Landfill Gas Monitoring Notes

Chapter 4: Monitoring of Landfill Gas

Introduction

Environmental health professionals may be asked to review and comment on landfill sampling and monitoring plans or interpret data to evaluate potential public health hazards.
This chapter provides information on monitoring program design, sampling and monitoring equipment, and data interpretation for different landfill gas sampling methods.
Monitoring data at landfills do not necessarily reflect actual exposure levels but offer insight into air quality, gas migration, or potential health hazards.
Landfill gas monitoring falls into five categories:
- Soil gas monitoring
- Near surface gas monitoring
- Emissions monitoring
- Ambient air monitoring
- Indoor air monitoring
Data from different monitoring activities have varying public health implications.
Mathematical modeling can help interpret landfill emissions data.

Landfill Gas Sampling Approaches: An Overview

Two important factors in selecting a landfill gas sampling approach are the sampling location and the sampling methods.
The sampling location and sampling methods are selected according to the data uses and questions to be answered by the overall sampling program.
Examples of location, or placement, of gas monitors.
Several methods of landfill gas collection can be used:
- Portable vs. stationary sampling equipment:
  - Portable monitors are hand-held instruments useful for initial screening or identifying methane leaks.
  - Stationary monitors are installed at fixed locations and usually generate higher quality data.
- Grab sampling vs. continuous monitoring:
  - Grab sampling is a one-time measurement (1 to 5 minutes) providing a snapshot of gas composition.
  - Continuous monitoring devices constantly sample and analyze gas concentrations, providing insight into long-term changes.
- Analysis of samples in the laboratory vs. analysis in the field:
  - Laboratory analysis may take days or weeks and can be expensive but generates highly accurate and precise results for many pollutants.
  - Real-time monitoring reports concentrations immediately but typically measures only one pollutant and is not as sensitive as laboratory analysis.
The features of a particular landfill gas sampling program vary from landfill to landfill, and the ideal sampling strategy for one landfill may not be appropriate for the next.
Regulatory requirements often dictate sampling features (e.g., EPA's soil gas monitoring requirements).

Soil Gas Monitoring

Soil gas monitoring measures concentrations of gases in the subsurface.

Why is soil gas monitored at landfills?

To meet regulatory requirements:
- EPA regulations under RCRA (Subtitle D) require MSW landfills to conduct soil gas monitoring for methane.
- Data provide information about the performance of landfill gas collection systems and characterize explosion hazards.
- Under Subtitle D of RCRA, MSW landfills must monitor methane around the landfill perimeter.
- If methane concentrations at the monitoring stations at the property boundary exceed the LEL, the landfill must report the exceedance to the proper state authority and develop and implement a plan to correct the problem.
- The methane monitoring must be performed not only while landfills are active, but after they close.
To characterize off-site fire or explosion hazards:
- Soil gas monitoring for methane is sometimes performed off-site to address concerns about gas migration.
To quantify off-site migration of chemicals:
- At some landfills, environmental agencies or residents might organize sampling efforts to identify the many contaminants in soil gases as well as their soil gas concentrations.
- Chemical-specific soil gas monitoring provides the most detailed information about levels of contamination in landfill gas.

How are soil gas samples collected?

Soil gas samples are collected from temporary monitoring probes (often labeled punch probes or punchbars), permanent soil gas monitoring wells, and landfill gas collection wells and vents.
Soil gas sample locations and sampling methods vary from landfill to landfill, depending on monitoring concerns or regulatory requirements.

What do soil gas monitoring data tell you?

Soil gas monitoring can characterize methane and other gases, such as NMOCs, in concentrations within the landfill and around its perimeter.
Important factors to consider when interpreting results include the sample location, frequency, and data quality.
Data can identify off-site subsurface pathways and on-site or off-site buildings that may be endangered by migrating methane and other gases.
This information may be used by decision makers to determine if and what soil gas collection and treatment is needed to protect public safety and health.
Soil gas monitoring data do not provide actual measurements of the gases and their concentrations that people living near a landfill may inhale.
Environmental regulations may require only methane monitoring. Other gases, such as NMOCs, may be present in the subsurface.
Environmental health professionals should consider the sampling locations in relation to potentially exposed populations and sample analyses conducted in relation to the gases, especially NMOCs, that may be present when reviewing soil gas data.
Understanding the pressure and water level during sampling provides additional information about the sampling results.
Pressure is important because it is key factor influencing landfill gas movement. Gases move from areas of high pressure to areas of low pressure.
Water level is important because water can be a barrier to gas movement. When a soil gas well/probe is filled with water, gases are restricted from moving into the well/probe.

Near Surface Gas Monitoring

Near surface gas monitoring measures gas concentrations within a few inches of the landfill surface.

Why is near surface monitoring performed at landfills?

To determine the need for and design of a LFG control system.
To determine if a LFG control system is adequately preventing methane and other landfill gases from escaping in high quantities through the landfill cover.
Large landfills required to install landfill gas collection and control systems by the NSPS/EG must perform near surface methane monitoring quarterly.
Corrective action must be taken if methane readings are more than 500 ppm above background.

How is near surface gas monitoring performed?

A common method uses a portable instrument such as an organic vapor analyzer-flame ionization detector (OVA/FID), calibrated for methane or other gases.
The sampling technician walks over the landfill surface, recording instrument readings and noting the location and surface conditions.
Measurements may be recorded as parts per million, percent by volume, or percent of lower explosive limit, depending upon the type of portable instrument used.
A grab sample may also be taken using a sampling device fitted with a Tedlar® bag or with a SUMMA®- polished canister.
Samples are taken to a laboratory for analysis, yielding results for many more specifically identified constituents of landfill gas than use of portable instruments.
A combination of a portable instrument and Tedlar® bag sample is sometimes used to provide a comprehensive analysis of gases emitted through the landfill cover.
The portable instrument is used to locate "hot spots," places in the landfill surface where relatively high concentrations of methane are detected.
A sample is then taken using the Tedlar® bag and sent to a laboratory for qualitative and quantitative analyses of several contaminants composing landfill gas.
Results should be reviewed in context of meterologic conditions at the time of sampling and with knowledge of the height of the probe inlet from the surface of the landfill. Even moderate surface winds of 5 to 10 mph will greatly dilute the gas sample taken at 4 inches from the surface.

What do near surface gas monitoring data tell you?

Near surface gas data provide the concentrations of gases, usually just methane, that are moving through the cover of the landfill into the atmosphere.
If laboratory analysis of samples is used, the results may help characterize the NMOCs being emitted by the landfill into the atmosphere.
Near surface gas data may indicate the location of point sources of relatively high concentrations of landfill gases such as cracks in the landfill cover.
Such information may be useful in locating permanent soil gas probes for long term monitoring or gas recovery wells to control the release of landfill gases.
Near surface gas monitoring is also useful inside buildings to locate sources of landfill gas movement into the building.
Cracks and openings into the buildings may then be sealed to reduce the amount and concentrations of infiltrating gases.
Near surface gas data do not indicate the concentrations of gases that people may be breathing because of the effects of rapid dilution that is normally expected of gases traveling from the surface of the landfill to the 3- to 5-foot height that may be considered the breathing zone for many people.
Additional dilution of gases will occur during the travel of contaminants from the landfill to nearby homes and businesses.
Near surface gas data may be used in computer air models that estimate the level of contamination in ambient air in adjacent communities.
The quality and validity of such models for public health purposes will greatly depend on the quality and validity of the gas data and site specific meteorologic measurements, as well as the validity of the assumptions and defaults values used in the computer model.
Air models and estimates that substitute too many default values for site specific measurements have very limited value for public health conclusions about breathing zone concentrations.

Emissions Monitoring

Emissions monitoring measures the rates at which chemicals in landfill gases are released from landfills.
Emissions sources at landfills that are most frequently monitored are the landfill surface itself and landfill gas combustion units (e.g., flares or other combustion devices).

Why are emissions monitored at landfills?

To comply with federal and state environmental regulations.
To judge the need for, or effectiveness of, a landfill gas control system.
To determine the general composition and volume over time of air contamination emanating from the landfill.
Emission rate estimates or monitoring may also be used to assess whether it is technically and economically feasible to recover and use the landfill gas for energy production.
Landfill gas can be collected and combusted in boilers to produce steam to heat a manufacturing process in a building, or it can be combusted in a gas turbine or internal combustion engine to generate electricity.
To comply with the CAA, large landfills (those that can hold at least $2.5 \text{ Mg }$ and $2.5$ million cubic meters of waste) that have estimated uncontrolled emission rates of $50 \text{ Mg NMOC/ year }$ or more must install landfill collection and control systems.
To estimate NMOC emissions, the landfill must use a model (described later in this chapter).
One input to the model is the NMOC concentration in the landfill gas, which can be measure through sampling and analysis procedures described in the NSPS/ EG (sometimes referred to as Tier 2 testing), or a default NMOC concentration provided in the rule can be used.
These large landfills must control emissions with (1) a well-designed and well-operated gas collection system and (2) a control device (usually flare or other combustion device) capable of reducing emissions in the collected gas by $98$ weight percent.
Depending on the type of control device, an initial stack test to measure the NMOC emission rate and percent destruction may be required.
To indicate whether the landfill gas collection system is operating properly, landfills must also periodically monitor surface methane concentrations (as described earlier in this chapter in the near surface monitoring section).
Temperature and nitrogen or oxygen levels also must be monitored at the landfill gas collection wells.
The small landfills, often owned and operated by local governments, may be exempt from the requirements of the NSPS. EPA estimated that 90% of landfills are exempted from the NSPS regulations.
The distinction between emissions monitoring and emissions estimation is reviewed below.

How are emissions measured?

Emissions are measured by the use of various combinations of field sampling techniques and laboratory analytical techniques (when laboratory analysis of samples is necessary).
When measuring emissions from the surfaces of landfills, field teams have used "flux chambers," which collect the passive release of landfill gases for later analysis, either on site or in a laboratory to determine the emission rate.
Surface emissions are also being measured by fourier-transformed infrared-red (FTIR) or ultra-violet spectroscopy (UVS) sampling techniques.
FTIR and UVS are spectroscopic sampling techniques that detect and identify contaminants in the air along a straight line (e. g., the boundary of a landfill or across the landfill surface).
The principle is that the infrared or UV light is generated and then passed to a receptor in a line-of-sight position along a boundary of concern.
The spectroanalysis can identify specific compounds and concentrations in the space between the source and the receptor.
However, the units are usually given in a concentration of volume per unit distance (e.g., ppm m), mass per area of the beam (e.g., µg/m2 ), or mass per volume sampled (µg/m3 ) rather than an emission rate
When measuring emissions from passive vents at landfills, field teams typically collect a sample over a given time frame in some type of enclosed device, such as a Tedlar® bag or a SUMMA®- passivated stainless steel canister.
When measuring emissions from a stack with a high flow rate (e.g., the effluent from a landfill gas incinerator), field teams typically insert a sampling probe directly into the stack.
The probe then draws a known volume of the stack air into another device, such as those mentioned above.
Various analytical methods can be used depending on the compounds being measured. For example, total NMOC may be measured at the inlet and outlet of an enclosed combustion device to show compliance with the NSPS/ EG. EPA Method 25C is used for this purpose.
Specific organic pollutants may also be measured as part of a local health evaluation or for a state air toxics rule. EPA Method 18, measurement of gaseous organic compound emissions by gas chromatography, is a method for measuring individual organic hazardous air pollutants.
Sometimes, sulfur dioxide $(SO<em>2 )$ and nitric oxides $(NO</em>X)$ , which are products of combustion, are measured initially or are continuously monitored as part of new source permitting requirements or to comply with other federal and state rules that apply to some boilers or other combustion devices.
Carbon monoxide (CO) may also be monitored as an indicator of good combustion.

What do emissions monitoring data tell you?

Emissions monitoring data have at least three inherent limitations to the environmental health professional:
- First, emissions monitoring data at landfills characterize environmental conditions on site, often far from where residents might be exposed to contaminants. Therefore, the emissions data might be a poor indicator of exposure concentrations.
- Second, emissions data typically (though not always) provide a one-time account or "snapshot" of landfill emissions. Because landfill emissions likely exhibit significant seasonal variations, the measured emission rates from one study might not be representative of emissions over the longer term.
- Third, emissions monitoring studies at landfills usually consider only one or a few of the landfills' sources. Because many landfills have numerous operations (e. g., composting, waste handling, transportation), each of which emits some pollutants to the air, most emissions monitoring data likely do not characterize the overall emissions from a given landfill.
Environmental health professionals ultimately must evaluate emissions monitoring data in perspective: the data indicate rates at which landfills release chemicals to the air, and they often indicate the relative quantities of chemical- specific emissions.
However, they do not provide a direct measure of breathing zone concentrations.
Breathing zone concentrations are characterized only by ambient and indoor air monitoring data.

Ambient Air Monitoring

Ambient air monitoring data can provide a better characterization of gases in the breathing zone.
Ambient air monitoring measures levels of contamination in outdoor air, or in the air that people breathe.
The levels of pollution measured in the ambient air reflect the combined influences of many different nearby sources, and even some distant ones.

Why is ambient air monitored at or near landfills?

The main reason ambient air monitoring is performed at or near landfills is to evaluate worker and community exposure concerns regarding releases of toxic chemicals to the air.
Federal regulations currently do not require ambient air monitoring to be performed in the vicinity of municipal solid waste landfills, so no ambient air monitoring data are available for many landfills.
In some cases, a landfill may be considered a hazardous waste site under federal and state regulations. At these sites, regulatory agencies or the landfill owner and operator may collect ambient air data.
At other landfills, states may operate ambient air monitoring stations near landfills to measure concentrations of some or all of EPA's criteria pollutants (carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide).
If organic compounds are of concern, continuous monitoring for total hydrocarbons is also possible.
These pollutants, however, originate from many sources in addition to landfills, and their monitoring data often are viewed as an indicator of general air quality, rather than as the influence of any one particular source (e.g., a landfill).

How are ambient air concentrations measured?

Ambient air concentrations are generally measured according to specifications set forth in an ambient air monitoring plan.
Plans typically address these critical elements of ambient air monitoring:
- Chemicals selected for monitoring:
  - If potential exposure to landfill gas is of concern, monitoring typically focuses on NMOCs, rather than on metals or particulate matter.
  - At sites where windblown dust is an issue, monitoring would likely also consider particulate matter.
- Sampling methods:
  - Results from soil gas, near-surface, and emissions monitoring data, if available, may be a useful guide for selecting chemicals to consider in air monitoring programs.
  - The programs should attempt to measure as many of the chemicals detected in the soil gas and emissions as possible, but especially the most toxic chemicals with the highest concentrations and emission rates.
  - Table 2-1 (Chapter Two) lists some of the more prevalent NMOCs in landfill gas. The EPA's compilation of Air Pollutant Emissions Factors (known as AP-42), Section 2.4, provides typical concentrations of more than 40 NMOCs and inorganic compounds in MSW landfill gas.
  - Ambient air monitoring for the scores of chemicals that landfills emit is a prohibitively expensive endeavor.
  - From a practical standpoint, selection of chemicals for monitoring is determined by weighing several factors, such as cost, chemical toxicity, and the availability of sampling methods that can reliably measure ambient air concentrations of a given chemical.
- Selection of chemicals for monitoring is determined by weighing several factors, such as cost, chemical toxicity, and the availability of sampling methods that can reliably measure ambient air concentrations of a given chemical.
Optical methods, such as FTIR discussed in the Emissions Monitoring section, may also be useful for ambient monitoring near the boundary of a landfill.
Use of EPA-approved methods is encouraged, because the approval is based on extensive testing of the accuracy and precision of ambient air monitoring.
- In some cases, however, EPA-approved methods might not be available for certain chemicals or monitoring frequencies, and use of other methods might be necessary.
- In these cases, extra care should be taken to ensure that the selected methods are capable of generating high- quality data.
The monitoring methods selected for use in a given program determine the detection limits for each chemical.
The detection limit is the lowest concentration at which the method can reliably measure a chemical's ambient air concentration.
For ambient air monitoring data to be useful to the environmental health official, all efforts should be made to use methods with detection limits that are lower than or comparable to ambient air concentrations that would be of health concern.
Ambient air monitoring locations:
- The goal of the sampling program. If the goal is to address community concerns, monitoring locations should include residential neighborhoods at downwind locations nearest the landfill and other places where people might be exposed to landfill gases (e.g., nearby parks, malls, and schools).
Many additional concerns should be considered when selecting monitoring locations.
- For perspective on the extent to which landfill emissions affect air quality, simultaneous monitoring at locations upwind and downwind of the landfill of concern is advised.
- It is equally as critical to review the surroundings of monitoring stations to ensure that local sources of air pollution will not bias a monitor's readings.
- Schools, parks, and churches generally make excellent choices for monitoring locations because they have few sources of emissions on their premises, they often have sources of electricity readily available, and they typically are located in or near residential neighborhoods.
Monitoring schedules:
* Ambient air monitoring plans should specify both the frequency and duration of the proposed monitoring, and both factors should be considered when interpreting data.
* The frequency of monitoring is often determined by the available sampling methods.
* Continuous methods provide an ongoing account of air quality, but these methods usually measure levels of only one pollutant; periodic monitoring is typically, though not always, conducted by collecting 24-hour averaged samples on either a 6-day or 12-day cycle.
* Sometimes 8- or 12-hour sampling is conducted. Though useful for occupational exposures, such as on the landfill, such sampling may miss significant off-site releases affecting nearby residents during non-working hours, such as the predawn hour when landfill gas odors are not diluted and diffused by strong wind.
The duration of monitoring is also an important consideration.
- Because landfill emissions might exhibit significant seasonal variations, monitoring for a year or longer is needed to accurately estimate the long-term average concentrations of air pollutants.
- Further, landfill emissions can change from year to year for various reasons, such as increases or decreases in daily disposal rates, changes in waste mix and moisture, landfill closure, and installation of pollution controls.
Data quality parameters:
* In ambient air monitoring plans, data quality objectives will be specified for the program.
* In general, ambient air monitoring programs should strive to collect and analyze air samples in accordance with their method's data quality specifications.
* Though these specifications vary from method to method, measurement accuracy and precision of better than 50% is usually feasible for most methods.
* A sampling completeness (defined as the percent of attempted sampling events that are successful) of better than 90% is desired.

What do ambient air monitoring data tell you?

Ambient air monitoring data characterize levels of contaminants in the air that people breathe.
Because these data are almost always the best metric for exposure concentrations at landfill sites, it is extremely important that environmental health professionals interpret ambient air monitoring data critically.

Indoor Air Monitoring

Indoor air monitoring measures air concentrations of contaminants in indoor or enclosed locations.
Sampling locations for indoor air monitoring efforts include, but are not limited to, basements of buildings (residential, commercial, and industrial), living spaces in homes, and office spaces at landfills.

Why is indoor air sampled at or near landfills?

Chemicals in landfill gas can transport directly into structures built on top of areas where soil gas contamination exists.
The reasons for conducting indoor air sampling at or near landfills are generally identical to those for conducting soil gas monitoring (e.g., to meet regulatory requirements, to characterize risks for explosions, and to characterize potential exposures to toxic chemicals).
According to EPA's RCRA regulations, owners and operators of landfills subject to these requirements must ensure that the concentration of methane gas does not exceed 25% of the LEL for methane $(1.25\% \text{ by volume})$ in indoor air samples collected in the facilities' structures.
If methane emissions repeatedly exceed the allowed limit, regulators might require that corrective action be taken, such as landfill gas control measures.
Indoor air monitoring at structures at or near landfills has been conducted:
- First, some studies have measured concentrations of methane in off-site structures to characterize the risks for explosion as a result of migration of soil gases beyond landfill property lines.
- Second, some studies have measured concentrations of many different NMOCs that are suspected of migrating with landfill gas and into residential properties.

How are indoor air concentrations measured?

The sampling and analytical methods listed for the ambient air monitoring sections can also be used to measure indoor air concentrations, but some of these methods involve the use of bulky equipment that is sometimes not suitable for indoor environments, particularly homes.
In these cases, environmental officials might rely on hand-held monitoring devices, surveying methods, or monitoring guidance provided by the National Institute for Occupational Safety and Health (NIOSH) or OSHA for use in occupational settings.
Other features of indoor air monitoring programs—chemicals selected for monitoring, monitoring locations, monitoring schedules, and data quality parameters—should be reviewed in the same manner as the features of ambient air monitoring programs.

What do indoor air monitoring data tell you?

Indoor air monitoring data characterize levels of contamination in indoor environments.
In cases in which only methane is monitored, for example, the data are useful only for evaluating risks of explosion.
In cases in which other chemicals are monitored, the data can be used for evaluating potential health risks.
You will need to consider the extent to which other indoor sources (e.g., cigarette smoke, losses from cleaning supplies, and emissions from stoves and furnaces) might have contributed to the measured concentrations.
Because indoor sources of contaminants can differ considerably from one house to the next, indoor air monitoring data from a given residence should not be viewed as representative of other residences in the area.