Cadmium pollutants were detected in the Longjiang River on 15 January after tests were carried out on dead fish.
The level then was 80 times the official limit, regional environment protection official Feng Zhennian said.
Teams have been putting neutralising chemicals into the river to treat the contamination.
Mr Feng, who is also a spokesman for the emergency response centre managing the spill, did not name the seven people at the press briefing on Monday.
But he said one of the companies involved was Jinchengjiang Hongquan Lithopone Material in Hechi city.
Officials said on Saturday that cadmium levels at the source of the pollution near the Lalang reservoir had normalised.
Local officials told Xinhua that they had stepped up efforts to ensure water safety for nearby cities.
Cadmium, commonly found in industrial usage such as the production of batteries, is a carcinogen which can damage the kidneys, bones and respiratory system.
Last week residents in Liuzhou city, located at the lower end of the river, emptied supermarket shelves of bottled water in panic buying.