Water and environmental authorities in the Guangxi Zhuang Autonomous Region are working to control cadmium pollution in a local river, while ruling out the possibility that water sources for Hong Kong and Macau have been contaminated.

Cadmium pollution may be double the restricted level in the downtown section of the Liujiang River in Liuzhou City, but authorities are making efforts to contain the contamination within the required standard, said Xu Zhencheng, an expert with the task force handling the emergency.

Xu is a deputy chief of the South China Environmental Science Institute under the Ministry of Environmental Protection.

Liuzhou Mayor Zheng Junkang said the water supply for residents in Liuzhou remained safe by mid-day yesterday.

"We will spare no effort in ensuring the safety of drinking water," Zheng said. "In case the tap water supply is suspended, the government will do its best to resume supply in the shortest time possible."

The mayor said the government activated a contingency plan to tap into the 3.5 tons of underground water reserves once the original drinking water source was polluted.

Cadmium is a carcinogenic chemical mostly used in industrial effluents.

The pollutant was detected in the Liujiang River on Thursday afternoon, more than 10 days after industrial waste from Guangxi Jinhe Mining Co Ltd spilled it into the Longjiang River, a tributary upstream of the Liujiang River, according to the city's environmental protection bureau.

Substances have been dumped into a dam on the Longjiang River in order to promote the flocculation of the pollutants. The chemicals can help facilitate the dredging of the pollutants when they clump together.

In an aim to curb potential sources of new pollution, environmental authorities in Hechi City, where the contamination originated, have ordered a suspension of production at all seven heavy metal producers in the upstream section of the Longjiang River.

More than 60 percent of cadmium content in the Longjiang River in Hechi, which was previously reported as five times higher than the restricted level, has been diluted and absorbed, according to experts.

They said no contamination has been spotted in the Qianjiang, Xunjiang and Xijiang rivers. The Xijiang is a trunk of the Pearl River, which is a major water source for Guangdong Province, as well as the Hong Kong and Macau special administrative regions.

The 500 million cubic meters of water at Honghua Hydropower Station on the Liujiang River is expected to substantially dilute the pollutants, bringing cadmium levels within the restricted level in the waters downstream of the station, Xu said.

However, the spill has caused panic buying of bottled water in parts of Liuzhou, which has a population of 3.7 million.

A supermarket salesperson said sales of bottled water had more than tripled in recent days.

"Some people bought 10 boxes of bottled water at a time," the salesperson said, adding that the store has ample stock, and there is no threat to supply.