Authorities in Lanzhou, capital of northwestern Gansu province, have resumed the supply of tap water to the city's urban districts on Monday after tests showed that the level of benzene in the water had dropped below national safety limits.

From Thursday evening to early Friday morning, Veolia Water, a Sino-French joint venture and the sole water supplier for urban Lanzhou, found between 118 and 200 micrograms of benzene per liter of water at their plants.

The company first detected high levels of benzene at around 5 pm on Thursday, but an announcement about the contamination was given 19 hours later. Both Veolia Water and the city government drew public criticism for the delayed announcement.