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Environmental health professionals are rarely required to design and implement sampling and monitoring plans at landfills, but you might be asked to review and comment on such plans. In addition, you might need to review and interpret sampling and monitoring data, when available, to evaluate potential public health hazards. To make such tasks easier, this chapter provides basic information (e.g., monitoring program design, sampling and monitoring equipment, and data interpretation) about the different types of landfill gas sampling methods that you are most likely to encounter.
It is important to remember that monitoring data taken at landfills do not necessarily reflect the levels of contamination to which people may be exposed. However, these data usually offer some insight into either general air quality, landfill gas migration, or possible health hazards. In general, monitoring of gases that emanate from landfills falls into the following five categories:
Table 4-1 presents a brief overview of the key features of each type of monitoring.
Data collected from these different monitoring activities have considerably different public health implications. Following an overview of landfill gas sampling approaches, this chapter reviews the five types of monitoring activities separately. In addition, mathematical modeling can be used to help answer questions about landfill emissions data. This chapter presents a brief summary of factors to consider when reviewing air modeling results.
Many different types of landfill gas sampling approaches exist —too many to review in this manual. However, two important factors in selecting an appropriate landfill gas sampling approach include 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. Some examples of location, or placement, of gas monitors are described in the box below.
Type of Monitoring |
Description of Monitoring | Typical Parameters Reported | Relevance to Public Health |
---|---|---|---|
Soil Gas | Soil gas monitoring programs measure the concentrations of chemicals in the vapor space of soils. Measurements of soil gas levels are taken at depth with the use of probes or wells. | Most landfills are required by federal law to report levels of methane around the landfill perimeter. Oxygen, carbon dioxide, and nitrogen are frequently reported. Sometimes H 2S and other specific NMOCs, such as vinyl chloride, might be reported if federal or state regulators suspect a significant problem. Pressure, in inches of water, is also frequently reported from permanent soil gas probes. | Because soil gas monitoring data at many MSW landfills typically (though not always) characterize levels of only methane, the data are generally useful for evaluating risks of explosion and for getting a qualitative sense of whether landfill gases are migrating in the soils to off-site locations. |
Near Surface Gases | Measures the concentrations of gases at a point no higher than 4 inches above the ground surface. | Methane is the most common gas monitored but VOCs and H 2S are sometimes reported. | Outside air methane concentrations do NOT pose an inhalation or explosion hazard. Near surface monitoring of methane on the landfill does not provide useful information to determine impacts on the health of adjacent residents. Monitoring can qualitatively indicate whether high levels of landfill gas are escaping from the landfill surface or whether the landfill gas collection and control system is working well to minimize emissions. |
Emissions | Emissions monitoring programs measure the rate at which chemicals are released from a particular source, such as landfill surfaces, flares, or stacks. | Landfill studies have measured emission rates for various pollutants, such as methane and NMOCs, from landfill surfaces and combustion by-products of flares and other treatment units. | Chemical-specific emissions data are useful for identifying potential contami- nants of concern at landfills, but they do not characterize the concentrations of chemicals that people actually breathe. Exposure concentrations can be estimated from emissions data, but such estimates can be highly uncertain. |
Ambient Air | Ambient air monitoring programs measure levels of pollution in outdoor ambient air, or the air that people breathe. | Ambient air monitoring can be conducted for a wide range of pollutants. Near landfills, air monitoring is most commonly conducted for EPA's criteria pollutants and NMOCs. | Because ambient air monitoring data characterize levels of pollution in the air that people breathe, they usually provide the best measure for air exposure concentrations in the vicinity of landfills. Of course, environmental health professionals still need to critically evaluate ambient air monitoring data to put them into proper perspective. |
Indoor Air | Indoor air monitoring programs meas- ure levels of contamination in indoor air spaces. | Indoor air monitoring for methane is required at structures on many landfill properties. Methane monitoring at off-site locations and NMOC monitoring is usually only performed to address site-specific concerns. Oxygen levels in confined spaces, such as buried utilities, are measured to determine if carbon dioxide and/or methane gases have replaced sustainable oxygen. | Indoor air monitoring data are useful for evaluating risks of explosions and exposures to contaminants within homes. Emissions from household products and tasks might confound these measurements, and levels measured in one home generally are not representative of levels in other homes, even nearby residences. |
Landfill gas monitors are typically placed in three types of locations at or near landfills; these are subsurface, surface, or enclosed space. The three types of monitoring locations address different landfill gas concerns and can be used either alone or together in a sampling program. Note that these systems generally do not measure landfill gas levels at points of human exposure. Subsurface Systems—Subsurface systems measure concentrations of contaminants in the soil gas at locations beneath the soil-air interface. The depth of sampling can range from a few inches to many feet below the surface. Surface Systems—Surface systems measure concentrations of gas within a couple of inches above the soil-air interface. Enclosed Space Systems—Enclosed space systems monitor gases in indoor air or confined areas overlying or adjacent to landfills, such as buildings, subsurface vaults, utilities, or any other spaces where the potential for gas buildup is of concern. |
In addition to the sampling location, several methods of landfill gas collection can be used in a landfill gas sampling approach. Examples of these methods, and their implications, follow:
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. For most landfills, regulatory requirements dictate the features selected for gas sampling (e.g., EPA's soil gas monitoring requirements are a major consideration for the sampling conducted at most MSW landfills).
This section defines soil gas monitoring and how it relates to landfills, discusses why soil gas is often monitored at landfills, and presents information environmental health professionals should consider when reviewing soil gas monitoring data.
As Chapter Two describes, decomposing waste in landfills generates gases containing many chemicals that transport through soils and may eventually be released to the surface. While in the soils, the landfill gas is typically referred to as "soil gas." Soil gas monitoring, therefore, is the measurement of concentrations of gases in the subsurface.
There are many reasons for monitoring levels of contaminants in soil gas at or near landfills. The three main reasons that such monitoring is performed are reviewed below, though soil gas monitoring might be conducted for other reasons. Information about sampling methods and the relevance of the monitoring results to public health is presented later in this chapter.
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.
Soil gas monitoring results may provide a great deal of information about landfill gases and how they are moving through the landfill. 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. Based on the location of soil gas monitoring wells or probes, 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.
However, soil gas monitoring data do not provide actual measurements of the gases and their concentrations that people living near a landfill may inhale. Soil gas samples are collected beneath the landfill surface, and gas concentrations will change as the gases move horizontally in the subsurface or vertically into the ambient air. In addition, environmental regulations may require only methane monitoring. Other gases, such as NMOCs, may be present in the subsurface. When reviewing soil gas data, environmental health professionals should be careful to 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.Some questions to consider in a review of soil gas data to ensure that they are truly representative of subsurface conditions are listed below. 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. As discussed in Chapter Two, gases move from areas of high pressure to areas of low pressure. Therefore, if the atmospheric pressure is higher than the pressure in the landfill, ambient air will enter the soil gas well/probe. Any samples taken under these conditions would not be representative of the landfill gas. 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. Samples collected from a water-filled well/probe would not be representative of the landfill gas. Appendix D provides a case example of a landfill at Wright-Patterson Air Force Base in Ohio where the filling of soil gas wells/probes with water was a problem.
Gases Selected for Monitoring
Pressure Monitoring
Sampling Methods
Sampling Equipment
Monitor Well Construction and Depth of Screened Intervals
Monitoring Locations
Other Sources
Monitoring Schedules
Data Quality Parameters
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General information about soil gas sampling can be found in the resources listed below. In addition, state and federal environmental officials are an excellent resource for site-specific insights.
This section defines near surface gas monitoring and how it relates to landfills. This section also discusses why near surface gases might be monitored at landfills, and presents information that environmental health professionals should consider when reviewing the resulting data sets.
Near surface gas monitoring is the measurement (usually by portable instruments) of gas concentrations within a few inches of the surface of the landfill.
Near surface monitoring of landfill gases may be performed to determine the need for, and the design of, a LFG control system. The near surface monitoring is also used to determine if a LFG control system is adequately preventing methane and other landfill gases from escaping in high quantities through the landfill cover. Under the Clean Air Act, large landfills that are required to install landfill gas collection and control systems by the NSPS/EG must perform near surface methane monitoring quarterly to show that the system is operating properly. Corrective action must be taken if methane readings are more than 500 ppm above background. (Other testing and monitoring requirements of the NSPS/EG are described later in this chapter.)
A common method of near surface gas monitoring is the use of a portable instrument such as a organic vapor analyzer-flame ionization detector (OVA/FID). Normally, the instrument is calibrated for methane but it can be calibrated for other gases commonly found in landfills. The OVA may be fitted with a funnel over the monitoring probe inlet. The probe inlet and funnel are then held within 2 to 3 inches of the ground surface and the measurement of gas is recorded by the sampling technician.
Using a method known as landfill gas sweeping or emissions screening, the sampling technician walks over the surface of the landfill in either a random method or over a pre-defined grid. The sampling technician records the instrument readings, making careful note of the geographic location of each measurement and the surface conditions. The 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. In both cases the samples are taken to a laboratory for analysis. The laboratory analysis may yield 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 from near surface gas sampling should always 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.
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.
However, 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. Furthermore, near surface gas monitoring is normally only performed on the landfill or at the boundary of the landfill. 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.
The CAA regulations (NSPS/EG) for MSW landfills can be found in the Code of Federal Regulations, at 40 CFR Part 60, Subparts Cc and WWW, available on the Internet at The NSPS/ EG surface methane monitoring requirements and methods are in section 60.753, 60.755, and Method 21 of Appendix A of Part 60. Additional summary information on the NSPS/ EG is available on the EPA Web site at http://www.epa.gov/ttn/atw/landfill/landflpg.html.
The Landfill Gas Operation and Maintenance Manual of Practice published in 1997 by the Solid Waste Association of North America (SWANA) provides detailed explanation of landfill gas monitoring and instrumentation. The published manual can be ordered via the SWANA Web site http://www.swana.org. (Note: The previous link will be opened into a new window, as the swana.org website prevents you from returning here.) A draft version is available online at the Department of Energy Information Bridge at the Web site http://www.osti.gov.
This section defines emissions monitoring and how it relates to landfills, discusses why emissions might be monitored at landfills, and presents information that environmental health professionals should consider when reviewing emissions monitoring data.
Unlike soil gas and near surface gas monitoring, which measure the concentrations of chemicals in landfill gas, 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).
Landfill gas emissions may be monitored for one or more of the following reasons: to comply with federal and state environmental regulations; to judge the need for, or effectiveness of, a landfill gas control system; and/or 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. For example, 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.
As mentioned in Chapter Two, to comply with the CAA, large landfills (those that can hold at least 2.5 Mg and 2.5 million cubic meters of waste) that have estimated uncontrolled emission rates of 50 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. However, the recent trend is toward larger landfills, so in the future a greater percentage of landfills may be subject to the NSPS/EG.
The distinction between emissions monitoring and emissions estimation is reviewed below.
Emissions Estimation vs. Emissions Monitoring
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Scientists generally use one
of two techniques to quantify air emissions from sources:
they either estimate the emissions or measure them. To estimate
emissions, scientists perform calculations or use models
to predict the rate at which sources may release chemicals
to the air. The uncertainty in the assumptions and input
values for these calculations make the estimated emission
rates uncertain as well. Though some models have been derived
from years of research on the transport of chemicals from
landfills into the air, models ultimately provide estimates
of emissions. Because the accuracy of these estimates cannot
be quantified, modeled emission rates should be carefully
scrutinized and viewed as somewhat uncertain.
In some cases, scientists will actually measure the air emissions from sources. Measuring emissions from an entire landfill is a challenging task, primarily because landfill emissions can occur over a surface that spans hundreds, or even thousands, of acres. Moreover, there are many different types of emissions sources at a landfill, such as evaporative losses through the landfill surface, mobile source emissions from dump trucks, and stack emissions from landfill gas treatment devices (e. g., flares). Monitoring studies rarely measure the emissions from all possible sources at a landfill. When reviewing emissions monitoring studies from landfills, environmental health professionals should critically evaluate all reported results, because they can be biased by poor study design and other factors. |
In a very few cases, a landfill might be identified as a hazardous waste site under federal or state regulations. In these instances, regulatory agencies might require landfill owners to perform limited emissions monitoring to address specific regulatory or enforcement actions, but monitoring for this reason is not common.
Although some landfill emissions are monitored as part of a regulatory process, often air emissions monitoring at landfills, particularly at MSW landfills, is conducted for nonregulatory purposes. These purposes can include addressing community concerns regarding potentially toxic emissions, conducting scientific research on air quality impacts of landfills, or validating the predictions of emissions models. More information about predictive models is provided later in this chapter.
Emissions are measured by the use of various combinations of field sampling techniques and laboratory analytical techniques (when laboratory analysis of samples is necessary). The techniques selected for a given monitoring effort depend on the type of source being evaluated. Some examples of sampling approaches follow:
The sampling strategy for a given study ultimately depends on other factors in addition to the type of emissions source. These other factors include cost, access, data quality needs, and chemicals selected for monitoring.
When working on most landfills, chances are you will not encounter emissions monitoring data. When you do, however, it is important to interpret these data in proper context. Though useful for characterizing the relative quantities of chemicals released from a landfill, emissions monitoring data have at least three inherent limitations to the environmental health professional, as discussed below.
First, like soil gas monitoring data, 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.
When considering these limitations, 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.
General information about landfill emissions and emissions monitoring can be found in the following resources. In addition, state and federal environmental officials are an excellent resource for site- specific insights.