HECHI - The official probe into an incident which resulted in cadmium polluting a South China river is making progress and local police have detained eight executives from two companies that are suspected of waste dumping.

Investigators have pointed to Jinhe Mining Co and Jinchengjiang Hongquan Lithopone Material Co Ltd as the two most likely sources of the cadmium pollutants which were first detected in Longjiang River on Jan 15, mayor of Hechi city in Guangxi Zhuang autonomous region said Wednesday.

The two companies are both based in Hechi city.

Police have detained eight company executives, including three from Jinhe Mining and five from Lithopone Company, and were still chasing four others, the mayor said at a press conference organized by an emergency response taskforce that was set up to handle the pollution.

At the press conference, He publicly apologized for the pollution, saying it had "a severe impact" on the lives of the local residents, particularly those living in the downstream areas including Liuzhou.

The pollution threatened water security in Liuzhou, a city with 1.5 million permanent residents in the city proper. Though water supplies were not cut off, it prompted panic buying of bottled water.

"On behalf of the Hechi city government, I apologize to those people," the official said, with promises to revamp local industries and seek more sustained development.

As of Wednesday, 145 firms have been scrutinized, of which 11 have been closed or suspended for revamps, the mayor said.

He said the local environment watchdog has also ordered closure of more than 90 dumping sites of minerals and tailings before Feb 5.

Lab test results published on Wednesday showed the cadmium concentration level in the Longjiang River was going steadily down, and drinking water produced by four major water plants in the downstream Liuzhou city was up to national standard, said Feng Zhennian, an official with Guangxi's regional environmental protection bureau.

He said the environment watchdog had been closely monitoring water quality in Liuzhou. Test results have been published every four hours since Monday.