All of the liquid chemical waste dumped into the Yangtze River in Hubei Province on Sunday has been removed and no water plants have been contaminated, local environmental authorities said Wednesday.

The picture taken on September 10, 2008 shows a sewage treatment plant in Zigui County, Hubei Province. The county has completed 14 garbage and sewage treatment projects in recent years with daily sewage treatment capacity reaching 31,100 tons and daily garbage treatment capacity reaching 1.48 million tons. These major investments are helping to protect the Yangtze River.

A nightshift worker at a porcelain factory in the town of Lanxi, Xishui county, witnessed 14 bucketfuls of waste being tipped into the river on Sunday night, the Hubei environmental protection bureau said.

The waste was found to contain trichloroacetic acid - which releases a poisonous gas when heated - ethyl benzoate and isopropyl naphthalene, it said.

Only a small part of the chemical waste flowed into the river thanks to the swift action of local authorities, the bureau said.

However, as a precaution, a local water plant has been told to stop drawing water from the river and to use its emergency supply instead.

The water at three other plants in the counties of Qichuan and Wuxue was not affected, the bureau said.

The quality of the water downstream is being tested every two hours, it said.

Public security authorities have joined the effort to catch the perpetrator of the "vicious incident", the bureau said.

The witness was unable to provide authorities with the number plate of the vehicle, which fled quickly after dumping the waste, the bureau said.

The incident is the latest in a series of water pollution cases in Hubei.

In September of last year, the Hanjiang River, a branch of the Yangtze, was badly polluted by wastewater from a paper mill.

The incident led to the water supply in some areas being suspended for a week.