Disciplinary authorities are investigating an environmental watchdog suspected of complicity in the illegal dumping of carcinogenic industrial wastes in Yunnan province, which threatens the water sources for tens of millions of people.

Realizing they would not earn any money by transporting chromium-contaminated waste to Guizhou as they were supposed to, two truck drivers dumped around 5,000 tons of the contaminated waste in the hills of Qilin district in Qujing near the Chachong Reservoir from April to June.

At least 3,000 people live near dump sites and runoff from the reservoir feeds into the Pearl River, one of China's longest waterways.

The Discipline and Inspection Commission of Qujing is investigating the case to see whether the local environmental protection department was paid to turn a blind eye to the dumping, as it claimed it was unaware of the transportation of the toxic waste even though it should have been, said He Hua, head of the local government's publicity department on Wednesday.

In addition to the two drivers, police have also detained a deputy manager and an employee from Yunnan Luliang Chemical Industry, which produced the chromium waste, and a deputy general manager of Sanli Fuel Co Ltd that was contracted to transport the industrial waste.

The dumping was uncovered when local villagers reported sheep were dying after drinking water from a pool.

The local water sources are now being monitored and the waste has been returned to Luliang Chemical Industry.

Chromium poses a severe health risk and drinking water polluted with it could cause acute poisoning. But the authorities say, so far, no chromium has been detected in the local water sources.

However, Liu Xiaoduan, a researcher with the Chinese Academy of Geological Sciences, said that people could have ingested chromium VI by eating plants grown in contaminated soil.

According to a report by Qianjiang Evening News, at least 17 residents in Xinglong village, which is 2 kilometers from the dumpsites, died from cancer in 2009 and many villagers suspect their illness was caused by contaminated drinking water.

The output of tobacco leaf, which was the main income sources of the local villagers, has dropped sharply in the past few years due to the pollution, said Wang Kaicai, a villager of Xinglong.

The villagers said they petitioned the local authorities about the dumping more than 1,000 times, but each time they were ignored.