Quick Dips

Landfills in Bad Odour

Back when people used long drop dunnies they knew you risked an explosion if you dropped a cigarette butt down inside. Methane coming off decomposing organic waste is explosive even at low levels, in a confined space. The buildup of methane and other gases from old landfills has been a concern to councils, as it can create a safety hazard to the people living above.

In July 1994 a complaint about an acrid odour inside two Wellington houses led to an investigation which had a dramatic outcome. The houses, situated next to playing fields, were actually built over a closed landfill and the gas had come up the piles and in through the service ducts into the house. The levels inside and under the house actually exceeded the safety guidelines set to prevent explosions and asphyxiations, and the houses had to be evacuated.

The Wellington City Council investigated the neighbouring houses and found landfill gas in the backyards of six of them. They were advised not to use naked flames such as barbecues until the council had put in pipes to remove the gas from this part of the closed landfill.

The council also investigated their other 27 closed landfills and found gas leaking hazardously from four of them. There is no reason to assume that the rest of New Zealand is different, but most of the territorial authorities have not investigated their closed landfills.

The organic rubbish inside landfills decomposes to produce landfill gas, which is actually a mixture of gases: about 65% methane, 35% carbon dioxide and small amounts of odorous and/or toxic compounds. The two main hazards from landfill gas are from injury or death from explosion or fire, and from asphyxiation from high levels of landfill gas inside a confined space.

It is produced in fluctuating volumes depending on the weather, and continues to be formed for 30 to 50 years after the landfill has been closed. Unfortunately this hazardous gas can migrate underground laterally as well as vertically, and follows cavities, service pipes or geological strata to emerge up to 400m from the boundary of a landfill.

Large modern landfills are collecting this gas for an energy source. However there are at least 700 old closed landfills in New Zealand which have uneconomic levels of gas and the gas is escaping into the environment, some in a potentially hazardous manner. The old sites are usually converted into recreational space and playing fields, but there are cases where houses have been built next to or over closed landfills.

Closed landfills are a recognised hazard overseas. In the UK during the last 20 years there have been 22 serious incidents reported, including children and workers being asphyxiated in culverts, and explosions destroying houses and other buildings, with many injuries and some fatalities. There are 500 sites in England and Wales identified as presenting a potential safety hazard to the properties and people living nearby.

Most people do not know where the old landfills are, and that is partly how mistakes of inappropriate land use arise. The things to watch out for are subsidence on flat land, brown staining leachate entering water ways or stormwater drains, and/or an acrid gassy smell. If you are concerned, contact the dangerous goods officer in your local authority. Don't drop a cigarette butt down to test it!

Jenny Easton is the Health Protection Officer at Hutt Valley Health