Water supply resumed Thursday at 11 a.m. in Yuhang District of Hangzhou, capital of Zhejiang Province, four days after contamination in the Tiaoxi River caused a water-buying panic.

Tests showed the river's water reached the standard of environmental regulations, according to experts of the provincial and municipal environmental protection departments.

Four companies are under investigation on suspicion of discharging waste into the river, local authorities said Thursday.

Seven monitoring stations along the river have been reported safe since 3 p.m. Wednesday, and pollution sources have been traced to Zhejiang Wanma Cable, Zhejiang Jinzhili Chemicals, Hangzhou Derunbao Oil, and Hangzhou Zhenxin Thermal Electric, said an official of the provincial environmental protection department.

The pollution has disrupted the operation of two downstream water treatment facilities and affected tap water supplies for the roughly 848,400 residens of Yuhang district.

The Pingyao Waterworks, which treats water from the Tiaoxi River and supplies drinking water for the nearby towns of Pingyao and Liangzhu, halted operation on Sunday.

Hangzhou residents bought bottled water in a panic following the news of contamination. The local government sent water trucks to the towns to distribute drinking water. Schools and kindergartens in the towns have been closed since Tuesday.

The city's Yuhang Waterworks had temporarily halted operations before switching to an alternative source, the Qiantang River.

More than 120 companies in Lin'an have halted production because of water shortages. An upstream reservoir has discharged more water to dilute the pollution.