The Shanghai city government is planning to pump 15 million tons of tap water underground this year to counteract land subsidence, the Shanghai Housing and Land Administrative Bureau said Thursday.

The amount is equivalent to the city's residential consumption of tap water each week.

From this year on, the city will set its amount of "pump-back water" according to the level of land subsidence annually.

"We plan to control the annual land subsidence within five millimeters by 2010 but it requires a lot of efforts," Liu Shouqi, a bureau official, said Thursday.

Next month, the bureau will construct 30 wells in areas around the Outer Ring Road to realize the pumping of tap water into the underground to support the ground surface.

According to Quan Guijun, the project manager, the wells pump back water every June to September when the soil's capacity for soaking is considered best.

In recent years, the city's subsidence has been around 10 millimeters - mainly caused by the overpumping of underground water and the rapid construction of skyscrapers, geologists said.

Most underground water is pumped for industrial and agricultural use, such as textile refrigerant, irrigation and food production.

Last year, Shanghai was reported by the Ministry of Land and Resources in its latest scientific survey as one of three Chinese cities whose accumulating land subsidence had exceeded two meters.

The current subsidence is far less severe than the 1960s when Shanghai was sinking by more than 100 millimeters a year - a rate that would have put the city below sea level by 1999 if it hadn't been stopped.

However, the ongoing subsidence is causing local geologists to worry.

"Although the current rate will maintain the city above sea level for quite a long time, it holds several potential dangers," said Li Qinfen, a professor with the Shanghai Institute of Geological Survey.

"It has already forced the city to raise its flood wall again and again and it will continue in the future," he added.

The problem is most noticeable in areas near the west side of the Nanpu Bridge and the Lujiazui financial district in Pudong. They sank around 50 and 40 millimeters, respectively, last year.

(Shanghai Daily July 16, 2004)