Thousands of residents of Harbin, the Chinese city left without water because of toxic chemicals in its water supply, were making their way out of the municipality today as the poisonous slick wound its way down the city's main river.

The 80km-long (50-mile-long) patch of benzene-polluted water entered Harbin late last night after local officials shut down the city's water supply at midnight on Tuesday. Officials said that the benzene, a carcinogen that is lethal in high doses, was expected to take 40 hours to pass through.

Jane Macartney, The Times China Bureau Chief, is in Harbin, the capital of the northeastern province of Heilongjiang and home to four million people. She said that the initial panic of the water shortage has given way to a busy but orderly exodus.

"The stations are crowded with people trying to leave," she said today. "A lot of trains are completely full, sold out even of standing tickets. The airport is the same, with all the flights booked out, even though the government has laid on extra flights. On our flight from Beijing into Harbin yesterday, there were only six other passengers coming into the city.

"The majority of people are just leaving Harbin and going home to the surrounding countryside. Many of the peasants who have come to work in the city are going back to their villages to see the shortage out."

Yesterday, the provincial government gave the first official explanation for the shutdown, saying benzene, an industrial solvent, had rushed into the icy Songhua River after an explosion at a chemical plant in nearby city of Jilin on November 13. Contamination in the river was 29 times higher than safety levels.

Macartney said that the government's response to the emergency had seen the city overwhelmed with bottled water. Harbin's taps will remain shut off for four more days.

"The place is absolutely barging with bottled water. I have never seen so many plastic bottles in my life. The supermarkets are overflowing with shopping trolleys full of bottles, the streets have filled with vendors and carts selling bottles. They are everywhere," she said. "There are water tankers in the streets and they are making their way to the apartment blocks."

Earlier, officials put Harbin's 15 hospitals on alert for cases of water poisoning. Short-term exposure to benzene can cause drowsiness, dizziness and unconsciousness while heavy consumption can cause leukaemia.

But Macartney said there were no reported cases so far: "There are no reported cases of poisoning yet but it would be impossible to verify them if there were," she said.

"That said, the government has improved its communication since the beginning of the shortage. In the beginning, the taps were just turned off and no one knew why. That's what led to mass panic-buying. But since then, the government has shared more information. Certainly the local newspapers are filled front to back with stories about the shortage. There's an overkill of information."

Senior officials in Beijing today put the blame for the pollution squarely on the shoulders of the China National Petroleum Corporation (CNPC), which runs the chemical plant in Jilin, 193km (120 miles) southeast of Harbin.

"We will be very clear about who’s responsible. It is the chemical plant of the CNPC in Jilin province," said Zhang Lijun, deputy director of the State Environmental Protection Administration, who told reporters that the company might be charged with criminal responsibility.

The explosion, which forced the evacuation of 10,000 people, occurred in a tower that processed benzene. The first sign of serious water pollution was a trail of dead fish, according to the official China Daily newspaper. On November 20, a monitoring station close to the accident found contamination levels 103.6 times higher than normal.

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