The toxic slick contains the chemical cadmium, caused by excessive discharge from a state-owned smelting works into the Beijiang River in Shaoguan city, Guangdong province, Xinhua news agency reported.
The Beijiang River is a major source of drinking water for northern Guangdong province. It flows into the Pearl River, which runs through Guangzhou, a metropolis of 10 million people.
Xinhua said the slick had reached northern Yingde, 55 miles from the site of the spill. Government broadcasts on local television warned residents not to drink the tap water, though officials said they had lowered a dam gate to keep the polluted water out of the drinking water supply.
Cadmium is a chemical used in protective plating. Serious exposure can cause liver and kidney damage and bone diseases.
Last month an explosion at a benzene plant caused a toxic spill in northeast China's Heilongjiang province, forcing Harbin and other cities to shut down water supplies for days.