Raw sewage pouring into Shenzhen's Dongjiang river could spell disaster for Hong Kong's drinking water, it was claimed yesterday. Water chiefs denied people were at risk, saying local supplies conformed to World Health Organisation standards. But experts said the problem was more serious than officials admit, claiming there had been a massive increase in pesticides and heavy metals flowing into the river. Nearly all of Hong Kong's water comes from the Dongjiang - or East River - at a cost of $2.60 per cubic metre. 'Around the Dongjiang river, population is now denser so domestic sewage is finding its way into the river basin,' Water Supplies Department assistant director Mak Kwing-yu told legislators. 'Since 1989 water quality has slightly deteriorated. It is not possible to say what will happen in five or 10 years' time.' Chief waterworks chemist Cheung Tze-leung said more minerals had been detected but denied there was any increase in bacteria from sewage or a significant change in levels of heavy metals or pesticides. But Professor Vincent Chen, of Guangzhou's Institute of Ecology, argued development of the Shenzhen and Dongguan areas had led to a massive increase in river pollution. The amount of untreated sewage had almost tripled between 1990 and 1995 to 847 million tonnes, he said. In addition, farms' use of chemical fertilisers had more than doubled while pesticides had almost tripled to 21,000 tonnes a year - with 70 per cent washed into the river. Head of Hong Kong University's Department of Ecology and Biodiversity, Dr John Hodgkiss, said mainland sewage contamination had shown up at the Sha Tin treatment plant. 'Undoubtedly it has got worse,' he said. Bacteria from sewage can be killed by chlorine, but the department's Professor Mike Dickman warned there was evidence that too much of the chemical was dangerous. 'As the East River gets more and more polluted you have to put more and more chlorine in to get the same effect,' he said. If China's raw sewage was contaminating water supplies, it was likely traces of toxic metals like mercury and lead were also reaching Hong Kong, he added.