered dangerous to humans was flowing at about 2 kilometres per hour, the bureau said.

Zhang Lanying, director of the Environment and Resources Institute of Jilin University, said that benzene which does not dissolve in water is a dangerous substance.

People who drink water with a little benzene can have oral festering. "Massive amounts can lead to the disorder of blood cells; in other words, leukaemia," she said.

"Harbin's move to cut off the water supply was not a knee-jerk reaction.

"If the contaminated water had been supplied to households, the result would have been unimaginable."

To speed up the flow of the river and dilute the contamination, Jilin city has increased the water discharge volume of Fengman Hydroelectric Power Plant, said a spokesman.

As for cities and counties downstream of Harbin, 90 per cent rely on groundwater as the main water source, said Dong Shuhua, director of the Heilongjiang Water Information Bureau.

"They will not be very much affected."

In Harbin, the panic buying of water and attempts to get out of the city were not as frantic yesterday as in previous days.

Adding to residents' relief was 15 hours of water supply to parts of the city and availability of bottled water in shops and supermarkets.

Zhao Wanxia, a saleswoman, said she had stored enough water to last for a few days. "I first wash vegetables, then use the water to clean the floor or flush the toilet."

More than 16,000 tons of bottled water was being transported to Harbin from neighbouring cities including 10 train carriages carrying 1,000 tons from Shenyang, capital of Northeast China's Liaoning Province.

The city's 918 wells are all in operation and teams from Daqing Oilfield are drilling another 100 wells, said Wang Zhengbang, deputy secretary of the Harbin municipal government.

The government has designated 10 hospitals to treat patients in case they drink contaminated water.

The government has earmarked 10 million yuan (US$1.2 million) to deal with emergencies; and allocated 1 million yuan (US$120,000) to help the poor, elderly and those living alone.

Wang urged residents to keep away from the riverbank for fear of contamination.

Despite the water cut-off, officials have assured residents of normal operation of the city's heating system.

But they told people yesterday not to take water from the system for household use.