Background

The process of methane oxidation reduces the emissions of methane and other volatile hydrocarbons from the surface of landfills. Quantifying methane oxidation is one of the major uncertainties in estimating national or global methane emissions from landfills. Landfill gas (LFG) that is not collected or vented passes through landfill cover soils prior to being released to the environment. Bacteria near the landfill’s surface consume methane and other volatile hydrocarbons produced by the decomposing waste by reacting it with oxygen. These bacteria harness the energy from these enzyme-catalyzed chemical reactions to fuel their respiration. A portion of the methane also is incorporated into the biomass of the microbial cells.

The current default value set by the Intergovernmental Panel on Climate Change (IPCC) and the U.S. Environmental Protection Agency (EPA) for landfill cover methane oxidation is relatively low — between 0 and 10 percent of emitted methane (set by the IPCC in 2006 and EPA in 2004). This value was based on seasonal results for a New Hampshire landfill as determined by studies published in 1996 by Czepiel et al. The 10-percent value was proposed at an IPCC workshop in Washington in 1995. At an international seminar in Chicago in 1997 it was agreed to use 10 percent as a standard value. The results of the comprehensive New Hampshire studies in New Hampshire were just being made available at that time.

To our knowledge, the EPA published the earliest government document making reference to a 10-percent value for landfill methane oxidation in 1998. In this document, the EPA cites the aforementioned New Hampshire studies, which included seasonally-averaged annual values of 10-percent methane oxidation. The 10-percent value for this landfillwas subsequently confirmed using air plume studies by Chanton et al. in 1999.

A 2004 EPA report stated that “average oxidation of methane (on a volumetric basis) in some laboratory and case studies on landfill covers have indicated ranges from 10 percent to over 25 percent with the lower portion of the range being found in clay soils and higher in topsoils.” Due to this general uncertainty and the lack of a standard method to determine oxidation rate, the EPA recommended the default factor of 10 percent by volume methane oxidation for landfill cover soils.

A value of 0 to 10 percent oxidation also was recommended by the IPCC in 2006 guidelines for national greenhouse gas inventories: “The use of an oxidation value higher than 0.1, should be clearly documented, referenced, and supported by data relevant to national circumstances.”

We reviewed the literature and compiled methane oxidation results from 42 determinations of the fraction of methane oxidized in landfill cover soils and reported that the average measured methane oxidation of all of the reviewed literature was 36 ± 6 percent. Roughly half of the methane oxidation results were from laboratory column studies and half were from field studies. If one looks only at the field studies results, the average measured percent oxidation is 35 ± 6 percent.

Our team also confirmed that, in addition to climate and soil type, the methane load on the soil cover from the waste below has a significant influence on oxidation. They reported that below a certain methane loading rate oxidation can be 100 percent. In this region, the soil cover can oxidize all of the methane coming from below. Above this loading rate, the soil is not able to oxidize all of the incoming methane.

The EPA does not recognize these values and has not revised its 10-percent value for landfill cover oxidation. It argues, “The percent of methane oxidized at the landfill surface is highly dependent on the velocity of gas flow. While areas of low flow are expected to have significant oxidation, areas of high flow will have little to no oxidation. Landfill gas will generally flow to the surface in fissures and channels that offer the least resistance to flow. These high volume flows will not have significant oxidation.”

In a sense, the EPA’s criticism is based on differences between the primary porosity and the secondary porosity (primary porosity refers to gas or liquid flow through the regular soil matrix, while secondary porosity refers to the flow through defects and cracks in the soil) of gas flow through landfill cover soil. The criticism is that measurements of methane oxidation, such as the FSU data, captured the flow of gas via primary porosity only, while the portion of the emissions dominated by secondary porosity (e.g., via plumbing leaks and cracks) is not oxidized. Our goal was to use a measurement and modeling approach to evaluate the validity of this criticism. We performed and evaluated isotopic measurements that included methane emissions from both the primary and secondary porosities.

The objectives of our project were to:

1. Use our extensive data of landfill ambient methane concentrations, collected at the landfill surface, and determine the percent oxidation of the ambient methane since it represents the total emitted methane (from both soil matrix and soil defects and cracks).
2. Conduct isotope measurements on all of the landfill’s emitted methane.
3. Introduce into our soil physics model (as designed by FSU) geo-statistical techniques to reproduce soil cracks in landfill covers and assess the effects of this secondary porosity (cracks) on gas transport and oxidation.