A mining company has been identified as the source of heavy metal pollution in a south China river, local authorities said Monday.

A mining company in the city of Hezhou in the Guangxi Zhuang Autonomous Region is believed to have used illegal production processes that resulted in excessive thallium and cadmium pollution in the Hejiang River, the Hezhou municipal government said in a press release.

The company's head, a man surnamed Gong, has been taken into police custody, said Li Weizhang, vice mayor of Hezhou.

Gong is suspected of adding indium production facilities to his iron ore processing plant without approval, Li said.

Indium refinement produces effluents that contain thallium and cadmium, both of which are toxic.

The pollution was discovered after local authorities in Fengkai County in Guangdong Province, which neighbors Guangxi, said Saturday that water in a section of the river had been tainted by upstream pollution.

The county warned local tap water plants and residents living on the lower reaches of the Hejiang and Xijiang rivers to avoid drinking water or consuming products derived from the river.

Cadmium and thallium levels at a reservoir located on the river were four times and 1.3 times the recommended level, respectively, as of Monday, according to Yang Zhongxiong, vice head of Hezhou's environmental protection bureau.

The Hejiang River winds across Guangxi before emptying into the Xijiang River in Guangdong. Xijiang is a major waterway in Guangdong.

Environmental authorities in Guangdong said Monday that they had detected elevated thallium levels on the Fengkai section of the Hejiang River, adding that water quality in downstream areas remains normal.