Authorities in a southwestern China city said on Sunday that it would relocate all smelting plants near downtown in five years after a spill of toxic cadmium in a river threatened drinking water supplies for millions of people.

Metals companies that refuse to move out of town would be shut down, Liao Jincheng, director of the development and reform commission of Hechi city in Guangxi Zhuang autonomous region, told Xinhua on Sunday.

Cadmium pollution was first detected in the Longjiang River on Jan. 15 in Hechi, and it later spread to the downstream Liujiang River, threatening water security in Liuzhou, a city with 1.5 million permanent residents.

Investigations found two factories, one producing a dye product called lithopone without a license, and the other a metallurgical chemical plant, to be responsible for the incident. They had illegally discharged highly contaminated sewage.

Liao said that Hechi, a city with 145 heavy metals companies, had been mulling to moving out those near downtown before the spill and plans for relocation were already written in the city's five-year plan starting 2012.

The mills will be relocated to two industrial parks dozens of miles away from downtown, said Liao.

Liao said that centralized production would make it easier for environmental monitoring and it would cut pollution treatment costs for companies as well.

Small smelting companies with annual production value of lower than 20 million yuan (3.17 million U.S. dollars) would be the first ones to move, Liao said, adding they would be closed if refused to go by the end of this year.