CONTAMINATED water leaking from landfills will be the subject of a $5.25 million study, the Environmental Protection Department (EPD) said yesterday. The dirty water, called leachate, is several times more contaminated than ordinary sewage and seeps out as the rubbish decays or when it rains. It ends up in drains, most of which empty into Victoria Harbour, and receives little or no treatment. EPD assistant director Mike Stokoe said yesterday the nine-month study signed with Binnie Consultants Ltd would look at how to apply on-site treatment to remove pollutants from the leachate. It follows a study completed earlier this year into environmental problems at five urban landfills that have been closed, and possible future uses for the sites. Methane gas drifting from these landfills was considered the most immediate threat. The gas was within safe levels, although methane can be explosive in the right combination with oxygen and can cause asphyxiation in confined spaces. The EPD has already decided to install flares at landfills to burn off the gas but until these are fitted, remedial measures are being carried out, for example, digging trenches around the landfills to trap the horizontally moving methane. Mr Stokoe said the department had found that the leachate could be intercepted by wells and pumping systems and that he wanted to do more detailed tests on the best way to treat it. ''Leachate at its weakest can be several times the strength of sewage so we feel it should be treated at the source,'' he said. When finished, it is hoped the findings will be applied to the closed urban landfills as well as three presently operating at Tseung Kwan O, Shuen Wan and Pillar Point. Three multi-billion dollar landfills due to open in the next two years will have special liners to catch the leachate, flares for gas disposal and other environmental features to minimise their impact. The closed landfills are at Ngau Chi Wan, Ma Yau Tong Central and West, Gin Drinker's Bay and Jordan Valley. Restoration of all the sites to parks or other recreational use will be carried out over the next four years as the leachate and methane gas are brought under control. Sai Tso Wan landfill, which is also closed, is being looked at separately in the present study because of an existing gas flare-off. Binnie's has been asked to look at leachate control and future uses for the site.