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One of Shanghai's two sources of fresh water could be at risk from sea water flowing into the mouth of the Yangtze River during the Spring Festival period.

About 30 percent of the city's fresh water comes from the Yangtze.

Hundreds of thousands of families in northern Shanghai could be affected in a worst case scenario. But water suppliers said they have sufficient backup reserves.

A salt tide is expected to take place between January 29, the first day of the Year of the Dog, and February 12, according to the Shanghai Municipal Raw Water Co Ltd.

The tide takes place at the mouth of the Yangtze every winter or early spring when water flowing from the river decreases.

It will last up to five days, forcing water suppliers to stop pumping fresh water from the Yangtze for a period of time each day.

A huge reservoir, on the bank of the Yangtze in Baoshan District, will therefore see its water level decrease. Pumps will not operate at full capacity and will be shut down when salt levels are too concentrated.

The reservoir has a capacity of 8.3 million cubic meters. The fresh water is stored and filtered for impurities before pumped to tap water plants for further treatment.

Without daily intakes, the reservoir at full capacity can only ensure water for 7 to 10 days for six water plants in northern Shanghai.

"That scenario is hardly possible for we have various contingency plans. It's worth noting the salt tide this year will not be serious," said an official with the Shanghai Municipal Raw Water Co. He would only give his surname as Wei.

The company manages the fresh water supply for all plants downtown. Wei said they have reached an agreement with Baosteel Corp for a supply of up to 100,000 cubic meters a day from the steel firm's reservoir, also along the Yangtze River.

In a worst case scenario, officials with the Shanghai Water Supply Administration said they will ask water utilities that use the Huangpu River as a source for fresh water to dispatch tap water to areas that may be affected by the salt tide.

"The tide this year comes later than usual, but it should be less serious than last year judging from current conditions," said Yin Rongqiang, an administration official.

A salt tide caused problem in 2004. The reservoir operator asked Baosteel to help for the first time.

(Shanghai Daily January 20, 2006)