Shuyang county authorities cut tap water to most of the county's more than 200,000 residents on Monday after supplies were found tainted with a dark, smelly substance. Authorities concluded there was 28mg of ammonia-nitrogen - an important measure of water pollution - per litre of water, well beyond the national standard.

According to Xinhua, Shuyang's water source, the Xinyi River, was polluted by upstream industrial waste released by chemical plants in Shandong .

An official at the Shuyang environmental protection bureau declined to identify the source of the pollution, but said an investigation had begun.

A Shuyang county waterworks staff member said the supply of tap water resumed on Tuesday to first- and second-floor apartments, but residents on higher floors had to wait because of pressure problems with the wells used to replenish supplies.

Supplies to all county residents were reconnected by noon yesterday.

Water was channelled from Anhui's Hongze Lake to dilute the levels of ammonia-nitrogen to 0.49 mg per litre, in line with the national standard.

Some contributors to the popular kdnet.net website claimed the situation was the result of poor government management, and insisted the chief of the Jiangsu environmental protection bureau should resign over two water scandals in the region.

On Saturday, at a meeting at Wuxi, Jiangsu, Mr Wen demanded that authorities exercise greater supervision of industrial waste, as well as speed up construction of waste-water treatment facilities in regions around Tai Lake in Jiangsu and Zhejiang, Chao Lake in Anhui and Dianchi Lake in Yunnan .

Mr Wen said the three lakes, the largest freshwater lakes along the Yangtze River, were all famous scenic sites but had become polluted due to rapid economic growth.

All three lakes saw high concentrations of blue-green algae last month, with the most serious outbreak occurring in Tai Lake, sending nearly five million Wuxi residents to buy bottled water.

Lu Jianjian, a water-resources expert from East China Normal University in Shanghai, said he would not be surprised to see more water crises exposed before September.

'Water pollution has accumulated around the country for many years. When the temperature rises, the problem will become prominent,' Professor Lu said.

'Whether it can be solved is determined by the implementation of the state authority's policies,' he added.

In 2005, residents of Harbin, Heilongjiang province, had their water supply shut down for four days because the Songhua River had been seriously contaminated by an explosion at a petro-chemical plant upstream in Jilin province .