Five people including managers of two companies in China have been charged in connection with a massive dumping of toxic waste into the Chachong Reservoir that could affect the drinking water for tens of millions of people.

The dumping of more than 5,000 tonnes of hexavalent chromium-contaminated waste allegedly occurred between April and June, and came to light after 77 livestock, mostly sheep, died after drinking from a local pond.

The arrests and charges were announced Sunday at a news conference in Qujing City by authorities in southwest Yunnan Province.

Chen Jun , deputy mayor of Qujing City, said among those charged are managers of the Luliang Chemical Industry Company Ltd. and the Sanli Fuel Company Ltd..

According to China Daily, a Luliang Chemical Industry employee and two employees from a coal company are also charged.

Government officials initially denied the contamination of the reservoir, which feeds into the Pearl River — one of China's longest and most valuable waterways.

At the time, officials said they had contained and cleaned the chromium-contaminated soil and water, but they now say the cleanup won't be complete until the end of 2012.

Hexavalent chromium is easily absorbed by the body, and can cause vomiting, abdominal pain, dermatitis and eczema. Short-term and long-term contact or inhalation poses a cancer risk.

Yunnan province is a major producer of vegetables and other farm products for Hong Kong and southern China.

The Hong Kong secretary of health says authorities are testing vegetables for any signs of contamination.