The chemical plants blamed for polluting Xinqiang River in central China's Hunan Province with arsenide have been closed down and their senior managers have been detained, China's top environment watchdog said Monday.

Haoyuan Chemical Company, a sulphuric acid manufacturer in Hunan's Linxiang City, and Taolin Lead-Zinc Ore Chemical Plant, both in the upper reaches of the river, were the major polluters, said an official with the State Environmental Protection Administration (SEPA).

The two companies had not passed any environmental assessments and had no pollution treatment facilities. They have been discharging waste water with arsenide content more than 1,000 times higher than the national standard directly into Xinqiang River for a long time, said the official.

Haoyuan Chemical Company discharged nearly 50,000 tons of waste water every month, and Taolin Lead-Zinc Ore Chemical Plant 280 tons, he said, without revealing over what time period.

Pan Yue, SEPA's deputy director, promised harsh penalties for the enterprises and the top managers may face criminal punishment.

Xinqiang River, a tributary of China's second largest freshwater lake, Dongting Lake, was found to be polluted last Friday when workers from the county's environmental monitoring center conducted routine testing of water quality and found the content of arsenide was 10 times higher than normal standards.

No casualties have been reported from the contamination. Dongting Lake has tested clear of the pollution, and the water quality at Dongting's estuary into the Yangtze River is also safe.

Arsenic trioxide is a highly toxic white powdery substance that can cause vomiting, stomach pains and convulsions, and can lead to coma or death.

A chronic intake of arsenide can also cause liver and kidney damage or lung and skin cancer.