Heavy rain loosened the foundation of a temporary leachate pool and its waterproof liner gave way, causing an unprecedented leak in the Ta Kwu Ling landfill in late July, according to an Environmental Protection Department probe.
Department deputy director Tse Chin-wan said the leak was caused by a one-square-foot hole in the liner, but the leachate was contained within the underground catchment area of the landfill and did not leave the site.
Earlier reports said leachate had overflowed into the nearby Kong Yiu River - a man-made concrete channel flowing into the Shenzhen River - but Tse said this overflow was just dirty water and soil running off the landfill site after heavy rainfall.
Tse maintained the leak was an isolated case and leachate lagoons in the other two landfills in Tuen Mun and Tseung Kwan O were in good condition.
"The damaged lagoon has been fixed and there is no sign of leaking in other landfills," said Tse, who briefed legislators on the progress of the probe during a site visit yesterday.
Leachate is the liquid that has percolated through solid waste. It is held in leachate lagoons in landfills before being sent to the leachate treatment plant in the landfill for treatment.
Since 2008, there have been 22 complaints about leaks from landfills, but the department said none of these was substantiated.
Most legislators said the incident had shaken public confidence in the department's landfill management and it would now be more difficult for the legislature to endorse the government's landfill expansion plans.
Labour Party lawmaker Cyd Ho Sau-lan, also the chairwoman of the Legislative Council panel on environmental affairs, said: "It is not realistic for Hong Kong to give up using landfills to treat waste. But we hope there will be a better monitoring and reporting system in place to avoid similar incidents in the future."
She also urged the government to step up efforts to reduce food waste, which was one of the major sources of leachate. Ho added her panel would call a special meeting this month to discuss the incident.