1

Date: June 7, 2013

To: Rachel Schmeltz, U.S. EPA

From: Jeff Coburn, RTI International

Subject: Uncertainty of Monthly Landfill Gas Methane Concentration Measurements

Purpose

This memorandum documents the uncertainty of the methane recovery when using a monthly

monitoring frequency with or without a minimum intervening period between “monthly” 

measurement events.

Background

On October 30, 2009, the EPA promulgated mandatory GHG reporting requirements for a 

variety of sources including municipal solid waste (MSW) landfills as 40 CFR Part 98 Subpart 

HH. For MSW landfills that have landfill gas collection, Subpart HH requires continuous 

monitoring of landfill gas flow and either continuous monitoring (if continuous monitoring 

equipment is present) or weekly measurement of landfill gas methane concentration. Industry

representatives have commented that the weekly measurement requirement is burdensome, 

particularly for closed landfills, and that monthly monitoring of methane concentration would 

provide an adequately accurate estimate of the methane recovery. To assist the EPA in their 

review of this issue, industry representatives provided their weekly flow and methane 

concentration measurements for the 2011 reporting year (RY2011). The data were provided on a 

landfill-specific basis with an indication of the landfill as either active or closed. 

The EPA evaluated the weekly measurement data, as documented in the memorandum “Review 

of Weekly Landfill Gas Volumetric Flow and Methane Concentrations” from Jeff Coburn, RTI

International, to Rachel Schmeltz, U.S. EPA, dated October 18, 2012 (see Docket Item No. EPAHQ-OAR-2012-0934-0014). On April 2, 2013, the EPA proposed to allow monthly monitoring

(rather than weekly monitoring) for methane concentrations at landfills with gas collection 

systems based on the results of the October 2012 data analyses. Specifically, the proposed 

amendments required that the methane concentration be determined “...at least once each 

calendar month; if only one measurement is made each calendar month, there must be at least 

fourteen days between measurements.” Industry representatives commented on the proposed 

rule stating that the 14 day interval was not necessary because: (1) the data showed little 

variability in methane concentrations; (2) it “significantly and unnecessarily complicates 

scheduling;” and (3) “it may not be possible to take a monthly reading at least fourteen days 

apart due to the operating schedule of the device.”

This memorandum evaluates the data originally provided by the industry representatives in light 

of these comments to determine if an intervening period between monthly samples is justified

because it serves the intended purpose of limiting the uncertainty of the methane recovery.
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Assertion 1: The data show little variability in methane concentrations

The previous analysis (as documented in the October 18, 2012, memorandum) used “Week 1” 

concentrations for each month and the total monthly flow to develop one estimate of methane 

recovery. A second estimate of methane recovery was developed using all “Week 2” 

concentration measurement. A third and fourth estimate of methane recovery was also 

developed using all “Week 3” and “Week 4” concentration measurements, respectively. In all of 

these cases, the recovery estimates were made using concentration measurement data that were 

approximately 28 days apart and had to be at least 22 days apart. If the rule did not include an

intervening interval between monthly samples, it is possible that the landfill owner or operator 

could determine the methane concentration just before midnight on January 31, wait a few 

minutes, and determine the methane concentration just after midnight on February 1. This would 

qualify as monthly samples as the samples are taken in different calendar months, even though 

they are minutes apart. 

In order to evaluate the impact of removing the intervening interval as suggested be commenters, 

the original data was again evaluated using a single methane concentration determination at the 

end of Month 1 for the cumulative flow for both Months 1 and 2, the methane concentration 

determination at the end of Month 3 for the cumulative flow for both Months 3 and 4, 

respectively. We note that subpart HH currently requires weekly samples to be at least 3 days 

apart, therefore using Week 4 for Month 1 and Week 1 for Month 2 would still indicate a 

minimum intervening period, so this approach was not used to evaluate the impact of removing 

the intervening period altogether.

The annual methane recovery was determined for each landfill based on “no intervening period” 

and compared to the annual methane recovery determined based on the weekly concentration 

measurements and flow rates. Figure 1 shows the distribution of the percent error in the monthly 

measurements as determined using the “no intervening period” values. Figure 2 shows the same 

distributional analysis determined previously using all “Week 1” methane concentration 

measurements. It is quite clear from a comparison of these figures that, while there is no 

significant bias (graphs still distributed around 0 error), there is a significantly broader range of 

errors. In the true monthly sampling paradigm (4 week intervals), there is only one landfill at the 

tails (outside ±6.25 percent error). For the “no intervening period” analysis, there are 

approximately 10 landfills at each tail. Under the true monthly sampling paradigm, the extreme 

values were -6.3 percent and +6.5 percent; under the “no intervening period” analysis, the 

extreme values were -21 percent and +13.2 percent. These results strongly indicate that more 

accurate methane recovery values will be achieved if there is an intervening period between 

monthly samples.

In October 18, 2012, RTI also performed a Monte Carlo analysis to project the impacts of the 

variability in methane concentration measurements on the accuracy of the calculated methane 

recovery value when going from weekly sampling (n= 52 ) to monthly sampling (n=12). In 

order to project the uncertainty of monthly sampling with no intervening period, the Monte Carlo 

analysis was repeated with n=6 (assuming the no intervening period is taken to its extreme). The 

95th percentile confidence interval for the annual average methane concentration based on 52, 12, 

and 6 measurements is ±3.8%, ±7.8%, and ±11%, respectively. 
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Figure 1. Distributional analysis of the percent difference in the annual methane recovery using 

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Figure 2. Distributional analysis of the percent difference in the annual methane recovery using 

monthly concentration measurements one month apart.
4

Table 1 summarizes the sum of squares approach for determining the overall uncertainty 

associated with the methane recovery values for a single landfill. As seen in Table 1, the overall 

uncertainty of the methane recovery values for a single reporter is expected to increase from 

±8% to ±10.5% when going from a weekly to a true monthly monitoring frequency. If there is 

no intervening period between monthly samples, the uncertainty increases to ±13%. Thus, the 

inclusion of an intervening interval in the monthly sampling requirement helps to limit the 

increase in the uncertainty associated with the methane recovery values when switching from 

weekly to monthly monitoring.

Table 1. Estimate of the Overall Uncertainty for Methane Recovery for a Single Landfill.

Parameter

Uncertainty Square of Uncertainty

N=52 N=12 N=6 N=52 N=12 N=6

Accuracy of flow meter ±5% ±5% ±5% 25 25 25

Accuracy of methane 

concentration

±5% ±5% ±5% 25 25 25

Variability of methane 

concentration

±3.8% ±7.8% ±11% 14.44 60.84 121

Sum of Squares 64.44 110.84 171

Overall Uncertainty 

(square root of Sum of 

Squares)

±8.0% ±10.5% ±13.0%

Assertion 2: The intervening period “significantly and unnecessarily complicates 

scheduling”

No data were presented by industry commenters as to how the intervening period “significantly” 

complicates scheduling. If the sampling is planned for approximately the 15th of every month, 

the subsequent month’s sampling can be conducted any time during the following month. Thus, 

if the landfill owner schedules sampling to be conducted near the 15th of each month, the landfill 

owner of operator has significant latitude to reschedule a certain month’s sampling event if an 

issue arises with the recovery system and the system cannot be sampled on the 15th. Therefore, it 

is not clear how this requirement “significantly” complicates scheduling. Furthermore, the 

weekly sampling requirement currently in subpart HH requires a 3 day interval between weekly 

sampling. As such, monthly sampling with a 14 day intervening period is much less burdensome 

or complicated than the existing weekly measurement requirement with a 3 day intervening 

period. As for “unnecessarily” complicating the rule, the “no intervening analysis” presented 

under Assertion 1 provides strong evidence that the intervening interval is necessary to limit 

measurement error and uncertainty associated with monthly sampling. 
5

Assertion 3: The intervening period may make it impossible to collect samples for some 

months at some landfills.

The majority of landfills operate their recovery system continuously. From the weekly 

monitoring data submitted from the industry, only 22 of 395 landfills had landfill gas flow for

less than 52 weeks per year and only 12 of 395 landfills had landfill gas flow for less than 50

weeks per year. If a landfill’s gas collection system is down for only one or two weeks a year, 

the 14-day intervening period will not hinder the collection of monthly samples. On the other 

extreme, if a landfill has no recovered gas flow for an entire month, then that landfill would have 

to use missing data procedures regardless of whether or not there is an intervening period 

between monthly samples (in fact, no concentration measurement is need since the cumulative 

flow for the month is zero). If the landfill owner or operator generally plans to sample early each 

month or near the 15th of the month, the next month’s sample can be taken anytime during the 

month. The only time the lack of an intervening period would allow a landfill owner or operator 

to collect a monthly gas sample is when there is only gas flow during the last week of a month 

and during the first week of the next month. The weekly flow periods for the 12 landfills that 

have landfill gas flow for less than 50 weeks per year were evaluated to determine how often the 

14 day intervening period would prevent a landfill owner or operator from collecting a monthly

sample that they could have otherwise collected if there were no intervening period between 

monthly samples. We could not identify one month for any landfill within the entire data set 

where missing data provisions would have to be used under the monthly sampling with a 14-day

intervening period but not under the monthly sampling with no intervening period. Based on the 

industry’s weekly sampling data and this analysis, we conclude that the 14-day interval between 

monthly sampling events will not increase the prevalence at which facilities must rely on missing 

data procedures.