HARBIN, China, Nov. 24 - The Chinese government on Thursday blamed the country's biggest oil company for a pollution spill that allowed a 50-mile slick of toxic benzene to reach this northern city of almost four million people on the river that normally supplies it with running water.
Residents continued to stockpile bottled drinking water on Thursday, the second day after the authorities shut down the municipal water system and stopped pumping from the Songhua River to minimize the risk of poisoning.
Schools and many businesses remained closed, and restaurants in the city center were mostly empty late Thursday as the environmental disaster led the authorities to mount an investigation that could prompt a criminal charges.
Authorities in Harbin, the capital of Heilongjiang Province, assured residents that adequate supplies of drinking water would be trucked in and warned them to watch for the symptoms of benzene poisoning.
They said that the slick was expected to have passed the city by Saturday and that normal water service could resume by Sunday.
China has also warned Russia about the toxic spill, which is being carried toward Khabarovsk, on Russia's border.
An explosion on Nov. 13 at a China National Petroleum Corporation plant in Jilin Province, 236 miles upriver from Harbin, spewed an estimated 100 tons of benzene compounds into the Songhua River, Chinese authorities said.
The state news media reported that five people had been killed in the blast, which was a few hundred yards from the riverbank. As many as 10,000 people were temporarily evacuated.
"We will be very clear about who's responsible," said Zhang Lijun, deputy director of the State Environmental Protection Administration, at a news conference in Beijing. "It is the chemical plant of the C.N.P.C. in Jilin Province," he said, referring to China National Petroleum.
Mr. Zhang said the investigation would consider whether there was any criminal liability for the spill.
PetroChina Company, a subsidiary of the state-owned China National Petroleum listed in New York and Hong Kong, is responsible for the company's domestic petrochemical production, the China National Petroleum Web site says. China National Petroleum holds 90 percent of PetroChina's shares.
The official New China News Agency reported that China National Petroleum had apologized. The company "deeply regrets" the spill and will take responsibility for handling the consequences, the deputy general manager, Zeng Yukang, was quoted as saying. The vice governor of Jilin Province, Jiao Zhengzhong, also apologized to the people of Harbin, The Beijing News reported Thursday.
The contamination of one of China's major rivers has drawn attention to the environmental price that the country is paying for a three-decade economic boom. Living standards have sharply risen in many regions of China, particularly the provinces on the east coast, but severe environmental degradation threatens further development.
China's major cities are among the most polluted in the world, and vast tracts of farmland are being lost to erosion, desertification and industry.
However, the pollution of vital rivers, lakes and groundwater in China, which is already short of water, looms as the biggest immediate threat, environmental specialists say.
Even before the benzene spill, the Songhua River had serious problems with water quality, the Asian Development Bank said.
The bank is helping local authorities develop plans for pollution control on the river.
The environmental impact of development is also leading China's authoritarian government to become increasingly open with a public that has become more vocal about pollution and ill-considered industrial projects. Private environmental organizations are flourishing and have occasionally persuaded authorities to block development or revise development plans.
Provincial and central government environmental agencies reported that benzene and nitrobenzene contamination much higher than allowed levels had been detected upstream from Harbin.
The official China Daily reported Thursday that specialists had said that the authorities had no choice but to cut water supplies to the city.
"Harbin's move to cut off the water supply was not a knee-jerk reaction," Zhang Lanying, an environmental specialist at Jilin University, was quoted as saying.
"If the contaminated water had been supplied to households, the result would have been unimaginable."
The spill could also have diplomatic repercussions as it heads toward the point where the Songhua River joins the Heilongjiang River, which then crosses the Russian border in the region of Khabarovsk, about 340 miles from Harbin.
Russian officials have begun testing water in the Heilongjiang River. Senior Chinese officials rejected suggestions that it had been unfair to wait until this week to inform Russia about the spill.
Mr. Zhang, from the Environmental Protection Administration, said it would be another 14 days before the contamination reached the Heilongjiang. "So we don't think we were late in providing information," he said.
A Chinese Foreign Ministry spokesman, Liu Jianchao, said that the Russian Embassy in Beijing was briefed twice this week and that the two sides had set up a hot line to share information.