"The delayed information release from the authorities is not a matter of procedures. It is a matter of informing the public or not," he said.

Ma said the incident has also brought the city's emergency response plan for environmental issues into question.

Tian Huaqiang, a spokesman for Veolia Water, a Sino-French joint venture and the sole water supplier for urban Lanzhou, insisted information was released in a timely manner.

"The detection of benzene is a time-consuming process," he told China Youth Daily.

Yu Haiyan, Lanzhou's Party chief, said during a work conference on Saturday that the authorities' measures after the incident were appropriate and the information disclosure was "open and transparent".

Wang Jinsheng, a professor at the College of Water Sciences of Beijing Normal University who was part of the investigation, said the investigation shows that the crude oil was left behind after an explosion involving the pipelines belonging to the Lanzhou Petrochemical, a subsidiary of China National Petroleum Corp.

"The residue from the explosion entered the water channel through cracks in the concrete, thus causing the pollution," he said.

As of Sunday, local authorities had canceled emergency water supplies to two districts in the city following tests showing the tap water there was safe. However, in the Xigu district, where the water pollution was most severe, the level of benzene in the tap water was still above the nationally permitted level.

Although the tap water has been declared safe in some districts, residents remain skeptical.

"I will not drink the tap water for the next month, whether they say it is safe or not. It will only be used for the laundry or washing," said a 30-year-old woman surnamed Xiong, who lives in the city's Chengguan district.

Xia Zhaolin, a toxicology professor at Fudan University, said residents who drink the water won't notice any immediate health problems.

"Based on the official statistics of benzene in the tap water, there will be no acute poisoning caused," he said.

Such level of benzene in the tap water, which is a major cause of cancer, can only be felt after 20 or 30 years, he said.