The 85-year-old farmer has grown vegetables for the village since 1952 on a plot next to the dump site. He said he expects the debris to cause flooding that will cover his field and destroy his crops.

Whoever dumped the debris apparently placed a 60cm-diameter concrete pipe under the rubble to try to keep the stream flowing. However, the farmer, who was unwilling to give his name, said the pipe was far too small to drain the entire valley. 'This will lead to the flooding of my land at least twice a year,' he said.

A group of angry residents has reported the dumping to the government, but it has continued.

They fear the natural environment of the area, a breeding ground for the endangered Romer's tree frog, could be irreparably damaged.

The pipe also reduces the efficiency of a new concrete flood channel built by the Drainage Services Department last year. The HK$8 million project widened the stream to 2.3 metres, but now the water must pass through the small pipe before reaching the channel.

District lands officer Choy Kin-lun said the department had already raised concerns about how well the concrete pipe would handle the runoff during rainstorms. In response to residents' complaints, various government departments took part in a fact-finding inspection at the site on April 2.

The Environmental Protection Department conducted five surprise inspections to assess the levels of noise and air pollution caused by the dumping, but they were considered to be negligible on all occasions.

'From our investigations, the areas where construction waste has been deposited are all within private lots owned by different people,' Peter Diu Chin-pong, an environmental protection officer, said.

Officers inspecting the site found a man dumping construction waste, but were unable to do anything as the owner of the land said it was taking place under his direction.

Mr Choy said his office had put posters on railings around the site warning people to stop the dumping. Local resident Theodoros Boulieris said he had seen contractors removing the posters before continuing to dump the waste.

'The police, when I reported it, said the owner of the land was allowed to dump on it,' Mr Boulieris said. 'They said it was not illegal until they start to build on it. But what about the leaching effect of this waste into the soil, which is used to grow the fresh produce we feed our children?'

New posters were put up on Friday morning.

Under the Waste Disposal Ordinance, construction waste must be disposed of in an environmentally acceptable manner.

A nearby lily pond and surrounding breeding grounds for the endangered Romer's tree frog have been all but destroyed by the waste.

Frog expert Nancy Karraker, of Hong Kong University, has watched frogs breeding at the site for three years. 'Without concerted and informed restoration of the habitat, this will never be a breeding ground again for the frogs,' she said.

She has reported the problem to officials at the Agriculture, Fisheries and Conservation Department, but has received no response.

Although no building plans for the site have been received by the planning office, residents worry that the intention of the dumping is to reduce the ecological value of the land in the hope of future re-zoning.

The owner has told police that the land was being used for temporary waste storage.

The environment department said it would continue to monitor the situation and carry out spraying to prevent mosquito breeding.