The China Economic Review says:

After days of heavy rainfall, water quality in the mouth of the Yangtze River was seriously polluted by flood waters flushed from the Liuhe River in Jiangsu province, according to Shanghai Morning Post (in Chinese). Two of Shanghai's tap water sources, the Changhang Reservior and the upper stream of the Huangpu River, were also affected, with ammonia concentration of Changhang Reservoir reaching the unstable level of 3.6 milligrams per liter, 20 times the normal level. Nitrite concentration in the upper stream of the Huangpu was also well above normal levels, due to sewage and agricultural and industrial effluent. Local officials said the city's water supply system is under strict monitoring and the quality drinking water has remained normal so far.

Source:

http://www.chinaeconomicreview.com/dailybriefing/2007_07_12/Flood_in_Jiangsu_may_affect_water_quality_in_Shanghai.html