Water quality concerns decreased after a salt tide at the mouth of the Yangtze River ended over the weekend, the Shanghai Water Bureau said.

The relief, however, may only be temporary as a second salt tide could again affect the city's water supply within seven days, according to officials from the bureau.

The salt tide started affecting the Yangtze River, the city's largest tap water source, on February 12. Chloride concentrations in the river mouth increased to a maximum of 1,000 milligrams per liter.

Bureau officials said people can taste salt if the chloride concentration reaches 200mg per liter. Water with a chloride concentration of more than 250mg per liter can not be used as a tap water source, according to the country's household water quality standards.

During the salt tide, the city cut water intake from the Yangtze River from 1.6 million cubic meters a day to 1.3 million cubic meters a day.

Water from local reservoirs was used to make up for the shortfall during the period.

With the water quality returning to normal, reservoirs such as Chenhang Reservoir along the Yangtze River in Baoshan District are increasing water intake to replenish levels to normal.

Waterworks branches around the city are also required to fill up water tanks on the roofs of commercial and residential buildings to prepare for a possible water shortage due to the chance of a second salt tide.

Salt tides usually occur annually in the city about this time of year as lower water levels in the Yangtze allow salt water to move further upstream.

(Shanghai Daily February 23, 2009)