Farther downstream, residents of the Russian border city of Khabarovsk bracing for the arrival of the polluted water vented their anger at China.
The shutdown Friday in Jiamusi, a city of about half a million people, came as China's chief environmental regulator resigned, taking the blame for the Nov. 13 chemical spill into the Songhua River in China's northeast.
The disaster has disrupted water supplies to millions of people living along the river and strained relations with Russia.
The benzene from a chemical plant explosion upstream is expected to reach Jiamusi on Tuesday, according to the government.
The city's No. 7 Water Plant "has been closed due to the possible contamination of the water supplies," said an official who answered the phone at the Jiamusi city government headquarters. He refused to give his name. The official Xinhua News Agency said the plant supplies 70-80 percent of the city's drinking water.
Jiamusi is the second-biggest Chinese city affected by the spill, after the major industrial center of Harbin upstream suspended running water for 3.8 million people for five days after benzene polluted the water supply.
Jiamusi also has access to deep wells that will not be affected by the contamination and so should be able to continue to supply drinking water, said an employee of the water company, who refused to give her name. But hundreds of villagers living near Jiamusi have also been ordered to stop using water from shallow wells on the river bank.
The contamination has prompted the Chinese government to ship thousands of bottles of drinking water to Jiamusi and other communities along the river and to send fire trucks and other vehicles to deliver water to residential neighborhoods.
Russian authorities expect the slick to cross the border Dec. 10 or 11, and three days later reach Khabarovsk, the largest Russian city in the spill's path and home to 580,000 people. Khabarovsk lies along the shores of the Amur river, which is fed by the Songhua River.
Natalya Zimina, spokeswoman for the Khabarovsk regional government, said authorities will shut down the water supply in Khabarovsk for about two days if toxin levels are deemed dangerous. The spill is expected to take about five days to pass through Khabarovsk.
The city is in a corner of Russia being forced to cope with China's dramatic economic development, and live with the runoff from some 80 million Chinese upriver. Anywhere from 60 to 93 percent of the Amur's pollution comes from China, said Boris Voronov, director of the Institute of Water and Ecological Problems in Khabarovsk.
"In China, there's wild economic growth," he said. "There are no measures to protect nature."
On Friday, residents of Khabarovsk pleaded with a Chinese diplomat for more information about the spill. About 15 demonstrators carried red signs outside the Chinese consulate in the city, chanting "The Amur isn't a Yellow River," a reference to China's northernmost river, which is plagued by high sediment.
China, anxious to control the diplomatic damage from the spill, on Saturday donated a railcar full of activated charcoal to Khabarovsk to help it purify the water. China's representative in this Far East city, Consul General Fan Xianrong, said China will help with whatever means we can.
"The Amur River is our common river and we of course have responsibilities that we need to take."
Chinese and Russian experts have set up a joint monitoring post on the river, Xinhua said.
The slick is slowing down and lengthening as the river freezes, the Chinese government says. Xinhua said Saturday that the slick, originally 50 miles long, now stretches for 90 miles.
Toxins are still some 18 times the allowable levels, Xinhua said.
On Friday, the director of China's State Environmental Protection Administration resigned after being blamed for the disaster, state media reported. The environmental agency "is responsible for the major losses that were caused," state television said.
There was no indication whether Communist Party leaders in Jilin province, where the plant was located, or neighboring Heilongjiang, which lies downstream, might face punishment. State media say leaders in Jilin withheld information about the spill and that once leaders in Heilongjiang learned about it, they failed to tell the public immediately.