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Landfills are sites where waste materials are disposed by burial, and the biogenic gases produced on these sites are known as ‘landfill gases’. Landfill gases are produced in the subsurface layer of landfills during the decomposition of buried waste materials. The gas is approximately made up of 60% methane (CH4) and 30% carbon dioxide (CO2), with trace amounts of other organic vapours and gases. The proportion of these compounds, as well as the overall quantity and rate of gas production, depends on the stage of decomposition, the operational conditions, density, composition, and age of the buried waste material.
The release of these gases and their constituents to the atmosphere is known to have some human health and environmental implications such as carcinogenicity and mutagenicity, air pollution, corrosion of monitoring facilities, fire and explosions hazards. For this reason, the integrated monitoring of landfill gas has been a conventional practise across Europe and North America.
Subsurface monitoring of methane production is one of the major components of these integrated landfill gas monitoring programmes because of the hazards associated with subsurface production and migration of methane. Although the integrated approach has succeeded in reducing the hazards ensuing from methane-related explosions in landfills, their scale of operation remains a limiting factor. For instance, the current practise in subsurface landfill gas monitoring that involves installing gas monitoring probes, or wells, might not be operationally efficient if implemented on large areas of landfills reclaimed for the establishment of forest vegetation.
This limitation is evident in the reclamation of open-mine pits in Alberta, Canada where the residual materials from oil sands mining (overburdens) that contain fragments of concentrated oil sands (lean oil sands) are used to fill the open-pit mines. The degradation of Lean Oil Sands (LOS) under anaerobic (absence of oxygen) conditions produces methane gas and other volatile organic compounds (VOC).
Since the major aim of open-pit mine reclamation in this region is to re-establish a boreal forest ecosystem on post-mined landscapes, the flux of methane from the subsurface layer forms a major limitation to the success of this reclamation attempt. The spatiotemporal variability in subsurface methane production in such a large area of landscape cannot be captured by the contemporary subsurface gas monitoring techniques. Thus, this article presents the need for developing large scale and spatiotemporal subsurface gas monitoring strategies that will be suitable for large areas of landfill sites such as the reclaimed open-pit mines in Alberta.
Lean Oil Sands (LOS) in landfill soils and ecological implications
The open-pit oil sands mining technique involves the removal of all vegetation, soil, overburden, and oil, leaving pits of up to 100m deep, which stretches to several kilometres across the landscape. Environmental regulatory standards require that disturbed landscapes are reclaimed to equivalent land capabilities relative to pre-disturbance conditions.
These pits are reclaimed through a variety of land cover treatments which should be capable of sustaining vegetation establishment on oil sands mine sites. Reclamation involves filling the open-pit mine with mineral overburden salvaged during mining operations and the surface capped with a peat/mineral mixture.
Unlike municipal landfill sites that contain a wide range of domestic and industrial waste materials, the mineral layer of reclaimed oil sands mines contain residual chemicals and LOS only. The LOS consists of about 8% Petroleum Hydrocarbons (PHCs), and it’s been established that the byproducts of PHC degradation will be detrimental to plant growth and establishment.
The breakdown of PHC in the mineral layer of the reconstructed soil by decomposer microbes will produce a wide variety of gases with trace organic compounds. The latter implies that the gases produced during decomposition of buried LOS are most likely to contain a wide variety of hydrocarbon compounds such as benzene, esters and some non-methanogenic VOCs that can contribute to the formation of ozone, a gas which, in addition to having deleterious respiratory effects, also can reduce plant growth and contribute to vegetation damage.
Additionally, the occurrence of methane in the soil layer around the rooting zone of plants displaces oxygen, resulting in plant stress and subsequent death. This explains why Brosseau and Heitz4, suggested that the state of vegetation in a landfill site may be used as an indicator of overall levels of landfill gas emission within the site. They further suggested that aerial photographs or remote sensing of vegetation can be used to obtain a snapshot of areas under ecological stress caused by methane gas burns.
As seen on reclaimed oil sands mines and farms near municipal landfills, methane emission can cause crop damage. But previous studies examining the problems associated with the release of landfill gases and trace compounds to the environment did not document the impact on ecological variables (e.g. vegetation); rather, they focused on the harmful affect of these compounds on human health, air quality and corrosion of landfill gas collection and combustion systems. Thus, the need to monitor the affect of landfill gas on ecological variables has created a new paradigm for landfill methane emission monitoring.
Subsurface production and migration of methane in landfills
Methane can be described as an odourless and colourless flammable gas that burns with a pale, faintly luminous flame. It is widely distributed in nature and the atmosphere naturally contains 0.00022% by volume (2.2 ppm). It is lighter than air and under constant atmospheric conditions has the tendency to rise through the air.
The release of methane from landfills involves three key processes: methane production, oxidation and transport mechanism. Methane is produced during anaerobic decomposition of the buried waste materials in a landfill. The process is mediated by a group of methane-producing archaea (methanogens) and is very efficient when the subsurface layer is saturated with water.
Methane is oxidised to CO2 in the oxic zone (where oxygen is present) of the subsurface profile by a group of bacteria known as methanotrophs. When produced, methane gas is transported to the surface layer through three major modes of transport: diffusion, ebullition and plant-mediated transport5. The availability of oxygen within the subsurface layer forms a major control on these processes. Thus, environmental factors that alter oxygen availability – such as soil temperature, aeration and water table elevation – within the subsurface layer will affect the rate of methane production and emission in landfill sites.
Methane emissions to the atmosphere or at perimeter locations adjacent to the landfill can occur through a combination of lateral and vertical migration. The subsurface and atmospheric pressure gradient drives the migration of methane gas from areas of high pressure to areas of low pressure. The production of methane during the decomposition of waste materials creates the high pressure conditions required for the gas to migrate to the surface. Relative changes in meteorological conditions such as barometric pressure can also affect the migration of methane by changing methane gas pressure gradients in the subsurface layer, which leads to an increased potential for vertical gas migration into the atmosphere.
Methane migration can also occur laterally along preferential pathways where higher permeable native soils are present, or along buried utility corridors filled with aggregate that are coarser than surrounding soils. The frozen winter soil conditions also enhance lateral methane migration, as vertical gas migration routes are impeded by thick frozen soil columns. Thus, as discussed in the later sections of this article, geometric properties and layering of landfill materials determines the dominant gas migration route.
Monitoring methane emissions in landfill sites
Landfill gas monitoring programmes were established to help reduce some of the environmental problems associated with landfill gas emissions. Methane monitoring is implemented at every landfill to ensure that elevated methane concentrations are detected before they present an explosion risk to the general public.
In the United States, for instance, the California Integrated Waste Management Board (CIWMB) implemented the landfill gas monitoring regulations after a series of fire and explosion incidents in structures near landfills were linked to subsurface fugitive methane gas emission6. Similar regulations have also been established in other North American jurisdictions like Alberta, British Columbia, Manitoba, Québec and Ontario, Canada.
A common attribute of these regulations is that they recommend the adoption of an integrated monitoring approach, which goes beyond the conventional surface trace gas monitoring to also account for subsurface production and migration of landfill gases to adjacent structures. Previous studies4 also solicited for further research in order to expand the current knowledge about air dispersion and migration of trace gases in the subsurface layer of landfill sites.
Subsurface monitoring of methane in landfills
Methane is the major landfill gas targeted in subsurface monitoring programmes because it has been implicated as the major cause of landfill explosion and fire incidence. Current landfill legislation recommends that methane monitoring should be a frequent operation in order to capture the potential for seasonal changes in gas concentrations due to ensuing changes in moisture, ground temperature and frost conditions.
Most of the legislation recommends three to four monitoring campaigns in a year, involving multiple sampling events. The essence of a multiple sampling event is to avoid making conclusions on site conditions at any particular time based on concentrations recorded from one point which may not fully represent the actual site conditions.
Presently, subsurface methane monitoring is being conducted with temporary monitoring probes or permanent gas wells installed to allow for multiple sampling events. The temporary monitoring probes are only suitable for measuring methane concentrations near the surface, as they can only be driven approximately one to two metres into the soil. Permanent gas wells consisting of perforated plastic casing are installed in some high risk sites because it affords the ability to monitor over a greater cross-sectional area of the buried waste, since the wells penetrate the entire thickness of the waste fill from the base of waste to a few metres below the ground surface.
Obtaining a representative concentration measurement with this method involves the installation of multiple wells and placing of sampling probes at set vertical intervals along the soil horizon. The concentration of methane gas migrating from the probes is determined using handheld gas analysers or by laboratory analysis of gas samples collected from the probes.
Portable gas analysers are capable of quantifying the concentration of methane gas instantly. Other specialised instruments have also been designed to offer continuous monitoring of subsurface gas migration from permanent wells. These devices are also capable of testing for other gases such as oxygen, carbon dioxide and hydrogen sulphide. The laboratory analyses of gas samples collected in the field using an evacuated canister is also in practise. This method provides more accurate results than the portable handheld gas analysers, especially at lower gas concentration thresholds. Field measurements with portable handheld gas analyser still have a time efficiency advantage, however.
In addition to subsurface methane monitoring, landfill monitoring legislation also recommends that the potential of methane migration be verified by monitoring the gas pressure gradient. This can be achieved simultaneously with gas composition by using the same gas monitoring well installation. Instant gas pressure measurements can be taken from the well with the aid of an electronic pressure gauge, or a U-tube Manometer.
Factors affecting subsurface migration and monitoring
The geometric layering and hydrogeologic properties of the buried overburden in landfill sites are the major factors that affect the migration of gas. These properties are associated with the level of ground water, and the textural characteristics of landfill geologic units, such as the transmissivity and permeability – primary and secondary – of geologic materials. Some of these properties will be determined by the landfill waste composition. With regards to level of ground water, landfills with shallow ground water will have a high anaerobic layer and high rate of methane migration to the surface. In such landfills, shallow monitoring wells are also installed.
Primary permeability is a function of the coarseness or fineness on the geologic material, which determines the ease of gas migration through subsurface layers. Water permeable geologic units are also permeable to gases. Highest concentration and migration rates of methane will be measured in coarser sediments, or even in secondary permeability structures such as rock fractures, which act as conduit and preferential gas migration pathways.
Although these factors are considered during the installation of subsurface monitoring wells, it is almost non-realistic to place the well in an area that will intercept all methane gases in the subsurface. Thus, the major limitation of the current methods used in subsurface landfill gas monitoring is that the probes driven into the potential areas of concern provide only point source measurements of methane gas concentrations in the local environment around the probe.
Improving current techniques
As noted in the preceding section of this article, the contemporary techniques used in monitoring subsurface gas migration are limited to point source monitoring, which essentially captures steady diffusive fluxes and often provides little information about the spatiotemporal distribution of subsurface gas migration.
Capturing the spatiotemporal variability of methane migration is critical, since the environmental and hydrogeologic variables – e.g. pressure gradients, temperature, and water table elevation – controlling subsurface methane fluxes also vary spatiotemporally. Another limitation of the probe method is seen in the disturbance associated with the installation of monitoring wells, as this may disrupt the in situ gas regimes and potentially create secondary permeability conduits within the landfill profile. Thus, landfill sites reclaimed for the purpose of forest establishment will require an efficient subsurface gas monitoring programme that is less destructive, and able to provide information on the spatial and temporal distribution of biogenic gases within the subsurface layer.
In view of these limitations, adopting the principles of ground penetrating radar (GPR) presents a potential geophysical approach that could overcome all the disadvantages of the current method used in subsurface gas monitoring. Comas and Slater7 applied this technology in peatland studies and reported considerable success. Comas et al, (2008) had previously used this method to image peatland stratigraphy and identify potential gas trapping layers, estimate the vertical distribution of biogenic gases within the peat deposit and monitor seasonal changes in gas concentration and ebullition fluxes in northern peatlands.
As described by Comas and Slater8, the principle of GPR is based on a transmitting antenna that generates a continuous, high frequency electromagnetic (EM) wave that penetrates the subsurface and is returned as a sequence of reflections from stratigraphic interfaces. The GPR technique is very sensitive to the water distribution in the subsurface because the velocity of the EM wave is primarily controlled by the relative dielectric permittivity, a geophysical property strongly dependent on water content, and thus gas content, and therefore changes in velocity can be related to changes in gas content within the subsurface layer.
Change in velocity is estimated by temporal changes in travel time to particular interfaces. The GPR method will be suitable for subsurface gas monitoring in reclaimed oil sands mines, and other re-vegetated landfill sites, as it can be applied non-destructively from the surface to determine the distribution of biogenic gases within the subsurface layer.
Synthesis
Through the review of literature and field practises, this article was able to confirm that the subsurface layer of landfills such as reclaimed open-pit mines are hot spots for the production of biogenic gases that finally migrate to the surface, where they pose a detrimental effect to the growth and survival of vegetation planted for ecosystem reclamation. Landfill methane gas has also been associated with explosions and fire incidents in structures adjacent to landfill sites. The hazards associated with methane production in landfill sites have led to the integrated approach of methane gas monitoring which incorporates surface and subsurface monitoring of methane fluxes.
The production and migration of methane in the subsurface layer of landfills are controlled by environmental and hydrogeological factors. These factors vary in a spatiotemporal manner that cannot be captured with the current method of landfill gas monitoring, which adopts a point source monitoring approach. This well/probe monitoring method is also limited due to the destructive aspects of the sampling technique.
This method is not suitable for landfill sites reclaimed for forest re-establishment because it cannot generate a high resolution spatiotemporal information required to guide the decision of reclamation experts on when and where vegetation can be successfully established on the reclaimed landscape without risk of plant burns by methane gas. Secondly, the destructive method of monitoring well installation is not considered a sustainable approach in ecosystem reclamation, as plant establishment requires a less disturbed environment.
For this reason, this article advocates the adoption of a non-destructive approach that can generate high resolution spatiotemporal information on the fluxes of methane gas in the subsurface layer of reclaimed landfill sites. GPR technology presents the potential to efficiently support these operational requirements.
There is need for further research into the development and commercialisation of this technology. This will be useful to ongoing large scale open-pit oil sand reclamation projects in Alberta, Canada. Environmental equipment manufacturers are encouraged to explore this sustainable innovative technology.
Published: 07th Mar 2013 in AWE International
