Benzene from an unknown source polluted the public watersupply for two million people in the north-western Chinese city of Lanzhou onFriday, reports said.The level of benzene was 200 micrograms per litre, or 20 times the national limit,and nearly double the 118 micrograms per litre detected late Thursday, the official Xinhua news agency said.The city government was investigating the source of the benzene contamination,which was recorded by Veolia Water, a US-based contractor for water treatment inLanzhou.Benzene is a colourless petrochemical that is used as a solvent for making plasticsand other products. It is carcinogenic if ingested.Lanzhou, an inland city on the Yellow River, is well-known for its chemicalindustries and its associated air and water pollution.