Tainted drinking water found on 14 Cathay Pacific planes may have been contaminated with bacteria that entered the water supply before or during the process of filling the planes' tanks, a joint investigation led by the Department of Health has found. But the department says it has yet to determine the source of the contamination. Cathay is providing bottled water for use on the affected planes and has sterilised nine of those jets' water tanks after samples of drinking water taken from them failed to meet hygiene standards in a recent test, the statement released last night said. The remaining five jets are due back in the city before tomorrow, and will be cleaned on their return. The investigation is being jointly conducted with Cathay Pacific, the Airport Authority and the Hong Kong Aircraft Engineering Company (Haeco), which operates the water tankers that filled the planes. The airline yesterday could not give further details about the findings, such as when the contaminated samples were found, if the samples did in fact show excessive amounts of bacteria and which routes were affected. "The tests are still ongoing. We will announce the results once they are available," a spokeswoman said. The authority said the water sample from the latest test on June 22 was pending, while the readings of all other tests this month were normal. A Cathay pilot who did not want to be named said some Boeing 777 planes, used for long-haul flights, were affected. E coli was not detected in the water samples in Cathay Pacific's internal test, the statement quoted the airline as saying. Dr Ho Pak-leung, a microbiologist at the University of Hong Kong, said contaminants in the water might have exceeded hygiene guidelines issued by the World Health Organisation. Coliform bacteria in drinking water posed a "small risk" for passengers, Ho said. But, "it's still a hygiene problem". An excessive level of other types of bacteria would lead to turbid and smelly water, but does not cause great harm to passengers except children, pregnant women and the elderly, he said. Engineer Warren Chim Wing-nin, of the Institution of Engineers' aircraft division, said that the water might have been contaminated during delivery, when trucks bring water to planes from supply points. "If the pipes are polluted, the water truck would get contaminated. On the other hand, it could be a problem with the truck even with clean pipes," Chim said. He said it was unlikely the waste-disposal system would contaminate the water supply, as the two systems are not linked.