Residents queue up to get safe water supplied by the government in Lanzhou, Gansu province on April 11 after the city's tap water was found to be polluted by benzene, a toxic cancer-causing chemical. [Photo/Xinhua]

The company is in charge of conducting comprehensive tests for other cities in Gansu and it just incidentally tested tap water in Lanzhou while doing the tests for other places, Yan added.

Tap water supply fully resumed in Lanzhou on Monday.

Lanzhou Veolia is a joint venture established in 2007, with Veolia Water owning 45 percent of its shares and the local government 55 percent. It is the only water supplier for downtown Lanzhou, capital of Gansu.

Benzene is a colorless carcinogenic compound used to manufacture plastics. The chemical is known to damage the human hematopoietic system, which produces blood.

The pollutants were initially believed to have leaked from a pipeline of Lanzhou Petrochemical, a subsidiary of CNPC, China's largest oil company, said investigators.