The ability of landfill sites to emit methane has been recognised for very many years by site operators who appeared until the mid-seventies to accept small fires, caused by ignition of methane, as one of the minor problems associated with waste disposal. Methane problems were of little consequence when sites were small and situated outside the boundaries of towns or villages.
However in recent years there has been great pressure to acquire land for building so that in many instances real estate has been developed close to old, and not so old, filled and disused landfill sites (tips).
All real estate buyers are recommended to take great care when considering purchases close to landfills, or land which may have once been landfilled. These landfills can continue to generate methane for very many years.
Many old landfills have been restored and their sites have been incorporated into urban areas. Often such sites have been levelled and grassed over to be used for recreational purposes, and indeed this can be a very satisfactory end-use especially as any settlement which occurs can be filled in.
However, problems can arise if the landfill gas migrates sideways or upwards, as in the case studies which follow. These examples are quite old – newer ones are hard to come by as no-one likes to admit such problems, or risk publicity. Although implementation of preventative measures is much better nowadays and pretty much the norm, all those in real estate should be aware that landfill gas fire and explosion risks can still exist today.
Case Histories from the USA
One of the first explosions thought to involve landfill gas in a building occurred in Atlanta, Georgia in December 1967. Originally the building was constructed with one storey and a basement and the latter was eventually bricked off thus isolating it. The only connecting passage from the basement to the upper storey was a 6 inch diameter pipe through which a 3 inch gas pipe ran. The gap between the pipes was not sealed, landfill gas escaping through the gap was ignited by a cigarette which resulted in an explosion. This completely demolished the building with two people being killed and two suffering serious injury.
In 1962 an armoury was built close to an operating landfill in Winston-Salem, North Carolina. The landfill was from 3-13 m deep and during the mid-60’s combustible gases accumulated in nearby sewers and minor fires/ explosions occurred. In September 1969 an explosion, thought to be caused by smoking, occurred in the building in which 25 people were injured and 3 died. Migration of landfill gas into the building may have been encouraged by the placement of additional cover material on the fill about a week before the incident. The building was abandoned and the site used for the evaluation of various designs of gas migration prevention systems.
In Los Angeles, at the Branford landfill, which was situated in a gravel pit concentrations of methane well above the lower explosive limit were found up to about 150 m from the edge of the fill. Initial attempts to control migration by installation of standpipes and trenches were unsuccessful. The reason for the failure is not clear but is probably due to the depth of the site being considerable.
Also in Los Angeles, at the Sheldon-Arleta landfill, similar gas migration problems were encountered to those in case 11. Since the Sheldon-Arleta site is in a residential neighbourhood it was essential to take rapid remedial action. A series of wells 8 m deep, spaced on 50 m centres were-installed just within the landfill boundary. Each well had a designed withdrawal rate of 200 cubic ft/min each and were linked together with the landfill gas being flared off.
This site is of particular interest since it demonstrates that the need to install a gas migration control system can give rise to a vigorous industry based on extraction of landfill gas with subsequent utilisation. This experience in California gave impetus to the new technology of landfill gas utilisation for power generation, which is noteworthy for the real estate buyer as it shows just how much energy is present in the landfill gas.
CONTROL OF LANDFILL GAS MIGRATION
The most important step is to recognise that if any development is to take place either on or near to a landfill site then problems can occur. Planners, architects and developers are normally aware of these risks and take measures to overcome potential hazards. Special features have usually been incorporated into building designs to reduce this risks to acceptably low levels before these buildings are put up for sale, nevertheless, real estate buyers should carry out their own independent checks.
If development has taken place in close proximity to landfill then a survey of methane concentrations on the site is recommended.
This can be done by drilling boreholes and fitting these with perforated liners and measuring methane concentrations after a few days. A cheaper but in many instances equally satisfactory option, is to dig pits with a mechanical excavator to a depth of 3-4 m, install perforated pipe, back-fill and again measure methane after several days. Measurements of gas concentrations can be made either on site using commercially available instruments such as a Gas Tec or Gas Scope and must be assessed by suitably qualified experts.
The astute real estate buyer will ensure that by understanding landfill gas aspects when buying property near landfills to avoid losses later.