Constraining the Effects of Secondary Porosity on CH4 Oxidation
Investigators: Florida State University
Project Background & Objectives
Recently, Chanton et al. (2009) reviewed the literature and compiled methane oxidation results for 42 determinations of the fraction of methane oxidized following and including Czepiel’s landmark (10% oxidation) study and reported a mean value of 36 +/- 6% for this parameter. Roughly half of these values were from lab column studies and half were from field studies. Fifteen seasonal field studies ranging from latitude 30o to 55o N yielded a similar value of 35 +/- 6%. In the US regulatory environment, the EPA prefers not to recognize these values and does not wish to revise the 10% value for landfill cover oxidation. To justify their decision, they point to a limited number of studies performed for the most part at single landfill that had a clay cover of only 10 to 15 cm, and no gas collection system. They note that because of its clay cover, this particular landfill is a more suitable representative of US landfills than the bulk of the determinations referenced by Chanton et al. (2009). They further argue, “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 private, EPA administrator further suggest that some portion of emissions come from leaks from aboveground structures associated with gas recovery and that this leakage would not be affected by methanotrophs either. In a sense, this criticism is based on differences between the primary porosity and the secondary porosity of gas flow through the landfill soil. The criticism is that most measurements of methane oxidation have to date captured the flow of gas via the primary porosity only, while the portion of the emissions dominated by the secondary porosity via plumbing leaks and soil cracks is not oxidized. Our goal is to use a measurement and modeling approach to evaluate this criticism. We will perform and evaluate isotopic measurements that include methane emission from both the primary and secondary porosity.