BEIJING, Nov. 24 - The Chinese government's decision to cut potentially contaminated supplies of fresh water to a major city has highlighted the threat that industrial pollution poses to public health and economic development across the nation.

Almost four million people in Harbin in northeastern China are expected to be without running water until late Saturday after a chemical plant explosion on Nov. 13 contaminated the upper reaches of the nearby Songhua River with toxic benzene.

A 50-mile stretch of the river carrying the benzene reached Harbin this morning, Shi Zhongxin, director of the city's water bureau, said on state television, according to the Associated Press. The contaminated water was expected to take 40 hours to make its way through the city.

State media reported Wednesday that the local government ordered the shutdown starting at midnight Tuesday in Harbin, which is internationally known for its annual ice sculpture festival in January.

China's Environmental Agency confirmed that the river, which supplies the city, had suffered "major water pollution," the official New China News Agency said late Wednesday. But contaminated water had not reached the city, it added.

Before water was disconnected, residents were encouraged to store water in buckets and other containers, while the local authorities trucked in thousands of tons of bottled water. In panic buying Monday and Tuesday, customers stripped supermarkets and stores of bottled water and other beverages.

The airport and railroad stations were reported Wednesday to be jammed as residents tried to leave.

The New China News Agency reported that schools would be closed until Nov. 30, while 15 local hospitals had been placed on standby to handle any poisoning cases.

On Wednesday evening, Harbin temporarily restored water supplies to allow residents to stock up.

The shutdown is a potential threat to heating systems in Harbin, one of China's coldest cities, where day temperatures are already below freezing as winter approaches. The local authorities have ordered heating companies to ensure that they have adequate reserves of water from wells to maintain supplies of hot water to buildings.

The chemical plant explosion, 236 miles upriver, killed 5 people and forced 10,000 others to evacuate, the state media reported.

The threat of contamination to Harbin is a reminder that with its booming economy, China is facing a huge environmental challenge.

The combination of rapid industrialization, a vast population and intensive agriculture has led to some of the world's worst air pollution, widespread shortages of fresh water and soil degradation.

Pollution and contamination have exacerbated China's water shortages, which environmental experts and even senior officials say could threaten economic development. Data from monitoring stations in the country's seven major river drainage zones showed that 44 percent of rivers were polluted.

"Many lakes and water courses contain an excess of nutrients and need treatment before they are suitable as freshwater sources," the Organization for Economic Cooperation and Development said in a Nov. 14 report on Chinese agriculture.

Senior Chinese leaders, including President Hu Jintao and Prime Minister Wen Jiabao, have adopted environmental protection as a government priority, and they have repeatedly called for China to switch to economically sustainable development policies.

Specialists say China has some of the best environmental laws in the world, but the sheer scale of development, inadequate planning, corruption and poor enforcement often result in uncontrolled pollution.