Landfill gas threat measured
Gas monitoring wells are being installed around Nelson city’s old landfill site to further gauge potential risks to landowners.
The old landfill, in the area bounded by Queen Elizabeth II Drive, Atawhai Drive, Weka St and Sovereign St, operated for about 40 years before it was closed in 1987, covered and buried. Under the ground, the natural process of rubbish breaking down is producing a chemical cocktail known as ‘‘landfill gas’’, a mix of carbon dioxide (CO2) and methane (CH4).
In the next few weeks, specialist consultants will be more accurately mapping and measuring the landfill gas, finding out how thick the cap over the landfill is, and installing nine monitoring wells.
Methane is measured throughout the area every six months. In March last year, Nelson City Council announced increased research and testing into potential landfill gas threats, after routine tests showed gas levels creeping up.
The council’s group manager, infrastructure Alec Louverdis said measurements had so far not revealed any areas of concern, though an extra stormwater vent was recommended and installed at Miyazu Gardens.
‘‘All gas monitoring showed methane levels were within acceptable standards,’’ he said.
Both CO2 and CH4 are heavier than air, so when the gas reaches ground level, it tends to stay low until it dissipates.
The effect of this became clear at the Nelmac nursery on Atawhai Dr a few years ago, when a workman accidentally sparked a fire.
A former nursery worker who didn’t want to be named said the fire gave everyone ‘‘a bit of a fright’’, and had to be put out by firefighters.
‘‘The flames were just licking across the ground,’’ the former employee said. ‘‘We [at the nursery] had processes in place for the methane – we used to open up windows in the mornings – but this worker didn’t know, and it hadn’t occurred to us as a potential issue.’’
Louverdis said there were two fires attributed to methane at the nursery site. ‘‘One involved the use of a welder in an enclosed below-ground chamber, and one happened when a grinder was being used to collapse an old well.
‘‘We now understand more about the risks associated with carrying out underground work on an historic landfill site, and projects are carefully managed to take these risks into account.’’
Louverdis said that when the gas was detected at the nursery in 2016, monitoring equipment was ‘‘immediately’’ installed, and Nelmac staff also now had procedures to manage potential risks related to methane.
For people living within the landfill footprint, the soil was safe for all usual uses, but work underground could be a little more risky, he said.
‘‘Residents have been advised if they want to do any substantial digging or excavations, they should contact council first to ensure there are no extra safety precautions they might need to take.
‘‘Councils all over New Zealand are tackling the same issues, and as a result much more is known about the long-term management of closed landfills.’’
‘‘All gas monitoring showed methane levels were within acceptable standards.’’
Nelson City Council group manager, infrastructure Alec Louverdis