China's top environmental agency has sent investigators to probe a chemical company's dumping of carcinogenic industrial chemicals possibly affecting a reservoir that feeds one of the country's largest rivers.

The investigators from the Ministry of Environmental Protection arrived at Qujing, in Southwest China's Yunnan province, on Thursday, the city's information office said on Friday.

They will investigate the pollution of the Chachong Reservoir in the town of Yuezhou and the Nanpan River, which feeds into the Pearl River, a major river flowing through southern China.

The investigators will focus on the impact of the carcinogenic industrial chemicals' on human health and the environment in the area. In addition, they will assess the safety of heavy industrial plants in Luliang county, which is home to 29 such companies, Quan Ende, deputy director of Qujing information department told China Daily on Friday.

Quan could not say when the ministry will release its report.

Since April, more than 5,000 tons of chromium waste from Yunnan Luliang Chemical Industry was illegally dumped near the Nanpan River, in the upper reaches of Pearl River.

The plant has suspended production, but about 148,000 tons of unprocessed chromium waste was dumped along the Nanpan River, the city said.

In June, rain washed some of the waste into the water supply, killing 77 goats, the Pearl River Water Conservancy Committee said on its website on Thursday.

The committee, which is under the Ministry of Water Resources, joined the investigation early this week and has found excessive hexavalent chromium (chromium VI), a carcinogen, at the company's dumping sites. No chromium VI pollution has been detected in major drinking water sources, including the Huangnipu Reservoir and the Nanpan River.

No human deaths have been attributed to the chromium pollution, but at least 14 residents have been diagnosed with cancer since 2002 and many of them suspect their illness was caused by contaminated drinking water.

At least 3,000 people live near the company's dumping sites.

The committee has recommended that authorities closely monitor the rivers to prevent potential pollution.

"Hexavalent chromium poses great health dangers. Drinking water polluted with it could cause acute poisoning symptoms. But there are no reports of deaths caused by it," said Liu Xiaoduan, a researcher with the Chinese Academy of Geological Sciences.

"People could have ingested some chromium VI by eating plants growing in contaminated soil, but it takes longer for the buildup of the poison to exceed safe levels."

She also said the contaminated waters can be purified through various methods, and the chromium VI can be kept in the soil without entering the ecological chain.

The two truck drivers who dumped the waste near the river have been arrested. And authorities in Qujing are investigating whether any public agencies and officials are responsible.