Excessive levels of benzene, a carcinogenic chemical, in tap water have affected more than 2.4 million people in downtown Lanzhou, Northwest China's Gansu Province, provincial authorities said on Friday.

Tests on 8:30 am on Friday showed tap water contained 160 micrograms of benzene, a colorless carcinogen used in the manufacture of plastics, per liter, far in excess of the national limit of 10 micrograms, according to the city's environmental protection office. 

Other tests on Thursday and Friday found between 118 micrograms to 200 micrograms of benzene per liter, according to Veolia Water, a Sino-French joint venture and the sole water supplier for urban Lanzhou, the provincial capital.  Local authorities said the levels at the plant fell to 78 micrograms of benzene per liter on Friday afternoon. Veolia Water began using activated carbon at 3 am to abate the pollution.

An initial investigation found problems in a three-kilometer channel that links the plant that preprocesses the water and the plant that supplies water to Lanzhou. Closure of the channel will cut the city's water supply by half, said Tian Hong, head of Lanzhou's water quality monitoring station. Fire engines will be used to carry water to affected areas until the supply is restored around noon Saturday.

The local government is investigating the source of the contamination, and sources from Veolia told the Global Times that they believed the benzene came from external chemical pollution sources, but declined to single out any particular plant.

The Gansu publicity department affirmed that the Yellow River, which is the original source of the water and runs through the city, is not contaminated. 

Lanzhou residents panicked upon hearing the news, rushing to supermarkets or community grocery stores to stock up on bottled water.

This is the second incident of its kind in Lanzhou in two months. On March 6, residents reported a strange odor when they turned on their taps, which was later confirmed to be a high concentration of ammonia, but still within national standards.