The Hejiang River in Hezhou in the Guangxi Zhuang autonomous region was found to be contaminated with thallium and cadmium, considered carcinogenic and poisonous pollutants, China Daily reported.
Xu Zhencheng, vice director of the South China Institute of Environmental Sciences of the Ministry of Environmental Protection, said Sunday 68 miles of the Hejiang River was polluted and the peak concentration of cadmium in the river was 5.6 times the allowed level.
One-hundred-12 illegal metal mines along the Maweihe River, a branch of the Hejiang, were ordered to suspend production Sunday by the Hezhou government. China Daily reported 79 of the mines were confirmed to be sources of pollution and their industrial waste flowed to the Hejiang River because of recent floods.
"The water will recover itself in about 20 days, and the situation is still under control," Xu said, noting it was not necessary to neutralize the pollutants by adding more chemicals to the river.
Until then, the situation keeps some 30,000 people downstream in Fengkai County in Guangdong province from using river water and also could put tens of millions more people on alert in the downstream Pearl River Delta.
It is unclear for how long the thallium and cadmium have been present in the Hejiang River.
While dead fish were found in some parts of the river starting last Monday, China Daily noted the Hezhou environmental protection bureau did not detect any problem with the water.
But as the fish deaths continued, tests conducted Friday showed cadmium levels exceeded standards by 1.9 times and thallium by 2.14 times.
"It is a pity the environmental watchdog and local residents were left in the dark while serious pollution that threatens public health was present," the newspaper quoted Zhao Zhangyuan, a senior researcher with the Beijing-based Chinese Academy of Environmental Sciences, as saying.
"Although the pollutants are steadily declining now, the expert team and local authorities must act to remedy the ecology and accelerate the recovery of the river water, rather than waiting for it to recover itself," Zhao said.
China's water authority in a report last February said as much as 40 percent of the country's rivers were seriously polluted. Furthermore, it said about 20 percent of China's rivers were so polluted the water quality was considered too toxic for contact.
In April, carcasses of thousands of pigs were found in Shanghai's Huangpu River.