SITTING at the lower reaches of the Yangtze River and Taihu Lake, Shanghai usually does not need to panic about emergent pollution incidents that occur in the upper reaches.

But, environmental science experts say, the city’s water generally is of low quality and requires constant diligence against problems.

Incidents like the suspicious smelly water in Jingjiang City of Jiangsu Province in early May and the thousands of dead pigs floating in the Huangpu River last year raised repeated questions about the quality and safety of drinking water resources in Shanghai.

The six water intakes at the upper reaches of the Huangpu River, which supplies about 30 percent of the city’s daily water use, will be merged into one, the Shanghai Environmental Protection Bureau announced last week, before the World Environment Day that falls today.

The idea is to transform the original open-water source region into a closed one, reducing the influence of problems at its upper-reach regions, such as the dead pigs floating downstream from Jiaxing in Zhejiang Province last year.

The change is expected to improve the water quality from Level 3 to Level 2 at the source, according to Fang Fang, vice president of the bureau.

A five-level system is used in China to rank water quality. Only Levels 1 to 3 are considered drinkable after treatment. Level 1 is considered very good and requires only simple treatment; Level 2 is slightly polluted but still drinkable with treatment. Level 3 is usually for swimming pools and fish farming, needing careful treatment to become drinkable.

Nothing else is drinkable. Level 4 is for industrial use and artificial scenery — no human contact. Level 5 is for agriculture. And there are also worse levels of water that are simply off the scale.

But some experts say there might be hidden danger of eutrophication if making the source a closed one, which would require additional treatment.

Shanghai faces the East China Sea, with the Yangtze River and Hangzhou Bay to the south. There is certainly abundant water. At one time, the city and its suburbs contained 23,000 rivers and creeks. But many small streams and ponds have been filled in and used for development. The city gets an average 1,200 millimeters of rainfall per year.

Despite all that water, Shanghai is listed by the United Nations as one of the six biggest cities in the world facing future shortages of safe water.

“Shanghai does not lack water, but lacks water of good quality,” says Gao Naiyun, professor of environmental science and engineering at Tongji University.

Sitting at the lower reaches of Taihu Lake and the Yangtze River puts Shanghai in a very passive position, receiving pollution from thousands of towns and farms in the upper reaches. This makes it very difficult for the city to solve its water-pollution problems.

Despite efforts Shanghai has made throughout the years such as closing and relocating polluting factories and excavating polluted river sediment in Suzhou Creek, which used to be lined with factories, the general water quality in Shanghai is quite poor.

According to an investigative report by the Shanghai Water Affairs Bureau, last year only about 3.4 percent of the surface water in Shanghai was rated better than Level 3 on average, 23.7 percent was  around Level 4, 20 percent reached Level 5, while 52.9 percent was even worse — off the scale.

The relocation of the water intakes was done after water officials looked at where the decreasing water quality in Shanghai was taking place over the past 200 years.

The city used to take water from the lower reach of the Suzhou River in the 1800s, then was forced to shift to the Huangpu River in the 1880s when the water quality in the Suzhou River was quickly ruined. Yangshupu Water works became China’s first water plant, set at the lower reach of Huangpu River in 1883. The water intake was relocated repeatedly upstream until 1998, when it reached the Songpu Bridge region.

Still, the water quality deteriorated, which triggered the Qingcaosha Reservoir project in Chongming County, taking water from the Yangtze River.

The reservoir became operational in 2011, and shares the supply burden for Shanghai with the upper reaches of the Huangpu River. It supplies drinking water for the downtown area, the Pudong New Area, and part of Minhang, Baoshan and Qingpu districts, supplying around 50 percent of the needs in the city.

“The water quality in the Yangtze River is certainly better than that in Huangpu River, with its larger water quantity and quicker flow enabling greater self-cleaning capacity,” says Professor Gao. “The organism and hazardous and noxious substances in the Yangtze River are generally two to three times less than that in the Huangpu River even though it is taken from the lower reach.”

Except for the upper reaches of the Huangpu River, three of the four major drinking water sources in Shanghai now or will take water from the Yangtze River. They include the Qingcaosha Reservoir, the Chenghang Reservoir in Baoshan District completed in 1992, and Xisha Reservoir in the southwestern part of Chongming County, which is to be operational this year.

So far, only part of Jinshan, Fengxian, Qingpu and Minhang districts are still using water from the Huangpu River. Considering the relatively poor water quality, projects to add additional water treatment are being carried out at the water plants in these regions.

The additional treatments include a pretreatment process before the conventional treatment process and an advanced treatment process after that to ensure good-quality water that meets the 106 indexes of the new National Sanitary Standard for Drinking Water.

“The additional treatments can help largely keep our drinking water safe from the pollution emergency at the upper reaches, as they can eliminate most of the organisms and hazardous and noxious substances,” says Professor Gao. “We don’t have to panic with the complete treatment process in our hands even if the dead pigs come again.”

The additional treatment process is already working in Minhang, Songjiang and Qingpu districts, while those in Jinshan and Fengxian districts will soon be completed.

Though the water quality of the Yangtze River so far is better than that of the Huangpu River, the long-term use of the Qingcaosha Reservoir is still questioned. Pollution in the reservoir from eutrophication occurred last summer. Activated carbon was spread as a treatment.

Similar cases might happen to the water source at upper reach of the Huangpu River if it is transformed into a closed reservoir.

Gao strongly advised additional water treatments for all water supplies in Shanghai.

“We cannot change the surface water quality in Shanghai within a short time since the pollution is already done and possibly still being done at the upper reach,” says Gao. “What we can do is to advance our treatment as an absolutely safe scheme to fight against any emergency in our water.”