Chapter 2 Continued: Landfill Gas Basics
The direction, speed, and distance of landfill gas migration depend on a number of factors, described below.
- Landfill cover type. If the landfill cover consists of relatively permeable material, such as gravel or sand, then gas will likely migrate up through the landfill cover. If the landfill cover consists of silts and clays, it is not very permeable; gas will then tend to migrate horizontally underground. If one area of the landfill is more permeable than the rest, gas will migrate through that area.
- Natural and man-made pathways. Drains, trenches, and buried utility corridors (such as tunnels and pipelines) can act as conduits for gas movement. The natural geology often provides underground pathways, such as fractured rock, porous soil, and buried stream channels, where the gas can migrate.
- Wind speed and direction. Landfill gas naturally vented into the air at the landfill surface is carried by the wind. The wind dilutes the gas with fresh air as it moves it to areas beyond the landfill. Wind speed and direction determine the gas's concentration in the air, which can vary greatly from day to day, even hour by hour. In the early morning, for example, winds tend to be gentle and provide the least dilution and dispersion of the gas to other areas.
- Moisture. Wet surface soil conditions may prevent landfill gas from migrating through the top of the landfill into the air above. Rain and moisture may also seep into the pore spaces in the landfill and "push out" gases in these spaces.
- Groundwater levels. Gas movement is influenced by variations in the groundwater table. If the water table is rising into an area, it will force the landfill gas upward.
- Temperature. Increases in temperature stimulate gas particle movement, tending also to increase gas diffusion, so that landfill gas might spread more quickly in warmer conditions. Although the landfill itself generally maintains a stable temperature, freezing and thawing cycles can cause the soil's surface to crack, causing landfill gas to migrate upward or horizontally. Frozen soil over the landfill may provide a physical barrier to upward landfill gas migration, causing the gas to migrate further from the landfill horizontally through soil.
- Barometric and soil gas pressure. The difference between the soil gas pressure and barometric pressure allows gas to move either vertically or laterally, depending on whether the barometric pressure is higher or lower than the soil gas pressure. When barometric pressure is falling, landfill gas will tend to migrate out of the landfill into surrounding areas. As barometric pressure rises, gas may be retained in the landfill temporarily as new pressure balances are established.
It is difficult to predict the distance that landfill gas will travel because so many factors affect its ability to migrate underground; however, travel distances greater than 1,500 feet have been observed. Computer models that use data about the landfill and surrounding soil conditions can predict the approximate migration patterns from existing landfills. More information about models available for assessing landfill gas is provided in Chapter Four.
A study conducted by the New York State Department of Health found that of 38 landfills, gas migrated underground up to 1,000 feet at 1 landfill, 500 feet at 4 landfills, and only 250 feet from the landfill boundary at 33 landfills. —(ATSDR 1998)
Gases migrating from a landfill may eventually reach buildings and homes. Foundation cracks and gaps, pressure differences between the inside and outside of the building or home, mechanical ventilation systems, and leakage areas (e.g., utility entry points, construction joints, or floor drain systems) provides entry points for gases. Buildings and houses with basements generally provide the most easy access for gases migrating in the soil. The amount of gases let into a building or home depends on a number of factors, including the construction and maintenance practices. The gas concentration in indoor air also depends on the building or home design, the rate of air exchange, and the distance of the building or home from the landfill. Chapter Three provides more information about how people are exposed to gases once the gases have entered buildings or homes.
Your community may have different types of landfills within it or nearby:
- Municipal solid waste (MSW) landfills are
used to dispose of household wastes and non-hazardous commercial
and industrial wastes. More than 6,000 MSW landfills exist across
the United States, although fewer than 3,000 of these are currently
active and accepting waste. Landfills constructed after 1979
are required, under Subtitle D of the Resource Conservation
and Recovery Act (RCRA), to be designed and operated to prevent
contaminant migration to the environment. This design may include
liners or collection systems. Landfills constructed before 1979
may not have such environmental safeguards.
- Open dumps are waste disposal areas that
were used before 1979 and constructed without any engineering
design and siting criteria, and few, if any, regulatory controls.
Open dumps do not meet the RCRA Subtitle D regulations. Open
dumps may have accepted household wastes, similar to MSW landfills,
as well as commercial and industrial wastes. These dumps did
not have liners and rarely used daily cover for sanitary wastes.
No precautions were taken to prevent contaminant migration to
the environment. Most open dumps were discontinued and covered
in the 1960s. Unfortunately, the locations of many of these
old dumps are not marked on local planning maps. Some of the
current operating MSW landfills began in the 1960s as open dumps
or are located adjacent to closed dumps.
- Construction and demolition (C&D) waste landfills
are used for the disposal of construction and demolition waste
such as wood, sheet rock, gypsum board, concrete, bricks, and
paving materials. As with MSW landfills, C&D waste landfills
containing nonhazardous materials are regulated under Subtitle
D of RCRA.
- Hazardous waste landfills are used to dispose
of wastes characterized under RCRA as "hazardous." These wastes
include solvents, industrial wastes, and construction wastes
such as asbestos. Operating or recently closed landfills containing
hazardous materials are regulated under Subtitle C of RCRA.
- Vegetation waste disposal areas, also known
as "yard waste and stump fill areas," are used to dispose of
vegetation wastes. In many states, these disposal areas were
unregulated prior to the 1980s. In areas where burning was prohibited,
these areas were used by land developers to bury trees and brush
cleared from land used for subdivisions and commercial developments.
- Animal waste landfills are areas where
massive amounts of manure and, possibly, animal carcasses are
disposed. There are no specific federal regulations for animal
waste landfills. State regulations vary among the states that
do regulate the animal waste landfills. As a result of the high
organic content, methane production can be significant. Decaying
manure and carcasses will produce strong odors. Fires have occurred
on some animal waste landfills, increasing health and safety
concerns of nearby residents.
This publication focuses primarily on MSW landfills. Of all the types of landfills, MSW landfills are the most significant source of landfill gas emissions, because approximately 60% of the waste in a typical MSW landfill is organic. The Web site of EPA's Office of Solid Waste (http://www.epa.gov/epaoswer/non-hw/muncpl/facts.htm) is a good source of basic information about MSW landfills. The Solid Waste Association of North America's (SWANA's) Landfill Gas Operation and Maintenance Manual of Practice is another source of general information about landfills; it can be accessed by a search of the U.S. Department of Energy's (DOE) Information Bridge at the Web site http://www.osti.gov or by placing an order for a hardcopy from SWANA's Web site at http://www.swana.org.
Prior to 1979, landfills were often merely open dumps with few or no controls to prevent contaminant migration to the environment. Open dumps posed significant environmental and public health hazards. They attracted flies and vermin, and fires that could burn for days often broke out. These dumps had no gas collection systems, nor did they have liners to protect groundwater. All types of waste, including hazardous wastes, could be deposited in landfills before 1979. Some of these dumps have been listed as "Superfund" sites and are now being remediated or are on a waiting list to be remediated. No longer legal, open dumps have been closed or converted into MSW landfills. Past dumps with no gas control systems are the landfill sites most likely to have gas emission concerns.
Many state and local governments have regulated landfills since the middle of the twentieth century; however, before 1979, regulation and enforcement varied widely from site to site. In 1979, the federal government began regulating the siting, construction, operation, and closure requirements for landfills under RCRA. Subtitle D of RCRA addresses MSW and nonhazardous landfills and includes requirements for methane monitoring at the landfill perimeter. Subtitle C of RCRA addresses concerns associated with hazardous waste landfills. In 1996, EPA finalized regulations under the Clean Air Act (CAA)—the New Source Performance Standards and Emissions Guidelines (NSPS/EG)—that address methane and NMOC emissions from MSW landfills. These regulations are described in more detail below, according to the type of waste received by the landfill.
- Municipal solid wastes. Subtitle D of RCRA
regulates the siting, design, construction, operation, monitoring,
and closure of MSW landfills. RCRA establishes standards that
MSW landfills must meet. These standards are enforced by the
state solid waste authority. States may also develop additional
standards that are more stringent than RCRA. RCRA requires that
owners and operators of MSW landfills ensure that the concentration
of methane gas generated by the facility does not exceed 25%
of the lower explosive limit (LEL), the lowest percent by volume
of an explosive gas in the air that will allow an explosion,
for methane in facility structures and that the concentration
of methane gas does not exceed the LEL for methane at the facility
property boundary. If methane concentrations exceed the LEL
at the property boundary, then RCRA requires the landfill owners/operators
to notify the proper state authority and develop and implement
a plan to correct the problem (see Chapter
Three for more information about LELs). The state solid
waste authority will determine whether the landfill has properly
addressed the problem.
In 1996, EPA promulgated regulations under the CAA—NSPS/EG—that also address emissions from MSW landfills. These regulations apply to MSW landfills that accepted waste after November 8, 1987. The NSPS/EG require landfills that can hold 2.5 million megagrams (Mg) or more of waste and annually emit 50 Mg or more of NMOCs to install landfill gas collection systems and control landfill gas emissions. The collection systems must meet specific engineering design criteria. Control devices (usually a flare or some other combustion device) must reduce the NMOC emissions from the collected landfill gas by 98% or to a concentration of 20 ppm by volume. Those MSW landfills that are required to install controls based on their NMOC emission rate must also moni-tor surface methane emissions. If methane emissions are found at concentrations exceeding background levels by more than 500 parts per million (ppm) between 2 and 4 inches from the ground surface, the gas collection system must be adjusted or improved to achieve the 500 ppm level. The NSPS/EG also contain various other testing, monitoring, and reporting requirements that landfills must meet. Figure 2-2 can help determine to what extent, if any, the MSW landfill(s) in the area must comply with the requirements of the NSPS/EG. The NSPS/EG can be found in the Code of Federal Regulations (CFR), at 40 CFR Part 60, Subparts Cc and WWW. Additional information can be found at http://www.epa.gov/ttn/uatw/landfill/landflpg.html.
- Construction and demolition wastes. Most
C&D waste is classified as nonhazardous and can be disposed
of in an MSW landfill or in a C&D landfill (a landfill that
accepts only C&D waste). The siting, design, construction, operation,
monitoring, and closure of landfills containing nonhazardous
C&D wastes are regulated under Subtitle D of RCRA. Air emissions
from C&D landfills are not regulated and are not generally a
concern, because C&D wastes do not contain much organic matter
(which is necessary to produce landfill gas). However, if gypsum
wallboard is present in C&D waste, hydrogen sulfide may be produced,
particularly if moisture is introduced into the waste. Because
of hydrogen sulfide's objectionable rotten-egg odor, C&D landfills
that emit hydrogen sulfide often find themselves facing numerous
complaints from the surrounding communities. Operators of these
landfills often find that they must install gas control systems
to reduce odors caused by the hydrogen sulfide gas. Some C&D
wastes may be classified as hazardous wastes because they contain
hazardous materials, such as asbestos. Hazardous C&D waste must
be disposed of in a hazardous waste landfill, as described below.
- Hazardous wastes. The siting, design, construction,
operation, monitoring, and closure of landfills containing hazardous
wastes are regulated under Subtitle C of RCRA. Hazardous waste
landfills are strictly regulated because they handle wastes
that pose a greater risk to the public than nonhazardous household
waste. Air emissions from hazardous waste landfills are not
specifically regulated under RCRA Subtitle C. However, Subtitle
C does address air emissions from the generation, storage, treatment,
and trans-port of hazardous wastes. For more information about
how U.S. landfills are regulated, visit the Web site of EPA's
Office of Solid Waste at http://www.epa.gov/epaoswer/osw/index.htm.
a The New Source Performance Standards (NSPS) is a federal rule that applies to landfills that started construction or increased their total design capacity after May 30, 1991. The Emissions Guidelines (EG) apply to older landfills and are implemented and enforced through state plans (or a federal plan in cases where states have not developed plans). The landfill gas control requirements are the same.
- CMHC. 1993. Canada Mortgage and Housing Corporation. Soil gases and housing: a guide for municipalities.
- EPA. 1992. U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards. Seminar Publication. Organic air emissions from waste management facilities. EPA/625/R-92/003.
- O’Leary P, Walsh P. 1995. C240-A180 Solid waste landfills correspondence course. Lesson 3: Landfill gas movement, control, and uses. Madison, WI: University of Wisconsin, Solid and Hazardous Waste Education Center (originally printed in Waste Age Magazine January 1991–March 1992). January 1995.
- SWANA. 1997. Solid Waste Association of North America. Landfill Gas Operation and Maintenance Manual of Operation. SR-430-23070. Available at the Department of Energy Information Bridge at the Web site http://www.osti.gov/bridge/product.biblio.jsp?osti_id=314156.
- ATSDR. 1998. Agency for Toxic Substances Disease Registry. U.S. Department of Health and Human Services. Investigation of cancer incidences and residence near 38 landfills with soil gas migration conditions. New York State, 1980-1989. Prepared by the New York State Department of Health, Division of Occupational Health and Environmental Epidemiology, Bureau of Environmental and Occupational Epidemiology. PB98-142144. June 1998.
- Crawford JF and Smith PG. 1985. Landfill technology. London: Butterworths. DOE. 1995.
- U.S. Department of Energy. Greenhouse gases 1987-1994. http://www.eia.doe.gov/oiaf/1605/95report/chap3.html.
- EPA. 1993. U.S. Environmental Protection Agency. Solid waste disposal facility criteria—technical manual. EPA 530-R-93-017. November 1993.
- EPA. 1995. U.S. Environmental Protection Agency. Compilation of Air Pollutant Emissions Factors, AP-42, Fifth Addition, Volume 1: Stationary Point and Area Sources. January 1995. http://www.epa.gov/ttn/chief/ap42/ch02/ Section 2.4-Municipal Solid Waste Landfills.
- Tchobanoglous G, Theisen H, and Vigil S. 1993. Integrated Solid Waste Management, Engineering Principles and Management Issues. New York: McGraw-Hill, Inc. pp. 381-417.