Massive amounts of the chemical benzene were released by a blast 11 days ago at a plant in Jilin, about 380km (230 miles) further up the Songhua river.
Authorities shut off water supplies to Harbin's 3.8m residents two days ago.
There is plenty of bottled water, wells are being dug and supplies are being driven in, says a BBC correspondent.
More than 16,000 tonnes of water are also being brought in by road, the Chinese state news agency Xinhua says - though this is less than Harbin's residents normally use in a day.