Shanghai began to build a new reservoir yesterday in its northern Chongming County to improve water quality for residents of the island.

The Dongfengxisha Reservoir off Chongming Island's southwest bank will store water from the Yangtze River, the Shanghai Water Authority said. The construction is Shanghai's second-largest water project after the Qingcaosha Reservoir and is scheduled for completion by January 2014, officials said. The new reservoir should supply clean tap water to the county's 700,000 residents, said Zhang Jiayi, director of the authority.

A 12-kilometer-long dam will encircle Dongfengxisha Island to form a 3.74-square-kilometer reservoir, which will separate the island from the southwest of Chongming Island. The project is designed to take no land from surrounding farms, officials said.

Residents are now drinking brackish water from wells and inland rivers, 90 percent of which comes from tides of the Yangtze River. When the Yangtze is in its drought period, usually from January to March, salt water from the sea flows backward in the river to the islands and severely downgrades water quality, the authority said.

Once the new reservoir is built, clean water will be stored in the reservoir and should provide the county a stable water supply of 215,000 cubic meters per day by 2020, officials say.