Effluent that might contain a high level of ammonia has leaked from a rubbish dump in North District into nearby rivers, contaminating water used by farmers to irrigate their crops.
The leak at the Ta Kwu Ling landfill, discovered last month, was disclosed by the government yesterday amid complaints it had hidden the news from the public for more than a month.
The leak occurred after a supposedly impermeable layer at the base of a holding lagoon at the landfill was damaged, sending contaminated leachate into the Kong Yui Channel, which flows into the Shenzhen River, and then to Deep Bay, where the Mai Po nature reserve is located.
Officials said yesterday that no one was thought to have been drawing drinking water from the concrete-lined channel, but that a few farmers were using it for irrigation.
They said the leak had been dammed and some effluent had been diverted into waste piles while the remainder had been transferred in trucks to an off-site treatment plant operated by the Drainage Services Department.
Assistant director of environmental protection David Wong Tak-wai refused to disclose the extent or type of the pollution, but said the government was considering prosecuting the contractor, Far East Landfill Technologies, for violating the Water Pollution Control Ordinance.
Professor Jonathan Wong Woon-chung, a waste specialist from Baptist University, said the impact of the leak was difficult to measure without knowing the scale of it.
But he said landfill effluent usually contained a high level of ammonia and low level of oxygen that would hurt vegetation and soils.
Calling for an investigation, Wong said: "If the lagoon is newly built, there might be a question about its construction quality."
Lawmaker Fernando Cheung Chiu-hung said it was "unacceptable" for officials to hide the leakage from the public. "There are people using the water for irrigation of crops that they consume themselves. This will affect their health badly," he said.
The leak was first reported by the contractor - a subsidiary of Suez Environment through SITA Waste Services - on July 28.
The contractor said one of seven temporary lagoons, built to store effluent until an on-site treatment plant had the capacity to handle it, had been leaking since the previous day.
At least two water samples were collected outside the landfill. One, taken on July 29, contained raised levels of pollutants that were still within the legal limit. But another, taken on August 5, exceeded the limit.
At a press conference in the absence of the contractor yesterday, Wong dodged questions about why it took the government a month to disclose the leak, whether there had been a regular water sampling check at the channel, and how big the lagoon was. "Under normal circumstances, no sewage leaks out of the landfill. If there is a leak, our officers at the landfills can find it in their daily inspection," he said.
The Ta Kwu Ling landfill, opened in the mid-1990s, is one of three built about the same time and is expected to be full by 2017.
After the leak, the environment department inspected the two other tips, at Tuen Mun and Tseung Kwan O, and found no irregularities.