¡¡LANZHOU, May 30 (Xinhua) -- A chemical plant blast that left four dead and 11 injured in Northwest China on Monday caused no contamination to the Yellow River, China's second longest, and drinking water, the local government said Tuesday.

    Emergency measures had been used to stop 80 tons of water used to extinguish the ensuing fire from flowing into the Yellow River,the area's principal source of drinking water, said Zhang Zhengmin,deputy head of the local environmental protection authority.

    Tests showed the water quality of the section of the river flowing through Gansu Province and the air quality were within national safety levels and posed no risk to residents in the area,Zhang said.

    The 5,464-kilometer Yellow River originates in Qinghai Provincein the country's northwest and flows eastward through Gansu and other regions before emptying into the Bohai Sea.

    The Gansu section, on the upper reaches of the river, stretches913 kilometers passing four cities and prefectures.

    The blast ripped through a factory owned by Lanzhou Petrochemical Co. Ltd. in the provincial capital on Monday afternoon, killing four workers and injuring 11, including four seriously.

    The firm is a subsidiary of PetroChina, one of the three big state-owned oil companies.

    An investigation into the cause of the explosion is underway.

    With memories still fresh of a pollution incident caused by a chemical plant blast in northeast China last year that led to water supplies being suspended, residents have been storing tap and bottled water amid widespread concern over the quality of drinking water supplies.

    About 100 tons of pollutants containing benzene spilled into a river in Heilongjiang Province after a chemical plant blast upstream last November, forcing cities along the river to temporarily cut off water supplies.

    Gansu, where a high density of heavy industries have been discharging pollutants into the river, is to conduct a major environmental clean-up campaign to control pollution and improve river conditions by 2010.

    About 237 million tons of sewage from the province is pumped into the river each year, of which just 34 percent was properly treated, said Yang Zhiming, vice-governor of Gansu.

    The local government plans to invest 4.97 billion yuan (612.8 million U.S. dollars) in 199 projects to assist heavy industries in reducing pollution, construction of sewage treatment plants andinstallation of monitoring facilities to ensure better water quality, according to the provincial environment protection authority. Enditem