Two dams have been closed in the Pudong New Area as a salt tide is affecting Shanghai for the first time this year, threatening water quality in rivers within the city, the authorities said yesterday.
A salt tide occurs when seawater flows into the mouth of the Yangtze River, usually between October to April, when discharge from the Yangtze is lower.
The Pudong Water Authority has closed the water gates of the Sanjiagang and East Zhangjiabang dams to stop seawater at the mouth of the Yangtze River flowing into city rivers and creeks.
City river water is mainly for industrial and agricultural use.
While tap water supplies are as yet unaffected, the city’s tap water mainly comes from the Qingcaosha and Chenhang reservoirs at the mouth of Yangtze.
If the salt tide affects water around the reservoirs, their water gates will be closed, the Shanghai Water Authority said.
Qingcaosha Reservoir contains enough water to supply the city for 68 days, while Chenhang Reservoir has a 10-day supply.
The city’s first salt tide of 2015 came over the weekend near the Sanjiagang Dam and was later but stronger than previous years, the Pudong Water Authority said.
The chloride density, a salt tide index, increased to more than 525 milligrams per liter on Sunday in rivers near the Sanjiagang Dam, and surged to more than 950 yesterday.
This is almost four times the World Health Organization standard of 250.
Less affected
The other two dams in Pudong further upstream on the Yangtze River are less affected by the salt tide. Their water gates remain open to add fresh water to reduce salinity, said officials.
“If the chloride density keeps increasing, the water authority will close all the dams along the Yangtze River but keep open those on the Huangpu River to add fresh water into the rivers,” the Pudong Water Authority said in a statement on its website.
