SHANGHAI'S major water supply source faces the risk of eutrophication as upstream factories keep discharging waste water, a local political advisor warned yesterday.

The condition of the water at the mouth of the Yangtze River, where the Qingcaosha Reservoir is located, is threatened mainly by chemical plants along the river, such as the Japanese paper mill in neighboring Jiangsu Province, said Lu Jianjian, who is also a professor at East China Normal University.

"The current good water quality around the reservoir can be kept for at most 20 years if the plants keep releasing waste water into the upstream river," Lu told Shanghai Daily.

He said eutrophication - a process where water bodies receive excess nutrients that stimulate excessive plant growth - would cause algae floods and even turn the water in the reservoir black and foul-smelling.

Even if the discharged waste water from the factories was treated according to Chinese laws, it would still affect the water quality of the river.

The Japanese paper mill, which saw its sea pipeline construction suspended late last month by the Qidong City government, was the biggest threat, according to Lu.

"Without the sea pipeline, the mill has to discharge water into the Yangtze directly and its waste water outlet is only 100 kilometers upstream of the Qingcaosha Reservoir," he revealed.

Apart from the paper mill, there are at least three water treatment plants and some other chemical plants along the Xuliujing section of the Yangtze in Jiangsu, or the worst polluted water area on the upstream of the reservoir.

Some 80 percent of pollution at the mouth of the Yangtze comes from the section that provides millions of tons of nitrogen, phosphorus and heavy metal to the downstream, Tu Jianbo, an official with the State Oceanic Administration told the 21st Century Business Herald yesterday.

Meanwhile, the city's top political advisory body said the reservoir is also being threatened by busy shipping traffic and salt tides on the Yangtze.

Oil leaks from ships and chemical cargoes are other potential dangers, the Shanghai Committee of Chinese People's Political Consultative Conference has said in a report.

In response to the eutrophication threat, the Shanghai Water Authority said it is taking preventive measures.

"Moreover, a 5-kilometer buffer area around the intake of the reservoir is monitored round the clock to ensure it can be shut in time should any emergency occur," said Meng Mingqun, director of the water supply division of the authority.

The Shanghai government invested some 17 billion yuan (US$2.68 billion) to build the reservoir in December 2010.

About 10 million residents - nearly half the city's population mainly in downtown - get drinking water from the Qingcaosha Reservoir.