Gas seeping from landfill site

By ANNE BESTON environment reporter

An emergency alarm system has been installed at a defunct landfill site leaking dangerous amounts of flammable methane gas near Motat at Western Springs.

The old landfill at Motions Rd between Westmere and Pt Chevalier is also home to the Museum of Transport and Technology's historic aviation display, which includes a hangar housing a World War II bomber.

The hangar is now alarmed with a gas detector and the museum is having to pay to seal a crack in the floor because of the danger of gas seeping into the building.

Other buildings on the site are also linked to the alarm system, which will activate if the methane builds to dangerous levels.

Auckland City Council began dumping 45,000 tonnes of soil at the Motions Rd landfill late last year after monitoring work showed the gas at higher levels than previously thought.

The landfill was used between the 1930s and 1970s and some of the refuse is up to 10m thick. When the rubbish interacts with rainwater, it forms toxic leachate and gives off methane gas.

The clay "cap", up to 1.5m thick, helps to trap the gas underground but the unpredictability of methane means earthworks have now stopped until council staff and environmental consultants decide what to do next.

"We've done the emergency capping. The next phase is additional bores to intercept the gas to minimise it coming up in buildings and to look at flaring or co-generation options," said council utilities adviser Garry Peters.

Motat director Jeremy Hubbard said the museum was happy with the work being done and did not consider the gas was a public safety issue.

"The site has been looked at by the Fire Service. There is nothing in the risk assessment to say we should vacate or leave," he said.

Mr Peters said there had been a number of fires at the site over the years, some of which could have been caused by cinders from Motat's vintage steam locomotive that runs round the landfill's perimeter, igniting vegetation.

Mr Hubbard said the train had been modified to make sure no cinders escaped and scrub around the site had been cleared in the past few months.

A permanent gas flare was being considered, said Mr Peters, but that might create its own problems.

"There is a danger of sucking oxygen into the landfill if you suck the gas out too quickly," he said.

"As soon as you do that it can ignite and we don't want that because basically it would mean we would have to dig the whole thing up."