NANNING - Seven people have been detained on suspicion of industrial waste discharges that polluted a south China river and threatened water supplies in a major downstream city, the local government said Monday.

All the seven were chemical plants executives who worked in Guangxi Zhuang autonomous region, including Jinchengjiang Hongquan Lithopone Material Co. Ltd. in Hechi city, said Feng Zhennian, an official with the regional environmental protection department.

Feng, who is also spokesman for an emergency response center set up to handle the incident, made the remark at a press briefing Monday evening.

He did not identify the seven people or the other suspected polluters. Feng took no questions at the press briefing.

Cadmium pollutants were first detected in Longjiang River, a tributary upstream of the Liujiang River, on Jan 15, when cadmium concentration near the Lalang reservoir was 80 times higher than the official limit of 0.005 milligrams per liter, said Feng.

Environmental protection workers have been dumping neutralizers, made from dissolved aluminum chloride, at six locations along the Longjiang River to dissolve the contaminants.

The source of the pollution was reportedly contained Saturday, as cadmium concentration at Lalang reservoir, where the pollution was first detected, had returned to normal.

The pollution belt was now near the downstream Luodong hydropower station and the cadmium concentration levels were still 25 times higher than the official limit, Feng said.

He said the pollutants were still flowing downstream and were close to a major drinking water source for Liujiang, a city with 1.5 million permanent residents in the city proper.

The local environment watchdog has set up 20 surveillance stations along 200 km of the river and more than 210 surveillance workers are at work to monitor the water quality on a real-time basis.

"The tap water company in Liuzhou has also stepped up surveillance and will strive to provide safe water for the residents," Feng said.

"Judging from the surveillance data, we're confident the situation is under control and water supplies to Liuzhou will remain safe."