Two environmental inspectors went on trial in south China on Tuesday over case of serious river pollution that had threatened the drinking water source of a city with 1.5 million people.

Lan Qunfeng and Wei Yi, former chiefs of a district environmental inspection team in the city of Hechi, Guangxi Zhuang Autonomous Region, were charged with dereliction of duty and bribery before a district court in the city of Liuzhou.

The two are suspected of failing to carry out their duty and allowing Jinchengjiang Hongquan Lithopone Material Co., Ltd., a chemical plant in Hechi, to dodge pollution inspection, prosecutors told the court.

The plant had been discharging cadmium-rich chemical waste without treatment since April 2011. In January, Longjiang River was contaminated by the substance. The pollution later spread to the downstream Liujiang River, threatening drinking water safety in the highly populated Liuzhou.

Cadmium is a carcinogenic industrial chemical.

The river's water quality was brought back to normal after one month of emergency treatment.

Lan and Wei were also accused of taking bribes -- 20,000 yuan (3,200 U.S. dollars) in each case -- from unspecific polluters, the prosecutors said.

The court did not immediately announce the verdict after Tuesday's trial.

Prosecutors on Monday charged seven stakeholders and executives of the chemical plant with the crime of impairing the protection of environment.