By Louisa Lim
BBC News, Beijing
The authorities in southern China say a toxic slick in the Bei river has been stopped by a dam, ensuring the water remains safe to use downstream.
Officials are reassuring inhabitants in the city of Guangzhou that its water supplies will be guaranteed.
The head of the smelting plant that discharged the poisonous waste has reportedly been removed from his post.
It is the second case of industrial pollution contaminating a major Chinese river in two months.
This latest environmental crisis emerged after a state-run smelting plant spewed toxic cadmium into the Bei river, which supplies water to southern Guangdong province.
Several cities had their water supplies turned off temporarily. But now local authorities say the slick has been stopped by a dam, and there are plans to release clean water from a second dam to dilute the cadmium.
chemical used in batteries
exposure can cause liver and kidney damage
affects central nervous and immune systems
leads to bone disease
The authorities say they will not reopen the dam gates until water returns to safe levels.
Local newspapers say the head of the smelting plant in the city of Shaoguan has been temporarily removed from his post, and 14 other smaller smelting factories closed down.
But in this manufacturing hub, factories line the rivers, often using it as somewhere to dump untreated industrial waste.
This is the second such disaster in two months, vividly illustrating how China's rampant development is polluting the environment.
The central government says it is trying to clean up the damage, but its regulations are often ignored by local governments who are more concerned with ensuring economic growth.