RTI International 1 

Date: June 17, 2015

To: Rachel Schmeltz, EPA

From: Kate Bronstein, Meaghan McGrath, Jeff Coburn, RTI International

Subject: Review of Oxidation Studies and Associated Cover Depth in the PeerReviewed Literature – Revised Memorandum 

1. Purpose 

This memorandum documents a review of the fraction of landfill methane oxidation presented in the peerreviewed literature and associated landfill soil cover depths. 

2. Background 

As methane generated in the landfill flows up through a landfill’s cover soil, a fraction of that methane will be 

oxidized or metabolized by methanotrophic bacteria near the soil surface. The relative amount of methane 

removed from the landfill gas stream as the gas passes through the soil surface layer is termed the fraction 

oxidized or oxidation fraction. The fraction oxidized depends on a number of factors including: 

• The flow velocity of landfill gas, or the methane flux, through the soil surface (higher flux rates means 

lower residence times in the aerobic soil layer and less oxidation), 

• The porosity of the soil layer (greater porosity leads to more air ingress and a thicker aerobic soil layer, 

which leads to more oxidation), 

• The number and types of microorganisms in the surface soil layer, and 

• The soil surface temperature and moisture, which may increase or inhibit microbial activity. 

For the first three reporting years under 40 CFR Part 98 Subpart HH of the EPA’s Greenhouse Gas Reporting 

Program, owners and operators of municipal solid waste (MSW) landfills were required to use an oxidation 

fraction of 10 percent when estimating methane emissions from MSW landfills. Industry representatives 

commented that the 10 percent oxidation fraction was too low and that many landfills have much higher 

oxidation fractions as shown in the recent published literature. Additionally, numerous U.S. landfills with 

active gas collection systems and clay soil or ‘‘other soil mixture’’ covers have been tested in the past decade to

estimate the oxidation fraction. The 10 percent default value is based off of a study1 conducted in the early 

nineties (Czepiel et al., 1996). 

Industry representatives compiled data from peer-reviewed studies and evaluated soil oxidation fractions 

(SWICS, 2012) to support their statement and provide options for using the data to estimate the oxidation 

 

1 Czepiel, P. M., B. Mosher, P. M. Crill, and R. C. Harriss (1996), Quantifying the effect of oxidation on landfill methane 

emissions, J. Geophys. Res., 101(D11), 16721–16729, doi:10.1029/96JD00222.
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RTI International 2 

fraction for MSW landfills reporting under Subpart HH. These options, a comparison of methane emissions that 

would be reported using these options based on methane flux data provided by industry representatives, and 

recommendations for selecting a specific option for inclusion in Subpart HH to replace the current 10 percent 

default oxidation fraction requirement are documented in a memorandum dated December 7, 2012 (see Docket 

ID No. EPA–HQ–OAR–2012–0934). Based on the analysis documented in the aforementioned memorandum, 

the following changes to the oxidation fractions were proposed on April 2, 2013 (78 FR 19802): 

• Three different default oxidation fractions depending on the methane flux bin (see Table 1) were 

proposed to provide a more realistic estimate of the role of methane oxidation in the surface soil on the 

methane emissions than the single default oxidation fraction. 

o For cases where the methane flux is projected to be high (greater than 70 grams/m2/day), the 

default oxidation fraction remains as 10 percent. 

o For cases where the methane flux is projected to be low (less than 10 grams/m2/day), the 

default proposed oxidation fraction is 35 percent. 

o For cases with moderate methane flux rates (10 to 70 grams/m2/day), the proposed default 

oxidation fraction is 25 percent. 

• A calculation method to determine the methane flux rate to be used for determining the oxidation 

fraction when Equations HH–5, HH–6, HH–7, or HH–8 are used was also proposed. 

Table 1. Proposed Table HH-4 to Subpart HH of Part 98—Landfill Methane Oxidation Fractions

If your methane flux rate for the reporting year is: Use this landfill methane 

oxidation fraction: 

Less than 10 grams per square meter per day (g/m2/d) 0.35

10 to 70 g/m2/d 0.25

Greater than 70 g/m2/d 0.10

The proposed changes to the methane oxidation fractions did not include cover depth requirements. The need 

for a cover depth requirement resulted from a public comment (Center for a Competitive Waste Industry 

[CCWI]) in response to the definition of the methane flux rate, which is defined as a footnote to Table HH-4 as 

the mass flow rate of methane per unit area at the bottom of the surface soil prior to any oxidation. The 

inclusion of the ‘bottom of the surface soil’ phrase in this definition triggered a comment regarding the need for 

a defined soil cover depth and specifically noted that landfills with geomembrane covers and no cover soil 

should be assigned an oxidation fraction of zero. CCWI also noted that “...oxidation only occurs in landfill 

covers that are comprised only of soil (with the necessary depth, porosity, temperature and microbes), and not 

through composite covers that include a low permeable geomembrane.” 

The 24 inch soil cover depth requirement was a logical outgrowth of these comments. To address these 

comments, peer-reviewed literature, as synthesized in SWICS 2012, was reviewed. The soil cover depths 

reported in the studies on which the higher oxidation fractions were based had soil covers of at least 24 inches. 

If one assumes the measurements were taken at representative areas of each landfill, it can also be assumed that 

the soil cover depths reported at the measurement locations were similar to the majority (i.e., > 50 percent) of 

the landfill surface. Therefore, a soil depth of 24 inches over the majority of the landfill was selected as the 

basis for using the higher oxidation rates. An industry expert was also consulted and confirmed 24 inches of 

intermediate cover was typical industry practice. Nonetheless, several of these studies indicated that soil 

oxidation tends to occur in the top 12 inches of the soil cover. Considering these data, we determined it would 
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RTI International 3 

be appropriate to allow landfills with geomembrane covers to use the 0.10 oxidation fraction. Releases of 

surficial landfill gas from landfills with geomembrane covers are expected to be isolated to areas where leaks 

from the cover may occur (e.g., seams or tears). We expect the flux rate through the soil to be high (in those 

areas where flux occurs); therefore, landfills with soil cover over a geomembrane cover may only use the 0.10 

oxidation factor. 

Based on public comments received and our review of the available published literature, the following 

oxidation fraction values and soil cover depths were promulgated (78 FR. at 71925, see Table 2). 

Table 2. Promulgated Table HH-4 to Subpart HH of Part 98—Landfill Methane Oxidation Fractions

Under these conditions: 

Use this landfill methane 

oxidation fraction: 

I. For all reporting years prior to the 2013 reporting year

C1: For all landfills regardless of cover type or methane flux 0.10

II. For the 2013 reporting year and all subsequent years

C2: For landfills that have a geomembrane (synthetic) cover with less than 12 

inches of cover soil for the majority of the landfill area containing waste 

0.0

C3: For landfills that do not meet the conditions in C2 above, and for which 

you elect not to determine methane flux 

0.10

C4: For landfills that do not meet the conditions in C2 above and that do not

have a soil cover of at least 24 inches for a majority of the landfill area 

containing waste 

0.10

C5: For landfills that have a soil cover of at least 24 inches for a majority of the 

landfill area containing waste and for which the methane flux rate is less than 

10 grams per square meter per day (g/m2/d) 

0.35

C6: For landfills that have a soil cover of at least 24 inches for a majority of the 

landfill area containing waste and for which the methane flux rate is 10 to 70 

g/m2/d 

0.25

C7: For landfills that have a soil cover of at least 24 inches for a majority of the 

landfill area containing waste and for which the methane flux rate is greater 

than 70 g/m2/d 

0.10

3. Summary of the Petition for Reconsideration 

Waste Management (WM) petitioned EPA for reconsideration in light of the revision to the oxidation fraction 

made to the rule affecting Subpart HH for MSW Landfills2. The petition states the soil cover requirement of 24 

inches over the majority of the landfill containing waste will needlessly limit the number of reporters who can 

use alternative methane oxidation fractions, and thus more reporters will be relegated to the default 10 percent 

default oxidation factor. 

 

2

See Docket I.D. EPA-HQ-OAR-2012-0934 in response to the 2013 Revisions to Greenhouse Gas Reporting Rule and 

Final Confidentiality Determinations for New or Substantially Revised Data Elements; Proposed Rule, 78 Fed. Reg. 

19802-19877, 19825 (April 2, 2013). 
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RTI International 4 

The following is a brief summary of the arguments in the petition: 

• The 24 inch soil requirement is arbitrary and not based on sound science (no correlation between depth 

and methane oxidation). 

• The oxidation issue was not raised in proposal comments received by the Center for Competitive Waste 

Industries and Covanta on which the change was based; these comments do not specifically mention 

soil depth or provide data for a logical outgrowth to 24 inches. 

• A soil cover requirement of 24 inches is not a logical outgrowth of comments received and peerreviewed literature and is “so distant from the Proposed Rule that WM could not have anticipated its 

inclusion in the Final Rule.” 

• The majority of WM landfills will be precluded from using the higher bins of methane oxidation and 

will suffer the burden of over-reporting emissions because the 24 inch soil cover requirement cannot be 

met. 

The petitioner requests EPA to convene a reconsideration proceeding to fully evaluate and correct this technical 

revision by either: 

• Removing the 24-inch soil cover requirements altogether OR 

• Allowing reporters to use intermediate cover as the soil cover requirement, which would effectively 

require a 12-inch soil cover requirement. The petitioner notes that 27 states in which ~77% of MSW 

landfills are located have intermediate cover requirements of 12 inches, either exclusive or inclusive of 

the federally mandated 6 inches of daily cover. 

4. Methods 

Peer-reviewed literature was reviewed to tally all mention of soil cover depth and use of direct quotes in the 

WM petition. The following steps were completed under this task: 

• Gathered copies of all studies explicitly mentioned by WM in their petition; 

• Compiled information on the fraction of methane oxidized, soil cover type, and/or depth in the studies 

included in the WM petition as well as the studies included in SWICS 2012; 

• Reviewed each study to verify that all quotations cited in the petition were direct quotes; and 

• Determined if any additional information in the cited studies contradicts or supports the quotes used in 

the petition. 

5. Findings 

5.1 Soil Cover Depth Included in Reviewed Methane Oxidation Studies 

• Multiple studies give reference to and/or support the Czepiel et al. (1996) study which estimated a 10% 

methane oxidation fraction for any given landfill throughout the entire year, considering higher 

methane oxidation during warmer summer months and lower rates during winter. 

• Other studies (e.g., Chanton et al., 2011) argue specifically for a higher default methane oxidation in 

landfills with gas collection, using their data to support their claims (700+ measurements, etc.). 
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RTI International 5 

• Temperature (in terms of seasonal variability) is consistently presented as a significant variable for 

methane oxidation fraction at landfills. 

• All studies include soil characteristic data (e.g., moisture content, soil type, pH, soil cover 

depth/thickness). While soil cover depths were neither presented in every study reviewed, nor explicitly 

mentioned as an influencing variable in every study reviewed, all studies were conducted at a landfill 

or a simulated landfill environment with some depth of soil cover. It can therefore be concluded that 

some amount of soil cover is important for methane oxidation to occur; however the studies do not 

allow for a conclusion on the optimum depth of that soil cover. Two recent studies, Bogner et al. 

(2011) and Abichou et al. (2006) note a relationship between cover thickness and methane oxidation: 

o Bogner et al. (2011) note that landfill methane emissions are reduced by three major 

mechanisms, one of which is “the thickness and physical properties of engineered cover 

materials” (p. 1010). They also note that “thicker cover materials retard transport and increase 

retention times to facilitate oxidation.” (p. 1010). 

o Abichou et al. (2006) investigated methane flux and oxidation at two types of intermediate 

landfill covers and found that methane oxidation was higher at the site with a thicker (45-cm or 

17.7 in.) cover over 7-year old waste compared to a 15-cm (5.9 in.) cover over 1-year old 

waste. The average flux for both landfill covers were in the intermediate range (22 g/m2/d for 

the thicker cover and 54 g/m2/d for the thinner cover. The authors note “The mean percent 

oxidation was significantly greater (25%) at the thick cover relative to the thin cover (14%).” 

(p. 1305). 

• 27 peer-reviewed studies, included in either or both the SWICS 2012 Addendum or in RTI 2011, were 

reviewed for soil cover depths in relation to the oxidation fraction. From these studies, a total of 90 

measurements were included, but only 63 data points were considered (including 49 field 

measurements and 14 column measurements) for the purpose of this memorandum. Only data points 

where both an oxidation fraction and a soil cover depth were presented were included for the purposes 

of this memorandum. Table A-1 summarizes key data from all 27 peer-reviewed studies, including the 

soil cover type and depth and the fraction of oxidized methane. 

• 52 of the 61 data points (or 85%) gathered from the studies that explicitly mentioned a cover depth and 

an oxidation fraction, reported a cover depth of 24 inches or above. When only data points from field 

studies are included (49 field data points), 89% of data points were associated with a cover depth of 24 

inches or above. Regardless of the column or field measurement, all studies reviewed were conducted, 

or can be assumed, to have been conducted at a landfill or landfill-type environment (for column 

studies) with a cover. 

• The Wilshusen et al. (2004) column study was performed on compost comprised of leaf litter and 

manure. For the purpose of this memorandum, this study data was not included as we consider it an 

emerging cover type and is not commonplace at closed or operational MSW landfills. 

• Fifteen studies, including the Wilshusen et al. (2004) study, were cited in the WM petition (see Table 

3). Five were column studies and the rest field studies. If the Wilshusen et al. (2004) study is not 
Review of Oxidation Studies and Associated Cover Depth in the Peer-Reviewed Literature June 2015 

RTI International 6 

considered, 13 of the 14 remaining studies collected measurements at one or more locations with soil 

cover depth meeting or exceeding 24 inches. 

• Eleven studies (Cabral et al., 2010; Chanton et al., 2008; Gebert et al., 2011a and 2011b; Huber-Humor 

et al., 2009, Knightley et al., 1995; Rachor et al., 2011; Scheutz et al., 2009; Schroth et al., 2012; 

Spokas and Bogner, 2011; Wilshusen, et al., 2004) measured methane and/or oxygen profiles in the 

surface cover soil. While the cover soils generally exceeded 24 inches in these studies and only three 

of the studies were conducted in the field, most of the methane oxidation appears to occur in the top 12 

to 15 inches (30 to 40 cm) of the cover soil. 

• Two studies (Liptay et al., 1998 and Chanton and Liptay, 2000) noted that methane flux in areas near 

passive vent pipes were much smaller than in other areas of the landfill. 

Table 3. Soil Cover Depth and Landfill Status of Landfills in the Peer-Reviewed Studies Included in the 

Petition

Author(s) Field or 

Column 

Open/Closed Cover Soil Depth 

(in) 

Fraction 

Oxidized 

Notes

Barlaz et al. 

(2004) 

Field No explicit 

mention 

• 39 (soil) 

• 45 (biocover) 

• 0.21 

• 0.54 

Biocover consisted 

of 0.15 m thick clay 

layer over the waste, 

followed by 0.15 m 

of tire chips, then 1 

m of yard waste 

compost 

Bogner et al. 

(1997) 

Column Closed (final 

cover 1980s) 

• 49 • Not 

reported 

 

Chanton and 

Liptay (2000) 

Field Closed • 43 

• 39 

• 0.26 

• 0.14 

Samples taken from 

3-15cm below the 

surface 

Chanton et al. 

(2008) 

Field No explicit 

mention 

• Site 1: 29 

(additional 

biocover) 

• Site 1: 51 

(additional 

biocover) 

• Site 2: 6 (deep 

biocover) 

• Site 2: 35 (deep 

biocover) 

• Site 2: 23 (shallow 

biocover) 

• Site 1: 0.40 

• Site 1: 0.29 

• Site 2: 0.08 

• Site 2: 0.12 

• Site 2: -0.01 

Measurements 

taken from multiple 

locations; Site 1 

sites had additional 

biocover; Site 2 sites 

had deep biocover 

and shallow cover 

Chanton, et al. 

(2011) 

Field No explicit 

mention 

• 12-48 • 0.06-0.82 Range of cover 

depth provided 

Christopherson 

et al. (2001) 

Field Closed • 39 • 0.28 

• 0.89 

 
Review of Oxidation Studies and Associated Cover Depth in the Peer-Reviewed Literature June 2015 

RTI International 7 

Author(s) Field or 

Column 

Open/Closed Cover Soil Depth 

(in) 

Fraction 

Oxidized 

Notes

Czepiel et al. 

(1996) 

Field No explicit 

mention 

• 39-79 • 0.1 Range of cover 

depth provided 

De Visscher et 

al. (1999) 

Column Closed and/or 

Abandoned 

• 24 • 0.61+ 

• 0.65 

Cover depth as 

presented in SWICS 

2012 

Hilger et al. 

(2000) 

Column Closed • 12 • 0.21 Cover depth as 

presented in SWICS 

2012 

Kightley et al. 

(1995) 

Column No explicit 

mention 

• 35 • 0.40 

• 0.41 

• 0.61 

Liptay et al. 

(1998) 

Field No explicit 

mention 

• NR • 0.0-0.68 Samples taken from 

3-15cm below the 

surface 

Scheutz and 

Kjeldsen (2003) 

Column No explicit 

mention 

• 35 • 0.96+ Aerobic experiments 

sampled at 15-20 

cm depth; anaerobic 

experiments 

sampled at 50-60 

cm depth 

Stern et al. 

(2007) 

Field Closed • 39-69 • 0.19-0.38 Range of cover 

depth provided 

Wilshusen et al. 

(2004) 

Column NA • NA • 0.19 This study was 

conducted on 

compost and 

manure and is 

atypical of landfill 

cover practice, and 

is not a 

representative 

choice for this 

analysis. 

Goldsmith et al. 

(2012) 

Field No explicit 

mention 

• 0 (no cover) 

• 6-12 (temporary 

cover) 

• 24-47 

(intermediate 

cover) 

• 39-79 (final cover)

• Not reported Measurements 

taken from multiple 

landfills across the 

United States. 

Methane fluxes are 

provided, but 

presented by 

area/state with no 

relationship between 

which 

measurements were 

taken at which cover 

depth 

NR = not reported 

NA = Not applicable 
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RTI International 8 

5.2 Citations in the Petition 

Within their petition to EPA, WM references a total of 15 peer-reviewed studies to support their central 

argument that soil depth is not a significant factor influencing methane oxidation at MSW landfills. The studies 

are incorporated as footnotes in the petition to support three different statements. These statements are 

presented at the beginning of Sections 5.2.1 through 5.2.3 of this memorandum, followed by a brief analysis of 

the quoted text and study presented in the petition’s footnotes. These brief analyses indicate whether the 

information presented throughout the study does or does not support the quote used in the petition. 

In general, all quotations taken from the literature and presented in the petition were direct quotations. Only 3 

citations were not direct quotations (Chanton et al., 2008 in response to quote in Section 5.2.2 in this 

memorandum; and Chanton et al., 2011 and Goldsmith et al., 2012 in response to quote in 5.2.3 in this 

memorandum), however these quotes accurately described the figures and/or tables to which they were 

referencing. 

Multiple studies indicate the complexity of methane oxidation through soil in the landfill setting (Stern et al., 

2007; De Visscher et al., 1999; Scheutz and Kjeldsen, 2003) and all studies that observed landfill flux behavior 

over the course of multiple seasons found a similar relationship between methane oxidation and temperature: 

oxidation increases in warmer, summer months and decreases in colder, winter months. In addition to 

temperature, Bogner et al. (1997) state that methane oxidation is important, at times, in deeper landfill settings 

as a result of the relationship between different species of bacteria in the landfill layers. 

Three studies show evidence of a 10% methane oxidation fraction (Liptay et al., 1998; Barlaz et al., 2004; 

Czepiel et al., 1996) when all seasonal averaging is taken into account at a site, however, Barlaz et al. (2004) do 

note that these values were obtained at specific landfills, and thus may not be applicable to all landfills. 

5.2.1 Page 17, paragraph 2: “Additionally, more detailed review of the studies referenced in the 

2012 SWICS Addendum does not support the conclusion that soil depth is a significant 

factor influencing methane oxidation.” 

Barlaz et al. (2004): “Several factors have been shown to influence the extent of CH4 oxidation in landfill soil 

cover soils, including CH4 and O2 concentrations, temperature, moisture content, soil composition, moisture 

holding capacity, pH and nutrient content.” 

• The study begins by referencing the aforementioned factors as a probable cause of influence to CH4

oxidation in landfill cover soils, but later concludes that “there was not significant correlation between 

either ambient temperature or pressure and CH4 flux” (pg. 4895) in laboratory scale experiments 

conducted during the course of the study. The study continues to state that “there was not a significant 

correlation between either temperature or pressure change and the measured flux” (pg. 4895). In terms 

of direct mention of methane oxidation values, the study notes that “when annual CH4 oxidation is 

considered on a seasonally averaged basis, values of 10% and 14% have been estimated for soil covers 

in the Northeast and Florida, respectively” (pg. 4891). The authors note these values are site-specific. 

Barlaz et al. (2004) state that 21% of the methane was oxidized in the soil cover study area (pg. 4898), 

and the study area was covered by 100-cm (~39-in) of clay (pg. 4892). 
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RTI International 9 

Bogner et al. (1997): “Limitations on physical transport become major controls in the field, where relative 

rates of CH4 and O2 transport to a given depth are restricted by rates of methanogenesis below and by O2

transport from atmospheric sources above. Other factors that directly control methanotrophic microbial 

activity in the field include temperature, moisture, organic carbon content, available nutrients, and predation 

by or competition with other soil microorganisms.” 

• Bogner et al. (1997) note that “model inputs [in the study] were limited to the generalized site geometry 

for portions of each site (length, width, depth, and soil cover thickness), mean values for soil moisture 

and gas-filled porosity, and the soil gas profiles (geometric mean) through the cover material” (pg. 

2511) with the soil cover thickness consisting of silty clay “approximately 100 cm thick overlain by 

topsoil...approximately 25cm thick.” (pg. 2505) Soil depth was not specifically listed as an influencing 

factor for methane oxidation, however all model outputs for the study were based off of this 125cm 

total soil cover.

Chanton et al. (2000): “in the landfill we studied, temperature was the main factor in controlling methane 

oxidation at specific sites (Figures 8a and 8b)” and that “methane oxidation occurs in the outer rind of the 

landfill and is subject to environmental temperature fluctuations which cause rates to be greater in summer and 

lesser in winter.” 

• Results from this study reveal a large difference between methane oxidation rates during winter (3-5% 

oxidation) and summer (maximum of 43 ± 10% in July) that are verified in the cited quotation (pg. 57). 

Chanton et al. (2000) conclude that the seasonally integrated value for methane oxidation for areas of 

the landfill covered with mulch/topsoil was 26 ± 4% of the total methane oxidation rate, while for clay 

soil it was only 14 ± 2% with the overall average of both soil types being 20 ± 3% (pg. 57). Soil 

samples were taken from the top 15-cm of the cover soil, however, the entirety of the final cover for the 

sampled landfill was 109-cm where mulch/topsoil was still in place and 100-cm where mulch/topsoil 

had washed away. 

Christopherson et al. (2001): “Methane oxidation is controlled by various environmental factors: temperature, 

soil water content, nutrients, sub-strate, and oxygen concentrations.” 

• Christopherson et al. (2001) generally support the notion of seasonal variation in landfill gas flux with 

higher fluxes in the summer months. The authors conclude that “from the figures discussed in the 

previous sections it is difficult to identify the factors governing lateral gas emission, but when 

statistical analysis is used, it is possible to investigate the influence of several factors at the same time. 

... In general, the regression analysis on the summarised fluxes supports what is illustrated in the 

figures in the previous sections: the fluxes depend on temperature, and barometric pressure, and the 

pressure gradient”, which is supported in WM’s citation. (pg. 605) 

• The study also notes that “the concentrations of landfill gas and the soil moisture content in the upper 

part of the soil profile have significant influence on the fluxes” (pg 605), however it is once again 

important to note that the soil studied was taken from a landfill with a final cover of 100-cm (80-cm 

sand and 20-cm mould) (pg. 596). 

Hilger et al. (2000): “Some of the factors that influence microbial CH4 oxidation in landfills include climate 

variables such as moisture and temperature, as well as CH4 concentrations, soil type, and pH.” 
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RTI International 10 

• The Hilger study does not contain any information regarding the soil cover of the landfill from which 

fresh core samples were taken (the Renaissance Park landfill in Charlotte, NC), however the study does 

indicate that the landfill is closed and has since been converted into recreational fields. (pg. 458) The 

study findings support the importance of the cited factors (temperature, CH4 concentration, etc.) on 

methane oxidation and microbial activity in landfills. 

Kightley et al. (1995): “In conclusion, our data show that soil microbial community adapts rapidly to the 

presence of elevated CH4 concentrations and can effectively reduce emissions of CH4 from the surface of 

landfill soil. The capacity for CH4 oxidation in soil is related to the depth of O2 penetration and to the nutrient 

status of the soil, which may regulate the extent to which the methanotrophic community can develop.” 

• This study permeated CH4 into four landfill soil core samples (in triplicate with a control) measuring 1-

m by 15-cm and packed to a depth of 90-cm (p. 592). Three soil types (minus any turf) were collected 

from the upper 15-cm of various landfills and were sieved through a 20-mm mesh (p. 593). The soil 

cores were saturated with CH4 for a period of 6 months to provide “an estimate of the relative biomass 

of the methanotrophic population in the soil, as well as an indication of the CH4-oxidizing capacity of 

each soil” (p. 593). 

• “Measurement of the CH4 oxidation potentials demonstrated that each of the soils had a capacity for 

methanotrophy at most depths (Fig. 8), even extending into the anoxic layer.” (p. 596). We believe this 

quote is referring to Figure 6 (and not Fig. 8), which shows depth profiles for methane, oxygen, 

nitrogen, and carbon dioxide for the coarse sand soil type after amending with various nutrient sources. 

Figure 6 shows that 3 of the 4 coarse sand soil columns became anoxic at a depth of 30-cm, while the 

4

th (amended with NH4NO3) was still aerobic at 80 cm deep. The authors suggest “that it was the 

development of a very active methanotroph community in the coarse sand which rapidly removed O2

and limited its penetration into this soil. In contrast, the development of equally active communities in 

the other two soils was prevented by other factors such as nutrient limitation.” (p. 598). 

• This study makes a link between soil type, depth, oxygen penetration, nutrient status, and CH4

oxidation. 

Liptay et al. (1998): “Soil oxidation potential can change with factors such as temperature, pH and moisture 

content.” 

• In terms of quantifying soil oxidation, it is important to note that Liptay et al. explicitly state that 

“[their] study is consistent with the modeling results of Czepiel et al. (1996) that the best estimate for 

the annual value for methane oxidation in the landfills considered is about 10%” (pg. 8243). Liptay et 

al. later reiterate this statement and conclude that “Czepiel et al. predicted that methane oxidation in the 

soil cap of a New England landfill varied from 20 to 30% between May and September, the period 

during which our isotopic measurements were made. The seasonal pattern produced as a result of their 

studies yielded an overall value of 10% over the whole year, as oxidation rates are reduced 

considerably at lower temperatures. Our isotopic determinations of methane oxidation in the landfill 

soil cap are fairly consistent with their model-predicted results and confirm the value determined in 

their study.” (pg. 8248). 

• The study does not give reference to a soil cover measurement, but does mention that “at least one area 

of a working landfill is active, which means refuse is being dumped and covered by a minimum of 15-
Review of Oxidation Studies and Associated Cover Depth in the Peer-Reviewed Literature June 2015 

RTI International 11 

cm of soil or synthetic equivalent on a daily basis,” (pg. 8244) indicating a relationship between soil 

cover depth and methane oxidation. 

Stern et al. (2007): “Methane oxidation is controlled by several factors including soil temperature, moisture, 

and texture, as well as pH and nutrient content. Previous studies have shown seasonal variation in CH4

oxidation, which is greater during warmer months. In addition, there appears to be optimum soil moisture for 

CH4 oxidation, 10-20% (w/w) at temperatures from 25 to 30 C...Soil composition is also an important 

parameter, as soil texture and grain size affect oxygen diffusion into landfill cover soils. Coarser grained soils 

and porous mulch have been found to be superior to finer grained soils and clays.” 

• Stern et al. state that “conservative estimates put the average annual percent CH4 oxidation at the Leon 

County MSW landfill from 19% for untreated areas of the landfill to 38% for mulch treated 

areas...Therefore, temperature is an important factor, but complex relationships exist between 

temperature, moisture and CH4 oxidizing capacity of cover materials, and their physical properties” 

(pg. 1255), which supports WM’s cited quote but also makes note of the intricate relationship between 

environmental parameters at a landfill and how they interact with the soil and waste’s chemistry and 

biology. 

5.2.2 In regards to methane oxidation, Stern et al. (2007) note that their data, after and 

including June 29, indicate a mean oxidation of 14% for control cells with only soil cover 

(pg. 5146) which ranged in depth from 40 to 110-cm of cover (pg. 5144). “In fact, many of 

the studies conclude that methane oxidation primarily occurs in the upper part of the 

cover, close to the surface, and several of the studies do not even calculate methane 

oxidation below a depth of 30 to 45 cm (15 to 18 inches).” Pg. 17-18 

Bogner et al. (1997): concluding that there is one group of microorganisms “that is continuously active in 

shallow landfill soils, which thus have the capacity for continuous CH4 oxidation that rapidly responds to a 

wide range of dynamic CH4 concentrations. The only exception would be during times of periodic surface 

saturation.” 

“As shown by previous literature, rates of CH4 oxidation are also dependent on temperature, moisture, and 

other variables. Nevertheless, in a well-aerated landfill soil over short time frames where a wide dynamic 

range of CH4 oxidation rates is possible, the primary controlling variable appears to be CH4 concentration 

and, secondarily, aeration status (O2 availability).” 

“Previous stable carbon isotopic measurements for deep landfill CH4 below the cover materials, suggest that 

CH4 oxidation is also important, at least periodically, in deeper landfill settings.” 

• WM’s selected quotes from the Bogner study are in reference to microbial activity in landfills, and how 

there are generally two distinct groups of microorganisms which influence methane oxidation through 

the landfill cover soil. The third quote cited by WM from the Bogner study notes that methane 

oxidation is important at times in deeper landfill settings. Bogner et al. “postulate that the previously 

described group (high CH4, low O2) [in moderate to deeper landfill settings], or a third group able to 

function very close to a lower limiting value for O2, may also be active at high CH4 concentrations [in 

deep landfills] (pg. 2510). 
Review of Oxidation Studies and Associated Cover Depth in the Peer-Reviewed Literature June 2015 

RTI International 12 

Chanton et al. (2000): “Methane oxidation occurs in the outer rind of the landfill and is subject to 

environmental temperature fluctuations which cause rates to be greater in summer and lesser in winter.” 

• To reiterate from the previous section, this Chanton study reveals a large difference between methane 

oxidation rates during winter (3-5% oxidation) and summer (maximum of 43 ± 10% in July) (pg. 57). 

Chanton et al. conclude that the seasonally integrated value for methane oxidation for areas of the 

landfill covered with mulch/topsoil was 26 ± 4% of the total methane oxidation rate, while for clay soil 

it was only 14 ± 2% with the overall average of both soil types being 20 ± 3% (pg. 57). Soil samples 

were taken only from the top 15-cm of the cover soil, however, the entirety of the final cover for the 

sampled landfill was 109-cm where mulch/topsoil was still in place and 100-cm where mulch/topsoil 

had washed away, which should not be ignored when considering the complex behavior of landfill 

soils. 

Chanton et al. (2008): at Figure 1 (demonstrating that C-13 gets more positive between a depth of 20 and 

60cm, which is indicative of methane oxidation) 

• A review of Figure 1 on page 668 of this Chanton study reveals that the trend mentioned in WM’s 

petition (C-13 getting more positive between a depth of 20 and 60-cm) can be further extended to a 

depth of 80-cm, or in some cases, 100-cm. Total soil covers ranged from 73 to 130-cm (with biocover) 

in Site 1 of the study area and from 15 to 58-cm and 88-cm (shallow and deep biocover, respectively) 

in Site 4. Overall, Chanton et al. found the percent oxidation to be 23± 3% for soil covers. 

Czepiel et al. (1996): “The soils at each location were sampled at 5-cm intervals from the surface to a depth of 

25cm...The distribution of oxidation in the soil column was similar in each profile with maximum occurring in 

the 5 to 10-cm depth interval in seven of the nine profiles (Figure 1).” 

• Soil samples were taken from the surface (25-cm) of the landfill; however it is important to note that 

the entire landfill was covered by 1-2 m of sandy-clay loam cover material (pg. 16721). Czepiel et al. 

(1996) also conclude that “the modeled estimate of annual whole landfill oxidation of 10% is consistent 

with the current low range of oxidation estimate,” however, “...a climate factor should therefore enter 

into any future estimates of landfill CH4 emissions in which cover soil oxidation is considered.” (pg. 

16728). 

De Visscher et al. (1999): “By combining this kinetic data with the soil gas composition data, the actively 

methane oxidizing zone in the soil column could be determined and an in situ assessment of oxygen limitation 

could be performed. Methane oxidation takes place mainly in the top 30 cm of covering soil.” 

• Prior to the above quotation, De Visscher et al. state that “concentration profiles in a landfill cover soil 

are hard to interpret. The gradients that develop are the result of diffusion, reaction, and gas flow. 

Furthermore, the gas flow rate depends on the depth of the column. This can be deduced from the 

stoichiometry of the process...” (pg. 1856). De Visscher et al. specifically note a link between gas 

flow rate and depth. 

Hilger et al. (2000): “The CH4 oxidation potential of soil from the laboratory soil columns is presented by 

horizon in Table 4...The average CH4 oxidation potential measured from the top three horizon of all columns 

[0 to 15 cm] significantly exceeded that of the bottom three horizons [15 to 30 cm] (p<0.001), where O2 was 

more limited (Table 1).” 
Review of Oxidation Studies and Associated Cover Depth in the Peer-Reviewed Literature June 2015 

RTI International 13 

• The Hilger study does not contain any information regarding the soil cover of the landfill from which 

fresh core samples were taken (the Renaissance Park landfill in Charlotte, NC), however the study does 

indicate that the landfill is closed and has since been converted into recreational fields (pg. 458) . 

Kightley et al. (1995): concluding that “our data show that soil microbial community adapts rapidly to the 

presence of elevated CH4 concentrations and can effectively reduce emissions of CH4 from the surface of 

landfill soil. The capacity for CH4 oxidation in soil is related to the depth of O2 penetration and to the nutrient 

status of the soil, which may regulate the extent to which the methanotrophic community can develop.” 

• See the discussion for Kightley et al. (1995) in Section 5.2.1 of this memorandum. 

• With respect to this section and in response to pages 17-18 of the petition stating that “many of the 

studies conclude that methane oxidation primarily occurs in the upper part of the cover, close to the 

surface, and several of the studies do not even calculate methane oxidation below a depth of 30 to 45 

cm (15 to 18 inches).” 

o The soil samples were collected from the top 15-cm of various landfills (minus any turf) and 

were then sieved and packed to a depth of 90-cm. 

o Figure 8 in this article shows rates of methane oxidation potential for the 4 subsamples of 

coarse sand (1 control and 3 amended with different nutrient sources). The control and the 2 

subsamples amended with K2HPO4 and sewage sludge did show a peak in methane oxidation 

potential at a depth of approximately 20-cm. Rates of methane oxidation potential were greater 

than zero at depths below 20-cm, although significantly less than that observed at 20-cm. 

Scheutz and Kjeldsen (2003): “Vertical soil gas profiles showed that both methane and oxygen were often 

present between 20- and 40-cm depths. From 60-cm depth, the soil gas consisted of almost pure LFG—with 

only very low or no oxygen present.” 

• The authors note that “in landfill top covers, methane and oxygen counter-gradients may appear due to 

emission of methane from the waste and diffusion of oxygen from ambient air. Landfill soil covers thus 

often appear with an upper aerobic zone of 20-50cm and a lower anaerobic zone of 20-50cm in a 1m 

top cover, providing living conditions for different bacteria,” (pg. 5143) and state that their study 

consisted of soil retrieved down to 90-cm below the surface of the landfill (pg. 5144). The authors 

additionally note the complexity of methane oxidation and physical gas transport within a column of 

soil (pg. 5146). 

Wilshusen et al. (2004): showing the change in CH4 concentration at various depths and demonstrating that 

gas concentration typically begins to decline (indicating methane oxidation is taking place) at a depth of 

40cm). 

• Wilshusen et al. performed experiments on columns composed solely of compost compromised of 

municipal leaf compost co-composted with manure from a local zoo (pg. 644) which does not represent 

a typical landfill. 
Review of Oxidation Studies and Associated Cover Depth in the Peer-Reviewed Literature June 2015 

RTI International 14 

5.2.3 “This is consistent with Lancaster’s actual oxidation value, which was conservatively 

determined to be an average of greater than 34%, with a low methane flux rate ranging 

from 0.90g CH4/m2/d to 3.96g CH4/m2/d. However, as an active landfill, it is primarily 

under intermediate cover of less than 24 inches and thus, under the Final Rule, would 

have to use the default oxidation fraction of 10%.” Pg. 23 paragraph 2 

Chanton et al. (2011): Table 2, 316 

• This Chanton study provided a wide range of soil covers during experimentation; “generally the covers 

investigated at these landfills were intermediate covers composed of a single 30-122-cm thick layer of 

native soils, most commonly clay. One daily cover was investigated” (pg. 313). 

• Chanton et al. state that “mean values for CH4 oxidation efficiency calculated from chamber-derived 

CH4 varied from 34-41% across the different landfills (pg. 316) and additionally state that their results 

are consistent with a recent literature review which reported a mean value of 35 ± 6% for the fraction of 

CH4 oxidized. It is important to note that the recent literature review mentioned is also another 

Chanton study. The study notes that “[their] data...indicate that landfill CH4 oxidation is, at a 

minimum, 25% for landfills with gas collection systems” (pg. 317). From the expansive range of cover 

depth indicated by Chanton et al., the reader is unable to determine if the 34-41% oxidation resulting 

from the study is indicative of plots evenly represented throughout this range (30-122cm) or weighted 

to one extreme. 

Goldsmith et al. (2012): Table 2 (Location C5), 192 

• The location WM cited in the petition is only one of multiple studies reviewed by Goldsmith et al. 

(2012). The location also has the second lowest flux measurements of all sites studied (pg. 192). 

Goldsmith et al. studied a large variety of landfill covers: open face, daily cover, temporary cover, 

intermediate cover and final cover. “Significant measurements were conducted on intermediate soil 

covers due to ease of availability and the fact that they often constitute the majority of the landfill 

cover” (pg. 194). The authors state that the oxidation capacity of the soil used in landfill covers has 

been found to be greater than 10% (pg. 185) and note in their conclusion that “the western 

Mediterranean landfills, for example, generally used just 30-45-cm of sandy clay soil for intermediate 

cover, as compared to 60-cm loam to clay soils for most of the humid continental sites. Final soil 

covers ranged from 1-2-m in soil thickness at all landfills” (pg. 194). 

6. Conclusions 

• 27 peer-reviewed studies were reviewed previously to promulgate a soil depth requirement of 24 inches 

in Table HH-4 to Subpart HH of Part 98. These studies were included in either or both the SWICS 

2012 Addendum or in RTI 2011 (a literature review of landfill methane oxidation studies). A total of 

61 data points where both the fraction of methane oxidized and soil cover depth are presented across 

these studies, including49 field measurements and 12 column measurements. Table A-1 summarizes 

key data from the peer-reviewed literature across all 27 studies (also presented in SWICS, 2012). 
Review of Oxidation Studies and Associated Cover Depth in the Peer-Reviewed Literature June 2015 

RTI International 15 

• 85% (or 52 of 61) of the data points gathered from these studies where both the methane oxidized and 

cover depth were reported had a cover depth of 24 inches or above. All of the experiments in these 

studies were conducted, or can be assumed, to have been conducted at a landfill or landfill-type 

environment (for column studies) with a cover. 

• 15 of the 27 peer-reviewed studies were referenced by the petitioner to aid in their arguments against 

EPA’s cover requirements in Table HH-4. The portions of these studies used by the petitioner are 

elaborated on in Section 5.2 of this memorandum. Of these 15 studies, 14 were conducted at landfill 

environments and 13 (or 93%) of these included an average cover depth of 24 inches or greater (in the 

original peer-reviewed journal article or in SWICS 2012).

• None of the 27 peer-reviewed studies specifically investigated the optimal cover depth for methane 

oxidation. A conclusion cannot be made that cover depth does not impact methane oxidation because a 

cover material of various depths was included in all measurements. However, several studies measured 

methane and/or oxygen profiles in the surface cover soil and these measurements indicate that the 

majority of oxidation occurs in the top 12 to 15 inches of the cover soil. 

• In areas near passive vent pipes, a significant portion of the methane emissions is expected to be 

released via the vent pipe (as suggested by low methane flux in soil areas near passive vent pipes as 

noted by Liptay et al., 1998 and Chanton and Liptay, 2000), and therefore bypass the surface soil. The 

oxidation factor for the methane released via the passive vent is expected to be 0, so it may be 

appropriate to limit the use of high oxidation fractions for landfills with passive vents. It is important to 

note that the Intergovernmental Panel on Climate Change 2006 Guidelines (IPCC, 2006) recommends 

the use of oxidation fractions ranging from 0 to 10 percent largely due to the fact that landfill gas will 

flow primarily through channels of least flow resistance, which one would expect the passive vents to 

be. If there are fissures in the soil cover (or passive vent systems), a significant portion of the landfill 

gas will be released without any oxidation occurring. More explicitly than the IPCC (IPCC, 2006), the 

EPA Climate Leaders (EPA Climate Leaders, 2004) note that “passive controls may reduce the 

potential for methane to be oxidized.” 

References 

Barlaz et al. 2004. Evaluation of a biologically active cover for mitigation of landfill gas emissions. 

Environmental Science and Technology, 38: 4891-4899. 

Bogner et al. 1997. Kinetics of Methane Oxidation in a Landfill Cover Soil: Temporal Variations, a WholeLandfill Oxidation Experiment, and Modeling of Net CH4 Emissions. Environmental Science and 

Technology, 31: 2504-2514. 

Cabral et al. 2010. Quantifying microbial methane oxidation efficiencies in two experimental landfill biocovers 

using stable isotopes. Water Air and Soil Pollution, 209(1): 157-172. 

Chanton et al. 2008. Improved field methods to quantify methane oxidation in landfill cover materials using 

stable carbon isotopes. Environmental Science and Technology, 42: 665-670. 

Chanton et al. 2011. Landfill Methane Oxidation Across Climate Types in the U.S. Environmental Science and 

Technology, 45: 313-319. 
Review of Oxidation Studies and Associated Cover Depth in the Peer-Reviewed Literature June 2015 

RTI International 16 

Chanton J. and Liptay K. 2000. Seasonal variation in methane oxidation in landfill cover soils as determined by 

an in situ stable isotope technique. Global Biogeochemical Cycles, 14: 51-60. 

Christopherson et al. 2001. Lateral Gas Transport in Soil adjacent to an Old Landfill: Factors Governing 

Emissions and Methane Oxidation. Waste Management and Research, 19: 595-612. 

Czepiel et al. 1996. Quantifying the effect of oxidation on landfill methane emissions. Journal of Geophysical 

Research, 101(D11): 16721-16729. 

De Visscher et al. 1999. Methane Oxidation in Simulated Landfill Cover Soil Environments. Environmental 

Science and Technology, 33: 1854-1859. 

Gebert et al. 2011a. Can soil gas profiles be used to assess microbial CH4 oxidation in landfill covers? Waste 

Management, 31: 987-994. 

Gebert et al. 2011b. Relevance of soil physical properties for the microbial oxidation of methane in landfill 

covers. Soil Biology and Biochemistry, 43: 1759-1767. 

Goldsmith et al. 2012. Methane emissions from 20 landfills across the United States using vertical radial plume 

mapping. Journal of the Air & Waste Management Association, 62: 183-197. 

Hilger et al. 2000. Methane oxidation and microbial exopolymer production in landfill cover soil. Soil Biology 

and Biochemistry, 32: 457-467. 

Huber-Humer et al. 2009. Approaches to assess biocover performance on landfills. Waste Management, 29: 

2092-2104. 

EPA Climate Leaders. 2004. Direct Emissions from Municipal Solid Waste Landfilling, Section 2: Methods for 

Estimating CH4 Emissions. Available at 

http://www.epa.gov/climateleadership/documents/resources/protocol-solid_waste_landfill.pdf. 

IPCC (Inter-Governmental Panel on Climate Change). 2006. 2006 IPCC Guidelines for National Greenhouse 

Gas Inventories, Volume 5: Waste, Chapter 3: Solid Waste Disposal. Prepared by the National Greenhouse 

Gas Inventories Programme, Eggleston H.S., Buendia L., Miwa K., Ngara T. and Tanabe K. (eds). 

Published: IGES, Japan. 

Kightley et al. 1995. Capacity for methane oxidation in landfill cover soils measured in laboratory-scale 

microcosms. Applied and Environmental Microbiology, 61: 592-610. 

Liptay et al. 1998. Use of stable isotopes to determine methane oxidation in landfill cover soils. Journal of 

Geophysical Research, 103: 8243-8250. 

Rachor et al. 2011. Assessment of the methane oxidation capacity of compacted soils intended for use as 

landfill cover materials. Waste Management, 31: 833-842. 

RTI 2011. Updated Research on Methane Oxidation in Landfills. Memorandum prepared by K. Weitz (RTI) for 

R. Schmeltz (EPA, January 14, 2011. 

Scheutz C. and Kjeldsen P. 2003. Capacity for biodegradation of CFCs and HCFCs in a methane oxidative 

counter-gradient laboratory system simulating landfill soil covers. Environmental Science and Technology, 

37: 5143-5149. 

Scheutz et al. 2009. Microbial methane oxidation processes and technologies for mitigation of landfill gas 

emissions. Waste Management and Research, 27: 409-455. 

Schroth et al. 2012. Above- and below-ground methane fluxes and methanotrophic activity in a landfill-cover 

soil. Waste Management, 32: 879-889. 

Spokas, K.A. and Bogner, J.E. 2011. Limits and dynamics of methane oxidation in landfill cover soils. Waste 

Management, 31(5): 823-832. 
Review of Oxidation Studies and Associated Cover Depth in the Peer-Reviewed Literature June 2015 

RTI International 17 

Solid Waste Industry for Climate Solutions (SWICS) Methane Oxidation Addendum 2012, dated November 

19, 2012 

Stern et al. 2007. Use of a biologically active cover to reduce landfill methane emissions and enhance methane 

oxidation. Waste Management, 27: 1248-1258. 

Wilshusen et al. 2004. Long-term behavior of passively aerated compost methanotrophic biofilter columns. 

Waste Management, 24: 643-653.
Review of Oxidation Studies and Associated Cover Depth in the Peer-Reviewed Literature June 2015 

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Appendix
Review of Oxidation Studies and Associated Cover Depth in the Peer-Reviewed Literature 

RTI International Table A-1. Summary of Oxidation Study Data: Cover Material, Study Type, and Cover Depth for Studies Included 

Cover 

Material 

Cover 

Material 

Detailed 

Study Type 

Method 

Oxidation 

Rate (mol m

-

2d1

) 

Std. Error 

Oxidation 

Rate 

Fraction 

Oxidized 

Std. Error 

Fraction Ox 

Gas 

Collection 

Cover Depth 

Comments 

Cover Depth 

(inches) 

LF was Open 

or Closed 

Other Material 

Sandy loam landfill cover 

Field Isotope 

fractionation 

0.456+ 0.2158+ 0.25 0.028 No No gas collection system, but 

measurements were taken at intermediate cover (one 45 cm sandy clay with wellvegetated topsoil, and the 2nd at 15 cm non-vegetated)

17.72 No explicit 

mention, but had intermediate cover 

Clay Sandy clay 

landfill cover 

Field Isotope 

fractionation (field isotope chamber collected) 

0.564+ 0.3420+ 0.14 0.039 No No gas collection system, but 

measurements were taken at intermediate cover (one 45 cm sandy clay with wellvegetated topsoil, and the 2nd at 15 cm non-vegetated)

5.91 No explicit 

mention, but had intermediate cover 

Sand Sandy soil Column Repacked 

column with CH4 advection 

NR NR 0.77 NR NA 

Organic Compost 

landfill cover 

Field Isotope 

fractionation (field isotope chamber collected) 

0.041+ 0.0331+ 0.54 0.136 Yes Biocover; 0.15 m thick clay 

layer over the waste, followed by 0.15 m of tire chips, then 1 m of yard waste compost 

51.18 No explicit 

mention 

Clay Clay landfill 

cover 

Field Isotope 

fractionation (field isotope chamber collected) 

1.216+ 0.8963+ 0.21 0.056 Yes soil cover, clay 39.37 No explicit 

mention 

Other Material 

European landfill covers

Field Isotope 

fractionation 

0.4239+ 1.5460+ 0.84 0.023+ Yes 39.37 

Clay Silty clay 

landfill cover 

Column Undisturbed 

column with 135 to 145 ppmv CH4 diffusion 

0.004+ 0.0005+ NR NR NA 49.21 Closed (fiNAl 

cover 1980s) 
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RTI International 

Cover 

Material 

Cover 

Material 

Detailed 

Study Type 

Method 

Oxidation 

Rate (mol m

-

2d1

) 

Std. Error 

Oxidation 

Rate 

Fraction 

Oxidized 

Std. Error 

Fraction Ox 

Gas 

Collection 

Cover Depth 

Comments 

Cover Depth 

(inches) 

LF was Open 

or Closed 

Clay Silty clay 

landfill cover 

Column Undisturbed 

column with 1 to 8 % CH4 diffusion 

1.156+ 0.2927+ NR NR NA 49.21 Closed (fiNAl 

cover 1980s) 

Sand Sand Scholl, 

CA 

Field Soil gas probe 

isotope method 

NR NR 0.48 0.016 Yes Final cover 30-244 cm 53.94 Closed 

(assumed from fiNAl cover) 

Sand Sand Marina, 

CA 

Field Field isotope 

chamber collected 

NR NR 0.30 0.020 Yes 30-120 cm (fraction oxidized 

not presented by landfill in source); avg cover depth presented here; cover depths ranged from daily, intermediate, to final cover at 2 sites 

29.53 Open (assume

from intermediate cover) 

Sand Sand Scholl, 

CA 

Field Field isotope 

chamber collected 

NR NR 0.52 0.180 Yes 30-275 cm (fraction oxidized 

not presented by landfill in source); avg cover depth presented here; cover depths ranged from daily, intermediate, to final cover at 2 sites 

29.53 Open (assume

from intermediate cover) 

Sand Sand Marina, 

CA 

Field Soil gas probe 

isotope method 

NR NR 0.40 0.032 Yes Intermediate cover 10-50 cm 11.81 Open (assume

from intermediate cover) 

Sand Sand landfill 

cover 

Field Isotope 

fractionation (field isotope chamber collected) 

0.172+ 0.2771+ 0.42 0.095+ No Reported as 0.3 to 0.8; 

average presented here 

21.65 

Other Material 

Sandy loam landfill cover 

Field Isotope 

fractionation 

3.795+ 2.6892+ 0.26 0.048+ Yes See Table Footnote a See cover 

depth comments 
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Cover 

Material 

Cover 

Material 

Detailed 

Study Type 

Method 

Oxidation 

Rate (mol m

-

2d1

) 

Std. Error 

Oxidation 

Rate 

Fraction 

Oxidized 

Std. Error 

Fraction Ox 

Gas 

Collection 

Cover Depth 

Comments 

Cover Depth 

(inches) 

LF was Open 

or Closed 

Organic Wood chips 

and sludge 

Field Plume tracer 

and isotope fractionation (isotopic analysis of whole landfill plume) 

NR NR 0.16 0.031 Yes See Table Footnote a See cover 

depth comments 

Organic Wood chips 

and sludge 

Field Plume tracer 

and isotope fractionation (isotopic analysis of whole landfill plume) 

NR NR 0.25 0.003 Yes See Table Footnote a See cover 

depth comments 

Other Material 

Sewage sludge and mineral soils 

Field Plume tracer 

and isotope fractionation 

NR NR 0.07 0.023 Yes See Table Footnote a See cover 

depth comments 

Other Material 

Sewage sludge and mineral soils 

Field Plume tracer 

and isotope fractionation 

NR NR 0.015 0.011 Yes See Table Footnote a See cover 

depth comments 

Other Material 

Mineral soil Field Plume tracer 

and isotope fractionation 

NR NR 0.40 0.026 Yes See Table Footnote a See cover 

depth comments 

Other Material 

Mineral soil Field Plume tracer 

and isotope fractionation 

NR NR 0.38 0.032 Yes See Table Footnote a See cover 

depth comments 

Other Material 

Mulch and topsoil landfill cover 

Field Isotope 

fractionation 

1.678+ 0.2114+ 0.26 0.040 No 42.91 Closed 
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Cover 

Material 

Cover 

Material 

Detailed 

Study Type 

Method 

Oxidation 

Rate (mol m

-

2d1

) 

Std. Error 

Oxidation 

Rate 

Fraction 

Oxidized 

Std. Error 

Fraction Ox 

Gas 

Collection 

Cover Depth 

Comments 

Cover Depth 

(inches) 

LF was Open 

or Closed 

Clay Clay landfill 

cover 

Field Isotope 

fractionation (field isotope chamber collected) 

13.517+ 1.8114+ 0.14 0.02 No 39.37 Closed 

Organic Compost Field Isotope 

fractionation (field isotope chamber collected) 

NR NR 0.29 0.065 No 43.31 No explicit 

mention 

Other Material 

Loamy landfill cover 

Field Isotope 

fractionation 

NR NR 0.12 0.025+ No 42.13 No explicit 

mention 

Organic Mulch Field Isotope 

fractionation (isotope chamber collected) 

NR NR 0.40 0.033 No 39.37 No explicit 

mention 

Organic Mulch Field Isotope 

fractionation (field isotope chamber collected) 

NR NR -0.01 0.045 No 27.56 No explicit 

mention 

Clay Clay landfill 

cover 

Field Isotope 

fractionation (field isotope chamber collected) 

NR NR 0.08 0.030 No 7.87 No explicit 

mention 

Sand Sand, Kirby 

Canyon CA 

Field Field isotope 

surface air 

0.024 0.0100 0.56 0.24 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Clay Clay, 

Altamount CA

Field Field isotope 

surface air 

0.056 0.0120 0.27 0.08 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 
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Cover 

Material 

Cover 

Material 

Detailed 

Study Type 

Method 

Oxidation 

Rate (mol m

-

2d1

) 

Std. Error 

Oxidation 

Rate 

Fraction 

Oxidized 

Std. Error 

Fraction Ox 

Gas 

Collection 

Cover Depth 

Comments 

Cover Depth 

(inches) 

LF was Open 

or Closed 

Sand Sand, Kirby 

Canyon CA 

Field Field isotope 

surface air 

0.078 0.0000 0.69 0.07 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Other Material 

Sandy clay DADS CO 

Field Field isotope 

surface air 

0.204 0.1370 0.64 0.12 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Clay Clay, 

Clearview MS

Field Field isotope 

surface air 

0.365 0.1480 0.15 0.09 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Clay Clay, Tricities 

CA 

Field Field isotope 

surface air 

0.505 0.1480 0.24 0.12 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Clay Clay, OL 7 KYField Field isotope 

surface air 

0.688 0.2390 0.28 0.12 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Other Material 

Silty clay loam Suburban OH

Field Field isotope 

surface air 

0.712 0.2690 0.06 0.02 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Clay Clay, 

Clearview MS

Field Field isotope 

surface air 

1.079 0.1470 0.14 0.08 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Clay Clay, Pine 

Ridge MS 

Field Field isotope 

surface air 

1.233 0.3830 0.11 0.05 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 
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Cover 

Material 

Cover 

Material 

Detailed 

Study Type 

Method 

Oxidation 

Rate (mol m

-

2d1

) 

Std. Error 

Oxidation 

Rate 

Fraction 

Oxidized 

Std. Error 

Fraction Ox 

Gas 

Collection 

Cover Depth 

Comments 

Cover Depth 

(inches) 

LF was Open 

or Closed 

Clay Clay, Tricities 

CA 

Field Field isotope 

surface air 

1.349 0.0000 0.33 0.04 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Clay Clay, 

Maplewood VA 

Field Field isotope 

surface air 

1.927 0.1500 0.31 0.16 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Clay Clay, OL 7 KYField Field isotope 

surface air 

1.982 0.0000 0.29 0.07 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Clay Clay, Pine 

Ridge MS 

Field Field isotope 

surface air 

2.295 0.5300 0.22 0.01 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Clay Clay, 

Redwood CA 

Field Field isotope 

surface air 

2.431 1.8200 0.51 0.23 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Other Material 

Silty clay loam + composted yard waste OL8 KY 

Field Field isotope 

surface air 

3.540 1.2200 0.59 0.23 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Clay Clay, New 

Spruce MN 

Field Field isotope 

surface air 

4.112 0.0000 0.95 0.03 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Clay Clay, 

Springhill FL 

Field Field isotope 

surface air 

6.274 1.8000 0.20 0.08 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 
Review of Oxidation Studies and Associated Cover Depth in the Peer-Reviewed Literature 

RTI International 

Cover 

Material 

Cover 

Material 

Detailed 

Study Type 

Method 

Oxidation 

Rate (mol m

-

2d1

) 

Std. Error 

Oxidation 

Rate 

Fraction 

Oxidized 

Std. Error 

Fraction Ox 

Gas 

Collection 

Cover Depth 

Comments 

Cover Depth 

(inches) 

LF was Open 

or Closed 

Clay Clay, OL 7 KYField Field isotope 

surface air 

6.512 0.0000 0.53 0.12 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Clay Clay, OL 5 KYField Field isotope 

surface air 

7.47 0.0000 0.43 0.06 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Clay Clay, Metro 

WI 

Field Field isotope 

surface air 

7.7220 2.8950 0.16 0.09 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Other Material 

Sandy loam Atlantic VA 

Field Field isotope 

surface air 

8.329 0.0000 0.34 0.04 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Clay Clay, Spruce 

MN 

Field Field isotope 

surface air 

10.079 0.0000 0.47 0.03 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Clay Clay, OL 7 KYField Field isotope 

surface air 

10.760 5.3000 0.31 0.15 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Other Material 

Silty clay loam Suburban OH

Field Field isotope 

surface air 

11.050 0.0000 0.25 0.04 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Clay Clay, 

Springhill FL 

Field Field isotope 

surface air 

11.100 0.7220 0.29 0.16 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 
Review of Oxidation Studies and Associated Cover Depth in the Peer-Reviewed Literature 

RTI International 

Cover 

Material 

Cover 

Material 

Detailed 

Study Type 

Method 

Oxidation 

Rate (mol m

-

2d1

) 

Std. Error 

Oxidation 

Rate 

Fraction 

Oxidized 

Std. Error 

Fraction Ox 

Gas 

Collection 

Cover Depth 

Comments 

Cover Depth 

(inches) 

LF was Open 

or Closed 

Other Material 

Silty clay loam Central IA 

Field Field isotope 

surface air 

12.340 0.0000 0.82 0.06 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Clay Clay, 

Atascocita TX

Field Field isotope 

surface air 

15.318 4.2800 0.13 0.08 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Clay Clay, OL 5 KYField Field isotope 

surface air 

15.627 3.8900 0.41 0.16 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Clay Clay, 

Maplewood VA 

Field Field isotope 

surface air 

16.700 0.0000 0.41 0.05 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Clay Clay, 

Atascocita TX

Field Field isotope 

surface air 

16.736 7.9200 0.30 0.15 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Clay Clay, OL 7 KYField Field isotope 

surface air 

22.820 0.0000 0.20 0.04 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Clay Clay, Metro 

WI 

Field Field isotope 

surface air 

22.920 8.6000 0.31 0.12 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Other Material 

Silty clay loam + composted yard waste OL8 KY 

Field Field isotope 

surface air 

29.750 0.0000 0.70 0.04 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 
Review of Oxidation Studies and Associated Cover Depth in the Peer-Reviewed Literature 

RTI International 

Cover 

Material 

Cover 

Material 

Detailed 

Study Type 

Method 

Oxidation 

Rate (mol m

-

2d1

) 

Std. Error 

Oxidation 

Rate 

Fraction 

Oxidized 

Std. Error 

Fraction Ox 

Gas 

Collection 

Cover Depth 

Comments 

Cover Depth 

(inches) 

LF was Open 

or Closed 

Clay Clay, Old 

Spruce MN 

Field Field isotope 

surface air 

30.729 8.0800 0.33 0.07 Yes 0.3 - 1.22 m b 29.92 No explicit 

mention 

Sand Sand adjacent 

to landfill 

Field Isotope 

fractionation (field isotope chamber collected) 

0.149+ 0.1372+ 0.28 0.152 NR Reported as "na" in SWICS 

2012 

39.37 Closed 

Sand Sand adjacent 

to landfill 

Field CH4 and CO2 

mass balance 

1.097+ 0.4969+ 0.89 NR NR Reported as "na" in SWICS 

2012 

39.37 Closed 

Other Material 

Sandy-clay loam landfill cover 

Column Incubation and 

model (enclosed sample with CH4 diffusion) 

1.830+ 0.2642+ 0.1 NR No Reported as 1 to 2 m; 

average presented here 

59.06 No explicit 

mention 

Other Material 

Sandy-clay loam landfill cover 

Field Chambers and 

estimated fraction oxidized

0.906+ 0.2506+ NR NR No Reported as 1 to 2 m; 

average presented here 

59.06 

Other Material 

Sandy-clay loam landfill cover 

Field Plume tracer 

and estimated fraction oxidized

1.031+ 0.0586+ NR NR No Reported as 1 to 2 m; 

average presented here 

59.06 No explicit 

mention 

Other Material 

Sandy loam landfill cover 

Column Repacked 

column with CH4 advection 

15.0000 0.2725+ 0.65 0.020+ NA 23.62 Closed and/or 

Abandoned 

Other Material 

Loamy agricultural soil 

Column Repacked 

column with CH4 advection 

8.236+ 0.2725+ 0.61+ 0.020+ NA 23.62 Closed and/or 

Abandoned 
Review of Oxidation Studies and Associated Cover Depth in the Peer-Reviewed Literature 

RTI International 

Cover 

Material 

Cover 

Material 

Detailed 

Study Type 

Method 

Oxidation 

Rate (mol m

-

2d1

) 

Std. Error 

Oxidation 

Rate 

Fraction 

Oxidized 

Std. Error 

Fraction Ox 

Gas 

Collection 

Cover Depth 

Comments 

Cover Depth 

(inches) 

LF was Open 

or Closed 

Other Material 

Sandy loam landfill cover 

Column Repacked 

column with CH4 advection 

3.535 0.0757 0.21 0.005 NA Cores were collected by 

driving a 30 cm long by 5 cm 

wide pipe into the landfill cover surface . . . About 20 cm of soil was recovered from each core. 

12.20 Closed 

Organic Compost Column Repacked 

column with CH4 advection 

9.423+ NA 1 NA NA 23.62 

Other Material 

Loam Column Repacked 

column with CH4 advection 

NR NR 0.42 NR NA 

Clay Clay landfill 

cover 

Column Repacked 

column with CH4 advection 

6.800 0.3000 0.40 0.018 NA 35.43 No explicit 

mention 

Sand Fine sand 

landfill cover 

Column Repacked 

column with CH4 advection 

6.9 0.3 0.41 0.018 NA 35.43 No explicit 

mention 

Sand Coarse sand 

landfill cover 

Column Repacked 

column with CH4 advection 

10.4 0.1 0.61 0.006 NA 35.43 No explicit 

mention 

Other Material 

Landfill "A" cover 

Field Isotope 

fractionation 

NR NR 0.28 0.108 NR No explicit 

mention 

Other Material 

Landfill "B" cover 

Field Isotope 

fractionation 

NR NR 0.19 0.014 NR No explicit 

mention 
Review of Oxidation Studies and Associated Cover Depth in the Peer-Reviewed Literature 

RTI International 

Cover 

Material 

Cover 

Material 

Detailed 

Study Type 

Method 

Oxidation 

Rate (mol m

-

2d1

) 

Std. Error 

Oxidation 

Rate 

Fraction 

Oxidized 

Std. Error 

Fraction Ox 

Gas 

Collection 

Cover Depth 

Comments 

Cover Depth 

(inches) 

LF was Open 

or Closed 

Other Material 

Sandy-clay loam landfill cover 

Field Isotope 

fractionation 

NR NR 0.68 0.093 NR No explicit 

mention 

Other Material 

Rochester landfill cover 

Field Isotope 

fractionation 

NR NR -0.03 

(reported as 0.00 in SWICS 2012) 

0.035 NR No explicit 

mention 

Other Material 

Springfield landfill cover 

Field Isotope 

fractionation 

NR NR 0.01 0.042 NR No explicit 

mention 

Other Material 

Wayland landfill cover 

Field Isotope 

fractionation 

NR NR 0.32 0.053 NR No explicit 

mention 

Sand Sand fractions 

0.24 to 4 mm 

Column nr NR NR 0.52 NR NA 

Sand Loamy sand 

landfill cover 

Column Repacked 

column with CH4 advection of 65 mol m^-2 d^-1 

23.725 1.7299 0.37 0.027 NA NR No explicit 

mention 

Sand Loamy sand 

landfill cover 

Column Repacked 

column with CH4 advection of 4.8 mol m^-2 d^-1 

4.608+ 0.0842+ 0.96+ 0.017+ NA No explicit 

mention 

Other Material 

Loam Column Repacked 

column with CH4 advection 

NR NR 0.81 NR NA Uncontrolled landfill with no 

liners or gas extraction system. Depth ranged from 5-90 cm, with measurements taken every 5 cm 

18.70 Open 

(assumed) 
Review of Oxidation Studies and Associated Cover Depth in the Peer-Reviewed Literature 

RTI International 

Cover 

Material 

Cover 

Material 

Detailed 

Study Type 

Method 

Oxidation 

Rate (mol m

-

2d1

) 

Std. Error 

Oxidation 

Rate 

Fraction 

Oxidized 

Std. Error 

Fraction Ox 

Gas 

Collection 

Cover Depth 

Comments 

Cover Depth 

(inches) 

LF was Open 

or Closed 

Organic Yard-waste 

compost 

Field Isotope 

fractionation (field isotope chamber collected) 

0.108 0.0338 0.38 0.031 No See Footnote c 19.69 Closed 

Other Material 

Fine sandy loam landfill cover 

Field Isotope 

fractionation 

0.141 0.0281 0.19 0.027 No See Footnote c 13.78 Closed 

Sand Sandy soil Column Repacked 

column with CH4 advection 

4.366+ 0.2634+ 0.44+ 0.033+ NA 47.24 

Other Material 

Sand with clay landfill cover 

Column Undisturbed 

column with CH4 diffusion 

3.813 0.2914 NR NR NA 3.94 

Organic Compost Column Repacked 

column with CH4 advection 

6.250 0.6250+ 0.19 0.019+ NA Compost study; not landfill 

environment 

19.69 Not a landfill; 

compost 

 a 6 Swedish landfills (2 closed, 4 open) investigated; fraction oxidized varied with cover type; the info presented here is as summarized in SWICS 2012. 

was 0-30 cm (Cover Soils: Common Characteristics)

b not presented by landfill in source; average cover depth presented c Experiment was set up over waste that had been covered for 8 yrs by 20-60 cm of sandy clay overlain by 20-50 cm of fine sandy loam; 10-cm thick laye

over the entire test cells, followed by a 50-cm thick mulch layer in 1 area. 

OX = oxidation rate NR = not reported NA = not applicable