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MEMORANDUM

To: Swarupa Ganguli, U.S. EPA, OAP, Climate Change Division 

 Docket EPA-HQ-OAR-2014-0451 

 

From: Eastern Research Group 

Date: July 2015

Subject: Establishing a Site-Specific Emission Threshold Alternative for MSW Landfills

The purpose of this memorandum is to document components of the Tier 4 approach in the proposed 

revisions to the Municipal Solid Waste (MSW) Landfills Emission Guidelines and discuss how EPA 

selected the surface emission thresholds for the proposal. 

1.0 Current Rule Framework 

Under the current Emission Guidelines (EG) and New Source Performance Standards (NSPS) for MSW 

Landfills, the timing of when a gas collection and control system (GCCS) must be installed is based on an 

emission rate threshold for nonmethane organic compounds (NMOC). The landfill has three methods for 

calculating whether its modeled emissions exceed the NMOC threshold. 

Installation Triggers: 

Table 1. NMOC Emission Threshold Determinations for GCCS Installation in Current Rule Framework

Tier 1: Use NMOC emission rate equation to calculate 

NMOC emissions and compare to 50 megagrams per 

year (Mg/yr) threshold (i.e., LandGEM approach). 

If fail Tier 1, then install GCCS 

OR go to Tier 2: 

Use Method 25 or 25C to determine site-specific 

NMOC concentration and re-calculate NMOC 

emissions and compare to 50 Mg/yr threshold. 

If fail Tier 2, then install GCCS 

OR go to Tier 3: 

Use Method 2E to determine site-specific k value1

and re-calculate NMOC emissions and compare to 

50 Mg/yr threshold. 

If fail Tier 3, then install GCCS. 

 

 

1

 A k-value is the methane generation rate constant, expressed in units of year-1. 
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The timing of when a GCCS can be removed is also based on the NMOC emission rate. 

Removal Triggers: 

The following three criteria must all be met: 

1. The landfill is closed AND

2. The GCCS must have been in operation a minimum of 15 years AND

3. The calculated NMOC emission rate at the landfill is less than 50 Mg/yr. The emission rate is 

based on actual measured flow rates and average NMOC concentrations in the landfill gas (LFG). 

2.0 Background on a Surface Emission-Based Emission Threshold 

The idea to measure site-specific surface emissions to determine timing of when a GCCS should be 

installed was received by the Small Business Advocacy Review (SBAR) Panel from small entity 

representatives (SERs) when EPA was conducting outreach to small entities during its review of the 

Landfills NSPS and EG in December 2013. The SERs recommended adding a new method to test direct 

surface emissions to demonstrate that site-specific emissions are low prior to GCCS installation.2 In 

follow-up outreach with the SERs in April 2015, the SERs provided additional feedback on more detailed

features of a site-specific surface emission based threshold.3 In addition, in its January 2013 white paper 

outlining policy recommendations for the NSPS/EG, the Environmental Defense Fund (EDF) presented 

the idea that a surface concentration threshold allows “a monitoring-based applicability prong that 

provides built-in incentives for landfill operators to adopt one or more innovative methane avoidance or 

mitigation strategies, including organic waste diversion, oxidative landfill covers, and early LFG control 

measures.” EDF noted that landfill experts state a surface methane concentration of 200 parts per 

million (ppm) provides further empirical confirmation that a landfill is ready for installation of a full gas 

capture system.4 EDF also pointed to the use of a surface concentration threshold in the California 

Landfill Methane Rule (CA LMR). 5

In the July 17, 2014 notices for the proposed NSPS and the Advanced Notice of Proposed Rulemaking 

(ANPRM) for existing landfills, the EPA solicited comments on a new Tier 4 surface emission monitoring

(SEM) demonstration to allow increased flexibility for landfills that exceed modeled NMOC emission 

rates if they can demonstrate that measured site-specific surface methane emissions are low. The EPA 

sought general feedback on the implementation and appropriateness of this alternative and specifically

sought comment on: (1) which areas of the landfill would be subject to SEM requirements because these 

areas would no longer be limited to areas with GCCS installed; (2) what number of exceedances over a 

specified time period would require GCCS installation; (3) what frequency of SEM demonstration is 

appropriate (e.g., quarterly monitoring for landfills accepting waste, annual monitoring for closed 

landfills); (4) what exceedance level is appropriate for determining if a GCCS must be installed (200 ppm 

or some other level); and (5) what landfills should qualify for using a Tier 4 option. The EPA received 

numerous comments in support of a Tier 4 alternative for both the installation and removal of a GCCS. 

 

2

 See Docket ID No. EPA-HQ-OAR-2003-0215-005 and EPA-HQ-OAR-2014-0451 

3

 See Final Report of the Small Business Advocacy Review Panel on EPA’s Planned Proposed Rule 

Emission Guidelines and Compliance Times for MSW Landfills. 2015. 

4

 See Docket ID No. EPA-HQ-OAR-2014-0451-0008

5 http://www.arb.ca.gov/cc/landfills/landfills.htm
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The EPA also received comments on specific implementation concerns, as discussed in the preamble for 

the proposed revisions to the Emission Guidelines.

3.0 Proposed Tier 4 Approach 

Based on public comments and feedback from SERs and other stakeholders, EPA has proposed 

regulatory text for a Tier 4 approach. Under the proposed Tier 4 option: 

• Area monitored: The SEM demonstration would be conducted around the entire perimeter of 

the landfill and along a traverse pattern at intervals no greater than 30 meters (98 feet), 

although EPA is requesting comment on whether to reduce the traverse pattern intervals to 25 

feet. All surface penetrations and areas where visual observations indicate elevated 

concentrations of methane must also be monitored. 

• Exceedances: The landfill owner or operator would be required to submit a design plan within 

90 days of the first measured concentration of 500 ppm or greater and would be required to 

install and begin operating a GCCS within 30 months of the first measured concentration of 500 

ppm or greater. Once the 500 ppm or greater threshold is exceeded, the owner/operator must 

install the GCCS within the specified time frame and cannot delay its installation by taking action 

to correct the measured exceedance. 

• Frequency of monitoring: Monitoring would be required quarterly. After four consecutive 

quarters, the frequency can be reduced to semi-annually at active landfill areas and annually at 

closed landfills or closed areas of active landfills if no exceedances were found. 

• Threshold: A surface concentration of 500 ppm was proposed. 

• Qualifying landfills: Any landfill exceeding the design capacity threshold can opt to use the Tier 4 

alternative after failing a Tier 1, Tier 2, or Tier 3 test. Once a landfill fails a Tier 4 test, a GCCS 

must be installed. 

Revised Installation Triggers: 

Table 2. NMOC Emission Threshold Determinations for GCCS Installation in Proposed Rule Framework

Tier 1: Use NMOC emission rate equation to calculate NMOC 

emissions and compare to 34 Mg/yr threshold (i.e., 

LandGEM approach). 

If fail Tier 1, then install GCCS 

OR go to Tier 2 or Tier 4: 

Tier 2: Use Method 25 or 25C to determine sitespecific NMOC concentration and re-calculate NMOC 

emissions and compare to 34 Mg/yr threshold. 

Tier 4: Use SEM to demonstrate surface methane 

emissions <500 ppm. If fail Tier 4, then install GCCS. 

If fail Tier 2, then install GCCS 

OR go to Tier 3 or Tier 4: 

Tier 3: Use Method 2E to determine site-specific k 

value and re-calculate NMOC emissions and compare 

to 34 Mg/yr threshold. 

Tier 4: Use SEM to demonstrate surface methane 

emissions <500 ppm. If fail Tier 4, then install GCCS. 
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If fail Tier 3, then install GCCS 

OR go to Tier 4. 

Tier 4: Use SEM to demonstrate surface methane 

emissions <500 ppm. If fail Tier 4, then install GCCS. 

Revised Removal or Decommissioning Triggers: 

The following criteria must be met in the proposal:

1. The landfill is closed or an area of an open landfill is closed AND

2. The GCCS must have been in operation a minimum of 15 years or landfill owner/operator must 

demonstrate that the GCCS will be unable to operate for 15 years due to declining gas flow AND

3a. The calculated NMOC emission rate at the landfill is less than 34 Mg/yr.6

OR

3b. The landfill or closed area of the landfill demonstrates for four consecutive quarters that there 

are no surface methane emissions at or above 500 ppm. 

4.0 Selection of a Surface Concentration Threshold 

4.1 Are the Proposed NMOC Emission Rate Threshold and Surface Emission-based Approach Equivalent? 

When examining options presented under Tiers 1 through 4, it is important to note that an SEM 

methane concentration and an NMOC emission rate are two different types of emission limits which are 

not directly comparable due to their inherent differences in measurement and estimation. An SEM 

reading in parts per million (ppm) is a concentration of methane (limit under Tier 4 option in proposal is 

500 ppm methane) in ambient air at a specific point at the landfill surface at a particular point in 

time. On the other hand, a modeled NMOC emission rate (limit in proposal is 34 megagrams/year) 

represents the estimated amount of NMOC emitted from an entire landfill over an entire year based on 

either default and/or site-specific modeling parameters and a first-order decay equation used to model

emissions over time. 

Unlike most emission sources, uncontrolled landfills emit gas through their entire surface area, not from 

a stack or exhaust vent. The amount of gas emitted from the surface of a landfill is site-specific and

varies spatially and temporally at any given landfill depending on several factors, including waste type 

and composition, waste age, waste moisture, waste temperature, cover practices, other avenues of gas 

extraction or venting, air temperature, and barometric pressure. An SEM concentration measures the 

methane in the ambient air 5-10 centimeters above the surface of the landfill at one location and not 

across the entire landfill. At a typical point source (e.g., manufacturing facility), you would measure a 

pollutant concentration and apply it to the volumetric gas flow rate (e.g., standard cubic feet per minute 

(scfm)) of a uniform and well-defined waste stream to obtain an estimated mass emission rate (e.g., 

Mg/yr). Because of the differences in how LFG emissions are released over time and space, correlating a 

surface methane concentration to a modeled NMOC mass emission rate is not practical for landfills. 

Commenters representing both industry and environmental groups noted the Tier 4 SEM option would 

encourage landfill owners and operators to implement alternative methane reduction practices such as 

oxidative covers, interim gas control measures, and organic waste diversion. The EPA agrees that such 

 

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 For landfills in the closed landfill subcategory, this third criterion remains less than 50 Mg/yr. 
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measures can directly affect surface methane emissions and would help a landfill owner/operator 

ensure that surface emissions are low, enabling the landfill to delay the regulatory requirement to install 

a GCCS. Commenters also expressed concern that a modeled emissions approach can overestimate 

emissions, particularly in arid areas. The EPA recognizes that modeling uncertainties can in some cases 

overestimate emissions and a surface-based approach can provide a more relevant and reasonable 

trigger for installing controls than modeling conducted under current emission threshold determinations 

(Tiers 1, 2, and 3). 

4.2 What Surface Concentration is Environmentally Protective? 

Due to the inability to correlate the modeled NMOC mass emission rate with a specific surface methane 

concentration, the EPA needed to select a surface concentration threshold for Tier 4 that is 

environmentally protective in order to make the Tier 4 approach viable. The Best System of Emission 

Reduction (BSER) for LFG emissions has been defined as a well-designed and well-operated GCCS. Once 

a GCCS has been installed and put in operation as required by the NSPS/EG regulations, the landfill 

owner/operator must conduct quarterly SEM and correct for exceedances of 500 ppm or greater in 

order to minimize surface emissions to the atmosphere. This level was selected in the original 1996 rule 

based on EPA’s evaluation of available information, including commenter feedback, to indicate proper 

operation of the GCCS.7 The EPA believes that it is appropriate to use this same threshold for 

determining when to install a GCCS in the first instance. As long as a landfill’s surface emissions of

methane remain below 500 ppm, the landfill’s emissions are consistent with the potential emissions of a 

landfill with a GCCS already in place and operating. Further, in addition to being environmentally 

protective, using the same 500 ppm threshold for determining when a GCCS is required to be installed 

under the EG, is consistent with the idea that voluntary systems installed or actions taken ahead of 

regulatory requirements must be well-designed and well-operated in order to be effective in minimizing

surface emissions to the atmosphere. 

The EPA reviewed other standards containing surface emission thresholds. The CA LMR retains the 500 

ppm level as an appropriate level for instantaneous SEM readings for areas already controlled by a 

GCCS. California ARB initially proposed a 200 ppm SEM threshold for both GCCS installation and for 

GCCS operation in its regulation, but finalized 500 ppm for GCCS operation because a lower threshold 

could cause an operator to overdraw the vacuum on the GCCS (to avoid a surface exceedance), which in 

turn could draw in too much oxygen and possibly cause fires.8 The EPA recognizes the concerns with 

setting the threshold too low, which may cause operators of voluntary GCCS to overdraw the vacuum on 

the GCCS, and has proposed a level of 500 ppm considering both environmental protection and safety. 

With respect to removal of GCCS, while the proposed rule allows for GCCS removal based on a surface 

emission-based approach, in practice a GCCS is rarely removed. Landfills would continue to operate the

GCCS for safety and LFG migration as part of their 30-year post-closure care obligations under the 

 

7 See Section 1.2.2.6 of Background Information Document at 1-43. Air Emissions from Municipal Solid 

Waste Landfills-Background Information for Final Standards and Guidelines, U.S. EPA (EPA-453/R-94-

021), accessed at http://www.epa.gov/ttn/atw/landfill/bidfl.pdf. 

8

 See Final Statement of Reasons. Page 30-31. 

http://www.arb.ca.gov/regact/2009/landfills09/landfills09.htm.
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Resource Conservation and Recovery Act (RCRA) regulations. However, once the GCCS requirements of 

the EG have passed, the GCCS may run more intermittently as gas flows build up enough to support 

combustion in a flare or other device. A surface emission-based approach would provide an incentive for 

landfills to maintain tight covers or install earlier final covers in order to keep the landfill’s emission 

below the threshold, thereby reducing SEM, recordkeeping, and reporting burdens associated with the 

EG requirements for landfills required to operate a GCCS under the rule. A 500 ppm threshold for 

removal would provide a consistent surface threshold to ensure that surface emissions venting to the 

atmosphere were minimized.