The tap water supplier, the Lanzhou Veolia Water Company, has denied a cover-up of the contamination. [Photo/Xinhua]

According to Yan, Lanzhou Veolia conducted comprehensive tests for all the 106 items for tap water in Lanzhou in accordance with the country's regulations in early March after a strange smell was reported in tap water.

All the indicators met standards and further tests by the Chinese Academy of Sciences found the smell came from decaying grass roots and algae in the source river, he said.

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.