As local authorities in Lanzhou, Gansu province, continued to investigate the city's tap water contamination on Sunday, experts and local residents have raised questions over the delayed response and lack of timely information from authorities and the water supplier.

The contamination was caused by a residual oil leak stemming from an oil tank explosion in 1987 and a fire in the residual oil pipeline, according to a preliminary investigation by the city government.

Authorities suspect the residual oil contaminated the underground water near a duct between two water works, bringing hazardous levels of benzene into the city's tap water, the city government said on Sunday.