Courts across the Guangxi Zhuang autonomous region sentenced 13 people on Tuesday stemming from charges of polluting a river in the region with heavy metals last year.

Three of those jailed were environmental inspectors who failed to perform their duties and allowed plants to dodge pollution inspections. The plants discharged industrial effluents containing cadmium into the Longjiang River from April 2011, the courts said.

The Dahua county court in the city of Hechi sentenced Zeng Juefa, former deputy director of the city's environmental protection bureau, to four-and-a-half years in prison for delinquency and taking bribes.

Liubei district court ruled that Lan Qunfeng and Wei Yi, former heads of a district environmental inspection team under the environmental protection bureau of Jinchengjiang district, were guilty of the same crimes as Zeng. The pair were handed jail terms of three-and-a-half years.

Zeng was found to have taken bribes worth 45,000 yuan ($7,335), while Lan and Wei accepted bribes of around 20,000 yuan each, the courts said.

Guangxi Jinhe Mining Co Ltd was found to have polluted the river and was fined 1 million yuan, as ruled by Jinchengjiang district court. The company's three managers were each sentenced to three years in jail, although the sentences were suspended in two cases.

The pollution of the Longjiang River was not discovered until January 2012, when dead fish were found near a power station located by the river. The concentrations of cadmium in the river near the power station were found 80 times the official safety standard.

The pollution endangered the quality of drinking water in Liuzhou city, which is located downstream of the power plant and is home to 1.5 million people. It took authorities one month of emergency treatment to restore water quality in the river.