A million people in Yancheng, Jiangsu province, were left without water on Friday after the supply became contaminated with toxic waste from a chemical factory.
Authorities turned off the taps at about 7:20 am, and the supply returned in some areas at 3 pm.
Supplies from the Yancheng tap water company, one of three firms serving 800,000 people in the Yandu and Tinghu districts of the city, were contaminated by carbolic acid, the government said.
Investigations showed that a factory in the Anhu community of Yancheng's Longgang township spilled carbolic acid into the Mangshe River, China National Radio reported.
Ding Changhui, head of the city's health inspection institute, confirmed the pollution was caused by the acid, but offered no further information.
Calls to the Yancheng tap water company on Friday went unanswered.
The municipal government has sent out four teams to investigate the incident and restore the water supply.
In the meantime, locals have been stocking up on bottled water, although online reports have said retailers have been profiteering from the accident, with the price of bottle of water soaring from 2 yuan (30 US cents) to 25 yuan.
A separate water company, located in eastern Yancheng, has been asked to increase its supply.
Local man Yin Ming said his tap water "had a chemical smell" Friday morning before the supply was cut.
Authorities have said the water supply to other parts of Yancheng is unaffected.
Jiang Yong, who lives in southern Yandu, said: "The government released a notice around noon, saying that water from the west tap water company was contaminated, but that ours was fine."