D-4-1 Assessing Methane Hazards from Landfill Sites
A guide for land use planning authorities and technical experts on how to test for the presence and potential harm from methane in soils around landfilled waste.
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1.1 This Appendix is intended for the use of Ministry staff who are familiar with problems associated with methane that is generated in landfills. The Guideline and this Appendix apply specifically where methane that has originated from waste exists in the subsurface, but also may be of some assistance in other circumstances. The document does not address problems associated with gases other than methane or with asphyxiation when oxygen is displaced by gasses produced from buried waste. The requirements of this Guideline are not intended to be prerequisites for a regulatory Approval, but rather to fonn a general basis for Ministry endorsement or advice.
1.2 Figure 1 shows a decision tree representing the procedure that should be followed in dealing with a potential methane problem. This decision tree should not be used unless the methane production in a landfill has peaked and is now declining. Therefore, unless there is evidence to the contrary, the Ministry considers the decision tree to apply only after at least ten years have elapsed since the landfill involved was closed. Note that where landfills have been encapsulated to prevent the entrance of water considerably more than ten years may be required before the peak rate of gas production passes. This decision tree must be used only in the context of the whole of this Appendix.
1.3 Discretion should be used by Ministry staff in applying this Guideline. It is expected that technology will improve and that this guideline will require updating. A flexible interpretation is therefore recommended.
General comments on technical assessments (2.0)
2.1 Methane cannot cause an explosion unless it accumulates to a concentration above its lower explosive limit (LEL) in an enclosed space where it can be ignited. (LEL is defined in Section 3.1). For this to occur, the gas that enters the enclosed space must have:
- a methane concentration sufficiently above the LEL, and,
- both a high enough entry rate, and a high enough accumulation time, such that the methane concentration will still be above the LEL, after dilution by ventilation of the enclosed space.
In theory therefore, a complete assessment of hazard should include consideration of the volume of gas containing methane at a concentration above the LEL that is moving into the enclosed space per unit time (i.e. the flux of methane at a concentration greater than LEL), and ventilation of the enclosed space. However at present it is not possible to confidently assess hazardous conditions in terms of flux. Therefore in practice safe conditions can only be assured by maintaining the methane concentration below the LEL. This is discussed further in Section 5.0.
2.2 It is a relatively straightforward matter to install monitoring and alarm devices for methane in, beneath, and immediately adjacent to structures, and in any associated utility conduits and trenches. Furthermore there is a high level of confidence that such devices will, if properly maintained, provide adequate warning. Therefore, these devices should be used where there is any doubt about whether or not methane could exceed 20% of the LEL at any time. Where gas control facilities are required to protect a structure, this type of monitoring array will be required, or recommended to insure that the control facilities do maintain the methane concentrations below 20% of the LEL.
2.3 The assessment of landfill gas "problems" is a specialised field and should be done by experienced professionals. However, Ministry staff should be aware of the following:
- It is important to use the proper instrument for measuring methane concentration in the subsurface. Most of the equipment commonly used to detect methane concentrations less than the LEL incorporates catalytic sensors. Such devices do not function properly in an anaerobic atmosphere, as is often present in landfill monitoring wells, without a special attachment. When methane concentrations greater than the LEL are expected, instruments using thermal conductivity sensors should be used. Such instruments are less sensitive than the catalytic type below the LEL.
- The concentration of methane and the landfill gas gauge pressure that will be measured in a monitoring well in a landfill may be influenced by changes in barometric pressure. There may be a delay of several hours before the landfill gas pressure and escape rate equilibrate to a changed barometric pressure. This should be considered when assessing monitoring data.
- Major changes in weather, such as thunder storms, may cause sudden increases in the concentration of methane at a point of concern. These changes may also cause power interruptions. Therefore back-up power should be provided to insure that methane detectors, and ventilation systems will continue to function as necessary.
2.4 Where studies are available that demonstrate that methane is not present in the landfilled waste at a concentration greater than 20% LEL, it can be assumed that methane from that landfill is not present on Adjacent Property at a higher concentration. Property near a landfill that might be threatened by landfill gas is called the Adjacent Property, even though other property may separate it from the landfill. Thus it may not be necessary to assess all the property in the vicinity of a landfill to establish safe conditions for development.
2.5 Various activities, such as the construction of utility conduits, ditches and trenches and groundwater pumping may provide new passageways for methane or change the rate at which methane is produced. Site assessment and facility decommissioning should consider the possible impact of such activities.
Rationale for the requirements of the guideline (3.0)
3.1 A mixture of 5% to 15% methane in air will explode if ignited. A concentration of 5% methane in air, is the "lower explosive limit" (LEL), and concentrations equal to or greater than the LEL are considered hazardous. To add a margin of safety, this Guideline considers that concentrations greater than 20% LEL may be associated with still higher concentrations, exceeding the LEL. Therefore, methane concentrations greater than 20% LEL warn of conditions which could be potentially hazardous, and gas control systems should be designed to maintain concentrations below this level.
Hazardous conditions are not considered to be present on a landfill, or on the property near a landfill, if the concentration of methane in the landfilled waste is determined to be less than 10% LEL. It is felt that if sufficient anaerobically decomposing organic material is present to be a threat, the concentration of methane would be more than 10% LEL. This provision is meant to eliminate shallow aerobic landfills, landfills that do not contain organic materials, and misidentified sites from further consideration or concern. The investigation necessary to demonstrate the presence of these conditions would likely be simple and would probably not require the accumulation of one year of monitoring data.
3.2 A number of factors may influence the migration and the concentration of methane in the subsurface, and several are dependent on both short term and seasonal weather conditions (e.g. barometric pressure trends, soil moisture, temperature, frozen ground). Therefore, to safely assess the influence of seasonal variations on these factors, except as noted in Section 3.1, at least one year and generally two years of monitoring, with several samples each year, are normally required.
3.3 The assessment of the concentration of methane in the subsurface on Adjacent Property is more difficult than is the assessment of its concentration within the waste. This is because the assessment of Adjacent Properties must consider the impact of weather conditions on both the production of methane and the migration of methane, whereas the assessment of methane within the landfill need only consider the impact of weather on methane production. It is for this reason that longer periods of monitoring are needed to assess Adjacent Property than are required to assess areas underlain by wastes.
Decommissioning and the installation of protective facilities (4.0)
4.1 The Ministry would consider that landfill gas control, alarm and monitoring systems could be safely decommissioned under the following circumstances.(Note Section 2.5 )
- For developments that overlie buried waste - Where the initial concentration of methane was greater than 10% LEL, monitoring systems may be decommissioned if methane concentration remains less than 20% LEL for three years, with any active gas control facilities not functioning.
- For developments that do not overlie buried waste - Where the initial concentration of methane was greater than 10% LEL, systems may be decommissioned if methane concentration remains less than 20% LEL for five years, with any active gas control facilities not functioning. Where the initial concentration of methane was less than 10% LEL, one year of monitoring, with any active gas control facilities not functioning, and showing methane concentration less than 10% LEL, would be needed.
Note however, that Ministry Guideline 07-07 "Land Use on or Near Landfills and Dumps", requires site conditions be assessed for landfill gas where there is a change in land use within 500 metres of the perimeter of fill of a landfill or dump, or where there are exceptional hydrogeologic conditions in the vicinity of a landfill or dump. Bear in mind also that a change in land use on the lands identified above may affect migration on adjacent lands (Section 2.5).
Passive gas control facilities, that is facilities that do not rely on air blower or gas suction equipment, cannot be "turned oft" and require maintenance or periodic inspection for proper operation. Therefore, monitoring facilities cannot be decommissioned at sites that rely on passive gas control facilities for safety, unless it can be shown that maintenance and/or inspection is not necessary.
4.2 There may be cases where a proponent wishes to proceed with development before all of the monitoring data that would be necessary to assess the site can be collected. Under such circumstances safe conditions can be achieved if protective facilities are installed that would warn of unsafe conditions and activate abatement. Initially, it should be assumed that worst case conditions are present and the concentration of methane is greater than 20% LEL outside the structures that are to be protected. Facilities must then be designed and installed that will operate to prevent concentrations greater than 20% LEL from occurring outside these structures. When it can be shown that the concentration of methane is less than 20% LEL, the facilities may remain on standby. Monitoring, as outlined in this Guideline, would still be required and the responsibilities must be assumed by an appropriate authority.
Alternatives to regulating by concentration (5.0)
5.1 The Ministry uses the concentration of methane as the main criterion for providing regulatory protection. This approach has been criticized in that it could occasionally prohibit developments where no danger exists, or require gas control facilities where none are needed. There are those who suggest that these restrictions could be avoided if a way could be found to provide protection by using the broader criterion known as flux (See Section 2.1) or by using some other approach. This matter has been examined and it was concluded that there are serious drawbacks associated with each of the alternate approaches that have been proposed. These approaches and their drawbacks are as follows:
Approach - Develop a means of measuring flux, and allow exemptions where the flux is less than some safe upper limit. For example, where a building code requires minimum air exchange rates for ventilation in dwellings, allow a methane flux that would not create a hazard provided the requirements of the building code have been met.
Drawback - Safe conditions cannot be related to the minimum number of air exchanges required for health and comfort in living areas. Even though the habitable space of a dwelling may have enough ventilation to dissipate a potentially hazardous flux of methane and provide safe conditions, there may still be enclosed spaces where explosive gas mixtures could develop. Examples are closets, cupboards, fuse boxes and basement cold rooms. Further, buildings are occasionally closed for a vacation or renovations. At such times, normal ventilation could be sufficiently reduced to allow dangerous concentrations of methane to develop in the interior. Therefore, the dividing line between non-hazardous and hazardous in terms of flux into living areas is not known.
As for monitoring methane flux in the underground, no instrument capable of doing this has yet been developed to a satisfactory stage. Even if it were possible to measure the magnitude of a flux of methane in the underground, it would still be uncertain how much of a hazard that particular flux constituted under various circumstances in various locations.
Approach - Seal the outsides of structures to prevent methane entry.
Drawback - Even if all methane entry points into a building could be sealed, additional openings might develop later. For example, cracks may develop in the basement wall, or openings may be made for such things as utility conduits.
Approach - Allow exemptions for sources of methane that, because of their size or the rate at which they produce methane, will not produce sufficient methane to be hazardous.
Drawback - The various types of landfill sites cannot be distinguished by the amount of organic material or methane they contain, with possible exceptions being ash disposal sites and hazardous waste disposal sites. No method is available to determine the minimum amount of buried organic material that could pose a hazard in a landfill site. Even small amounts of buried organic material, such as soil or putrescible wastes, can produce methane at concentrations above the LEL. Such concentrations can occur in otherwise relatively clean soil fill. Therefore, the Ministry is unable to suggest a way to provide exemptions from safety restrictions on the basis of a minimal content of organic material.
Approach - Allow exemptions where the soil type would prevent the movement of sufficient methane to be hazardous.
Drawback - Hazardous amounts of methane will probably not move through saturated soils and unfractured clayey soils. However, near-surface clayey soils are commonly fractured. All soils are subject to possible de-watering by construction. For example, where a high water table previously blocked methane migration, the construction of a new utility trench could lower the water table, and allow methane migration.
For these reasons, the Ministry does not believe that any of these proposed approaches provides reliable protection against explosion hazards from landfill gas. Therefore the Ministry must continue to regulate methane hazards on the basis of concentration even though in some cases this approach may be too restrictive. Where an applicant believes that restrictions based on concentration can be safely relaxed, arguments will be considered by the Ministry. Where there are uncertainties, the Ministry will allow them to be resolved through monitoring before development proceeds.