SWANA

This article presents highlights of a research project that was conducted this past year by the SWANA Applied Research Foundation’s (ARF’s) Disposal Group to compile and summarize what is known about the actual long-term environmental risks associated with Subtitle D landfills and to introduce long-term care alternatives that can be used to manage and minimize these risks.

The report on which this article is based was prepared by the SWANA ARF staff with input and guidance provided by the ARF Disposal Group Subscribers, who are listed in Table 1.

Background
In 1991, the US Environmental Protection Agency (EPA) promulgated new regulations for landfills used for the disposal of municipal solid waste (MSW). As the regulations were issued under Subtitle D of the Resource Conservation and Recovery Act (RCRA), these landfills are referred to as “Subtitle D” landfills.

Some Subtitle D landfills have now been in operation for many years and are approaching the end of their service lives. A small number have already been closed and have entered their postclosure periods.

Subtitle D regulations require that the postclosure care period—during which the landfill site is maintained and the environmental protection systems are managed and monitored—be 30 years in length. During the service lives of Subtitle D landfills, landfill owners are required to set aside or otherwise ensure that funds will be available to cover the costs of these postclosure management, maintenance, and monitoring tasks.

Of primary importance in this regard is the responsibility (and authority) given to state governments to extend or reduce the postclosure period—as warranted—to ensure that human health and the environment are protected for as long as the landfill poses a threat.

To date, the EPA has not provided any guidance to state governments on how to make this determination or how to promulgate regulations that will ensure that funds are available—should the postclosure period be extended—to cover the required management, maintenance and monitoring tasks beyond the prescribed 30-year postclosure period.

Certain environmental groups have claimed that closed Subtitle D landfills constitute major ongoing environmental risks to the communities in which they are located and that the postclosure care period should never end.

Performance to Date of Subtitle D Environmental Protection Systems
One of the major objectives of Subtitle D landfills is to prevent the discharge of pollutants from the landfilled waste to the atmosphere, surface water, or groundwater. Subtitle D regulations require the installation of a number of such environmental protection systems as the bottom liner system, leachate collection, and removal system, landfill gas collection and control systems, and final cover systems to accomplish this objective.

Based on data and findings from a number of recent reports, the following general conclusions can be drawn regarding the performance of the environmental protection systems of Subtitle D landfills.

• Bottom Liner Systems substantially prevent leachate from leaking out of the landfill and are enabling the vast majority of the leachate to be collected and treated. The life of the geomembrane in the bottom liner system, however, can be significantly shortened—from on the order of 900 years to 100 years or less—by high landfill temperatures. In some cases, landfill gas has been found to exit the bottom liner system through the leachate collection system piping and cause groundwater contamination.
• Leachate Collection Systems generally perform well and are collecting the vast majority (over 99%) of the leachate generated. Clogging problems have been experienced in a small number of cases.
• Landfill Gas Collection Systems collection system efficiencies of 97% should be representative for a landfill with a Subtitle D liner system and an active LFG collection system installed as early as possible. Additional methane oxidation is achieved in the cover soils used in the landfill.
• Final Cover Systems perform satisfactorily and can effectively isolate landfilled waste from the environment.

In summary, as concluded by one major recent study: “Based on as much as 20 years of observations, the committee concluded that most engineered waste-containment barrier systems that have been designed, constructed, operated, and maintained in accordance with current statutory regulations and requirements have thus far provided environmental protection at or above specified levels.” (National Research Council. 2007. Assessment of the Performance of Engineered Waste Containment Barriers. Washington, DC: National Academies Press. p.1.)

Long-Term Concerns
Studies have found that Subtitle D landfills, in general, are operating in compliance with federal and state environmental and public health regulations. Since the overall purpose of these regulations is to protect public health and the environment, it can be concluded that the environmental and public health impacts of Subtitle D landfills have—to this point—been minimal and acceptable from a societal standpoint.

If Subtitle D landfills are currently mitigating environmental and public health risks, why would these facilities be characterized as representing significant long-term risks with respect to public health and the environment? 

Simply put, the long-term risk is related to the landfilled waste that is still biodegradable at the time of closure and the finite service lives of the landfill’s environmental protection systems.

For example, the breaching of the final cover system can be caused by a number of factors, including vandalism, burrowing of rodents, and puncture by plant roots. Even if none of these occur, the final cover system will ultimately fail as it reaches the end of its service life.

When the final cover system is breached, moisture will reenter the landfill and the waste decomposition process will be reinitiated.  As a result, leachate and landfill gas will begin to be generated—both of which will ultimately require management in order to minimize public health and environmental impacts.

The general consensus is that biodegradable wastes in closed Subtitle D landfills that have not decomposed will represent a potential environmental and public health risk for hundreds of years and possibly into perpetuity.

Dr. John Skinner, SWANA’s executive director and chief executive officer, summarizes the issue as follows: “The problem with the dry-tomb approach to landfill design is that it leaves the waste in an active state for a very long period of time. If in the future there is a breach in the cap or a break in the liner and liquids enter the landfill, degradation would start and leachate and gas would be generated. Therefore, dry-tomb landfills need to be monitored and maintained for very long periods of time (some say perpetually), and someone needs to be responsible for stepping in and taking corrective action when a problem is detected.”

Environmental Characteristics
The long-term environmental risks associated with Subtitle D landfills are directly related to the physical, chemical, and biological environments within the closed landfills.

At the end of the 30-year postclosure period, a closed Subtitle D landfill will typically have the following characteristics:

• An anaerobic, reducing chemical environment
• A partially saturated and decomposed waste mass
• Little or no leachate or landfill gas production
• Mature leachate characteristics

When a Subtitle D landfill is closed, the majority of the landfilled waste is likely to be in what is referred to as the “stable methanogenic” phase of waste decomposition, which occurs in an anaerobic, reducing chemical environment (HOFGAS—Know How 07 Landfill Gas—What is it? Peter Spillmann. http://www.fackel.ch/default.asp?file=KnowHow_Knowhow_07e.htm). While studies have shown that leachate pollutant concentrations in the early acid phase of decomposition are high, the long methanogenic phase produces a more stable leachate, with lower pollutant concentrations. (“BOD” stands for “Biochemical Oxygen Demand” and is a parameter that is used to indicate the biological pollutant strength of a wastewater. Similarly, COD stands for “Chemical Oxygen Demand” and refers to the chemical organic strength of a wastewater.)

The landfilled waste mass in a closed Subtitle D landfill can be characterized as being partially decomposed as well as partially saturated with moisture.  The extent of waste decomposition is directly related to the amount of moisture that was available during the active life of the landfill which, in turn, is dependent on the design and operational practices implemented at the facility (leachate recirculation, disposal of sludges, etc.), the local climate (arid, temperate, etc.) and other factors. Despite these variables, it is safe to say that—for most Subtitle D landfills—a substantial portion of the biodegradable waste mass will not have been decomposed by the time the landfill is closed.

The landfilled waste mass can also be characterized as being partially saturated with liquids. This characteristic is important with respect to the potential development of small holes or tears in the final cover system. In such cases, the relatively small amounts of moisture that enter the landfill through these openings may be absorbed by the waste mass rather than drain to the bottom of the landfill and form leachate.

The placement of an impermeable final cover at closure prohibits the further addition of moisture to the landfilled waste mass. (It is assumed that, for most Subtitle D landfills, standard impermeable top liners will be installed.) As a result, leachate and landfill gas production will decline and eventually cease altogether early in the 30-year post-closure period.

Long-Term Risk Scenarios
The long-term environmental and public health risks of Subtitle D landfills are directly related to the long-term performance of the landfill’s final cover system.  In this regard, the long-term risks include the following future scenarios:

• A Functional Final Cover System—A functional final cover system will minimize the introduction of liquids into the waste mass, resulting in negligible amounts of
• Minor Breaches in Final Cover System—A minor breach could result from holes or tears in the final cover system, caused by rodents, plant roots, vandalism, or accidents. If the holes or tears are not found and fixed, this would lead to relatively minor amounts of moisture and air entering the closed landfill and would result in small quantities of leachate and landfill gas being generated. The resulting landfill gas would most likely exit the landfill through the breach. The leachate is likely to be absorbed within the waste mass and not exit the landfill.
• Deterioration of the Final Cover System At the End of Its Service Life—A major breach in the final cover would be caused by the disintegration of the final cover system at the end of its service life (which can typically be expected to be on the order of hundreds of years). Under this scenario, major quantities of moisture and air could enter the landfill, resulting in significant quantities of leachate and landfill gas being generated. Ultimately, oxygen from the atmosphere will also permeate the landfilled wastes to increasing depths, changing the environment from anaerobic to aerobic.

Characteristics of Mature Landfill Leachate
The leachate generated during the stable methanogenic phase typical of a closed landfill is sometimes referred to as “mature” leachate. The pollutants in mature leachate can be grouped into four categories:

• Dissolved organic compounds
• Inorganic macrocomponents
• Heavy metals
• Xenobiotic organic compounds

Published concentrations for the constituents of these groups in mature leachate are provided in Tables 2 through 5, along with relevant surface water and groundwater standards. (It should be noted that the concentrations presented in Table 4-1 include both average concentrations for dissolved organic matter and inorganic macrocomponents and high median concentrations for xenobiotic organic compounds and heavy metals.)

• Dissolved Organic Compounds—Dissolved organic compounds generally include the chemical byproducts of the biodegradation of decomposable organic waste. Their concentrations, which are presented in Table 2, are measured indirectly through such parameters as biochemical oxygen demand (BOD) and chemical oxygen demand (COD). 
• Inorganic Macrocomponents—Inorganic macrocomponents in mature leachate include positive metallic ions from dissolved salts such as calcium, magnesium, sodium, and potassium, as well as their negative ionic counterparts, such as chlorides, sulfates, and carbonates (see Table 3). The major inorganic constituents of mature leachate include sodium, potassium, chloride, and ammonia. Of these, ammonia nitrogen is viewed by many as the pollutant of most concern in the long term, as the concentrations of ammonia do not decrease over time (Kjeldsen, P., M.A. Barlaz, A.P. Rooker, A. Baun, A. Ldin, and T.H. Christensen, “Present and Long-Term Composition of MSW Landfill Leachate—A Review,” Critical Reviews in Environmental Science and Technology, 32, 4, p. 297-336).
• Xenobiotic Organic Compounds—Xenobiotic organic compounds (XOCs) are man-made organic compounds contained in such household products as cleaners, pesticides, and solvents.  These compounds are generally present in mature leachate in relatively low concentrations (see Table 4). XOCs can be categorized into three subgroups: hydrophobic XOCs, chlorinated solvents, and aromatic hydrocarbons. The hydrophobic (“water-fearing”) XOCs strongly absorb onto or adsorb into solid waste and have a low potential for migrating with landfill leachate. The chlorinated solvents generally react within an anaerobic environment, such as occurs in the stable methanogenic phase to form nontoxic end products (Barlaz, M., Rooker, A, Kjeldsen, P., Gabr, M., and Borden, R.2002. “Critical Evaluation of Factors Required To Terminate the Postclosure Monitoring Period at Solid Waste Landfills.” Environmental Science and Technology., 36, 3457-3465). For these reasons, the XOCs of greatest concern from an environmental standpoint are the aromatic hydrocarbons. While these compounds do biodegrade under anaerobic conditions, they can persist for many years. A recent publication concluded that “xenobiotic organic compounds are affected by volatilization and degradation as well as leaching and are in most cases not believed to be a long-term problem.”
• Heavy Metals—The concentrations of heavy metals in leachate vary over a wide range, depending on a number of factors, including waste composition, landfill age, and moisture availability. A 2004 SWANA report addressing heavy metals in Subtitle D landfills indicated that heavy metal concentrations in leachate have been reported in numerous recent studies to be relatively low (SWANA Applied Research Foundation. 2004. The Effectiveness of Municipal Solid Waste Landfills in Controlling Releases of Heavy Metals to the Environment. Silver Spring, MD: SWANA). This conclusion agrees with the data presented in Table 5, which also indicates low concentrations of heavy metals even when high median concentrations are reported.

Potential Environmental Impacts of Mature Leachate Discharges
The potential environmental impacts of discharges of mature leachate to the environment can be characterized by comparing the strength of the diluted leachate to groundwater pollution standards.

The EPA has established “Maximum Contaminant Levels” (MCLs) for allowable discharges of leachate pollutants to groundwater from Subtitle D landfills that are designed with alternative liner systems. The EPA established the MCL limits—which are identical to drinking water standards—by assuming that the leachate pollutant concentrations would be “diluted and attenuated” (and therefore lowered) by a factor of 100 before reaching the relevant point of compliance (Federal Register, Vol.55, No. 61, Thursday, March 29, 1990, p. 11827.  SWANA is aware that the EPA has updated its pollutant dispersion modeling approach since 1990 and may have developed constituent-specific dilution attenuation factors for certain metals. See 60CFR 66344, Dec. 21, 1995).

The same dilution and attenuation factor (DAF) is applied to high median concentrations of XOCs and heavy metals reported in the literature for leachate and the results are presented in the “Diluted Concentration” column in Tables 2–5.

As indicated, the resulting diluted groundwater concentrations of XOCs and heavy metals would comply with the groundwater MCLs established by the EPA for each of the pollutants in these categories. Therefore, it can be concluded that, even if mature leachate exits the bottom liner of the landfill, it is not likely to result in significant levels of groundwater pollution at the relevant point of compliance with respect to heavy metals and XOCs.

Put another way, the pollutant concentrations in mature leachate would most likely be diluted and/or attenuated to comply with their respective MCLs before any uncontained and/or uncollected landfill leachate reaches the relevant point of compliance. The MCL is measured at the “relevant point of compliance” (i.e., a location that is required to be no more than 150 meters (164 yards) from the waste management unit boundary.

Based on this analysis, it can be concluded that the discharge of relatively minor quantities of mature leachate to groundwater is not likely to cause groundwater MCLs at the relevant point of compliance of the landfill to be exceeded. Such quantities would be generated under the scenarios of a properly functioning final cover system or even a final cover system that has been breached to a minor degree.

The environmental impacts of a major breach of the final cover system—such as would occur at the end of the service life of the final cover system—would depend on the ability of the groundwater beneath the landfill to dilute and attenuate the pollutants in the mature leachate. Even under this scenario, it is not at all certain that groundwater MCLs would be violated for the pollutants currently regulated under Subtitle D regulations.

In light of these findings, it can be concluded that the long-term environmental risks associated with the discharge of small quantities of mature leachate to the groundwater beneath a Subtitle D landfill can be categorized as relatively minor. These risks increase as the quantities of leachate discharged increase due to major breaches in the final cover system due to the system reaching the end of its service life. 

The most prudent long-term management strategy for closed Subtitle D landfills may be to monitor and ensure the integrity of the final cover system over its design life and to install a new final cover when the end of the original final cover’s design life is reached. As indicated above, the required time frame for these monitoring activities is likely to be on the order of hundreds of years.

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Conclusions
Based on a review of the recent literature as well as data collected during the conduct of the SWANA ARF research project, the following conclusions were offered with respect to the long-term public health and environmental risks of Subtitle D landfills.

• In general, properly designed, constructed, and operated Subtitle D landfills are meeting environmental regulations and protecting public health and the environment.
• A small percentage of existing Subtitle D landfills have experienced problems with: pollution of groundwater by landfill gas escaping through the side slope edges of the bottom liner system, clogging of the leachate collection and removal system, and leakage of leachate through punctures or tears in the bottom liner system.
• Final cover systems have been shown to be effective at isolating waste, as long as periodic maintenance is performed. However, once the final cover system reaches the end of its useful life, precipitation will not be prevented from coming into contact with the remaining biodegradable portion of the landfilled waste, which will result in leachate and landfill gas production from the undecomposed waste.
• Overall, the period during which a Subtitle D landfill will potentially release contaminants may be on the order of hundreds of years. The threat of leakage can be mitigated through long-term cap maintenance to minimize the migration of liquid into the landfill.
• The risk posed by closed Subtitle D landfills consists mainly of the generation and possible leaking of a leachate of moderate biological strength, high inorganic macrocomponents, varying concentrations of xenobiotic compounds and low
• For a closed landfill with a fully functional final cover system or one where only minor breaches have occurred, the environmental and public health impacts are likely to be relatively minor. This is because the leachate pollutants regulated under Subtitle D—which consists of certain XOCs and heavy metals—are likely to have relatively low concentrations in the mature leachate that is generated and will, in addition, be diluted and/or attenuated by soil beneath the landfill and the receiving groundwater body.
• As the final cover system of a Subtitle D Landfill reaches the end of its service life and major breaches of the final cover system occur, the environmental and public health risks associated with leachate discharges from the landfill will depend on the capability of the receiving groundwater body to dilute and attenuate the pollutants in the leachate. Even under this scenario, it is not at all clear that the groundwater MCLs established in Subtitle D regulations will be exceeded at the relevant point of compliance.
• Long-term care beyond the current 30-year minimum is likely to be needed. However, the type and level of care needed is not clear. At a minimum, the final cover system needs to be monitored and maintained to ensure its integrity throughout its
• Funding is not required under Subtitle D regulations to be available to support the long-term monitoring and maintenance of the final cover system and, as a result, is not being accrued by Subtitle D landfill owners.
• One option under consideration by the industry is the reduction of postclosure monitoring expenses through the development of “indicator” parameters that would be sufficient to track the landfill’s postclosure performance. The savings in monitoring costs could then be used to fund site monitoring and maintenance activities following the initial 30-year post-closure period.

Author's Bio: Jeremy K. O’Brien, P.E., is the director of applied research for the Solid Waste Association of North America (SWANA).