The official Xinhua News Agency says authorities detected unsafe amounts of the substances in a section of the Hejiang River in Guangdong province after dead fish turned up in the water.
It said today that the Fengkai county government has warned the local water plant and residents against using the water or eating anything from the river. Cadmium is considered an environmental hazard, and thallium is extremely toxic.
Xinhua said the pollutants were believed to have come from upstream, but didn't say how they got into the river.