Xinhua news agency, citing Lin Qiang, spokesman for the Heilongjiang Provincial Environmental Protection Bureau, said: "The latest inspection result shows that by 6am on Sunday no benzene was found in the water at Sifangtai, the water source site in the upper reaches of Harbin section.
"The nitrobenzene level has met the national standard with a concentration of 0.0050mg per litre."
The 3.8 million residents of Harbin in China were due to have their water reconnected on Sunday, five days after supplies were cut as the 80km toxic spill hit the city.
Public water supplies to Harbin were cut on Tuesday as the government scrambled to ship bottled water into the city to avoid a health crisis.
Residents were expected to have water back before midnight.
Reporting pollution levels
Prime Minister Wen Jiabao, who arrived in Harbin on Saturday, ordered officials to begin directing their attention towards the lower reaches of the Songhua river where the toxic slick was heading.
He said that pollution levels should be promptly and publicly reported.
Environmental inspectors check
pollution levels in the Songhua
"Local governments must follow the polluted waters through the cities, towns and villages along the river and make prompt public reports on the monitoring results," Wen was quoted by the Heilongjiang Morning Post as telling officials.
"Every available measure must be taken to ensure the drinking needs of the people and the quality of the water."
As the spill passed Harbin on its way downstream to Russia, China made a formal apology to its neighbour for the benzene spill heading towards the Amur river - known in China as the Heilong river - that marks the border between the two.
Nearly 100 tonnes of benzene was dumped into the Songhua river, a main tributary of the Amur, on 13 November after an explosion at a chemical plant in Jilin province, 380km up river from Harbin and 1000km from the Russian border.
The spill is due to cross into Russia in the coming days.
Kamil Iskhakov, President Vladimir Putin's envoy to the Far East, told regional media outlets on Saturday that there was no reason for residents living along the Amur river to panic.
He said: "We will know at least a week before the chemical spill reaches Khabarovsk (a city of 650,000 people)."