Six approaches to measure and quantify landfill gas: Choosing the right solution

The rate at which vapor-phase chemicals, such as those found in landfill gas, cross the soil-air interface is called the landfill gas "flux" rate, which is measured as mass per unit area per unit time (e.g., micrograms of landfill gas per square meter of soil surface per minute). Direct measurement using the isolated flux chamber method is often used for landfills to quantify this rate of flux. Landfill gas is captured by flux chambers placed on the landfill surface so that the changes in concentrations of methane (and other constituents) in the landfill gas can be measured. 

Advantages:

• Easy to operate
• Able to detect smaller fluxes than other methods
• Suitable for both large and small areas if performed properly
• Highly portable and can be moved easily from one sample location to the next
• No power supply is required
• Low equipment cost, compared to other methods 

Considerations:

• Specific in time and location than other methods – Not capable of capturing long-term flux variation
• Sensitive to weather conditions  
• Labor intensive 

Eddy covariance technique (EC) is the most widely used methodology to measure carbon fluxes at a landscape level and is one of the most direct and defensible methods for flux measurement. EC uses an assembly of sensors arranged in a tower to measure emission flux based on the number of molecules moving upward and downward over time, as well as the travelling speeds of these molecules.

Advantages:

• Direct, near real-time measurement

• Capability to measure flux variation over time  

Considerations:

• Higher capital cost due specialized equipment
• Improved performance hardwired power connections
• Sensitive to low wind speeds conditions (<1 m/s, specifically during nights).
• Suited for permanent or semi-permanent installations.
• This method is widely used to measure carbon at a landscape level

The radial plume mapping technique is a relatively new technique and is based on the U.S. Environmental Protection Agency test method (OTM 10) for characterizing emissions from area and fugitive sources. An open path, path-integrated Optical Remote Sensing (ORS) system is employed to identify “hot spots” of emissions and quantify emission fluxes. Open path-Fourier-transform infrared (OP-FTIR) spectroscopy is the most common system using this technique with these common uses:

1. Horizontal radial plume mapping: Used for mapping gas concentrations in a horizontal plane. Hot spots near the ground can be located.

2. Vertical radial plume mapping: Designed to determine the emission flux of contaminants in a vertical plane downwind from an emission source.

3. One-dimensional radial plume mapping: Suitable for obtaining the profile of pollutant concentrations along a line-of-sight (e.g. fence line monitoring ).

Advantages:

• Characterizes fugitive emissions from large area sources
• Provides 3D gas profiles

Considerations:

• Suitable for diffuse sources and does not able to quantify point emission sources 

• The capital costs for the system are expensive due to specialized equipment

• Detailed technical understanding, training, and experience is required

4. Tracer correlation

The tracer correlation method is used to produce two-dimensional profiles of landfill gas emissions. It relies on the controlled release of a known rate of a tracer gas, compared with a suspected gas. The concentration of the tracer is measured downwind with a mobile monitor and simultaneously measures the source gas concentration. By knowing the emission rate of the tracer and the downwind concentrations of both tracer and target, the target emission rate can be calculated.

Advantages:

• Detailed accurate meteorological measurements are not required.
• Good for quantifying emissions from area sources and provides insights on emission pathways.

Considerations:

• Careful selection of tracer gas requires balancing considerations background conditions project emissions and health and safety. 

• Useful for understanding emissions from smaller areas; sampling complexity increases for with area size. 

Remote sensing techniques including drones, aircraft, and satellites are very promising for detecting emission hotspots over large area sources from a distance such as well site emissions.  Using remote technologies coupled with modeling software provides a high-resolution overview of the landfill and path map of the emissions which is then used to determine the total emissions and/or emission rates.

Advantages:

• Less labor intensive than other methods
• Reduces the time necessary to complete quarterly monitoring; improving efficiency and lowering labor costs. 

Considerations:

• May not accurately detect small scale changes in emissions.

Ambient air monitoring measures the air quality conditions around the landfill or at the facility fenceline. It can help to estimate the emission rates of LFG constituents through inverse plume modeling (“reverse modeling”), can help confirm dispersion modeling predictions, or to quantify the concentration of individual chemical compounds. Ambient monitoring can be carried out by using either fixed monitoring stations or using mobile platforms. Fixed stations are generally suitable for long term studies, while mobile platforms are more flexible and allow sampling at a variety of downwind distances and plume directions but are constrained by the limits of vehicle operations.

Advantages: 

• Determines actual concentrations of pollutants that can cause impacts to public health.

• Helps understand and confirm if air quality standards are being met. 

Considerations:

• Often requires permanent or semi-permanent  station. Best suited for long-term monitoring programs.