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The waste industry needs science-­based, field-­validated methodologies to provide realistic emission estimates for annual greenhouse gas (GHG) inventory reporting at national, regional, and site-­specific scales. Predating the majority of field measurement campaigns, the current methodology for landfill CH4 emissions1 has not fundamentally changed over the last 20 years-relying on a first order kinetic model to estimate CH4 generation from the annual mass of landfilled waste, then partitioning the generated CH4 into fractions recovered, oxidized (maximum 10%), and emitted. Field data on landfill CH4 emissions have failed to confirm a robust relationship between the mass of waste-­in-­ place and site-­specific CH4 emissions-­-­thus the current method yields misleading guidance for climate change policy decisions.  Importantly, the current methodology excludes the 3 major drivers for landfill CH4 emissions, now known from literature:

  • Area, thickness, and physical properties of site-­specific cover soils;;
  • Seasonal variability of methanotrophic CH4 oxidation rates in site-­specific cover soils;; and
  • Direct effect of engineered gas recovery on soil gas CH4 profiles in cover soils.

Executive Summary

Like other soil-­based GHG emissions, site-­specific landfill CH4 emissions are highly variable due to soil gas transport and oxidation processes related to the seasonal interaction of local soils with local climate at a specific location on the surface of the earth. Moreover, current field campaigns and modeling in many urban areas, which are attempting to partition seasonal CH4 emissions from multiple, complex anthropogenic and natural CH4 sources, require a more realistic modeling strategy for landfill CH4. Thus it is time to reconsider and replace the current methodology, relying on technical literature and modeling tools now available.

This study focused on the international field validation of a site-­specific annual GHG (greenhouse gas) inventory model for landfill CH4 emissions that incorporates both site-­ specific soil properties and microclimate modeling coupled to 0.5° scale global climate models.  Based on 1-D diffusion, CALMIM (CAlifornia Landfill Methane Inventory Model) is a freely available JAVA tool which models a typical annual cycle for CH4 emissions from site-specific daily, intermediate, and final landfill covers at any landfill site worldwide. CH4 oxidation is scaled to maximum rates based on soil temperature and moisture at 2.5 cm depth increments and 10-­min time-­steps.  In addition to embedded default values for general GHG inventory purposes, CALMIM can accept user-supplied values for critical parameters for more specialized uses including oxidation & emissions research, scheduling of field campaigns to observe seasonal emissions, providing a decision support tool for alternative cover designs, simulation of regional emissions variability, and prediction of future emissions under climate change scenarios.

This new approach, which is compatible with Intergovernmental Panel on Climate Change (IPCC) “Tier 3” criteria, was originally developed and field-­validated for the state of California during the first CALMIM project in 2007-­2010 funded by the California Energy Commission.  That project included model development with independent field validation at two California sites and limited field validation at three additional California sites (see Spokas et al., 2011;; Bogner et al., 2011;; Spokas and Bogner, 2011).

The current project, funded by EREF during 2011-­2013, significantly improved the CALMIM model and internationally field-­validated the revised model for broader U.S. and international applications.  Now compatible with PC, MAC, and UNIX platforms, the updated model (CALMIM version 5.4) contains numerous structural and cosmetic improvements as discussed herein. Direct comparisons between modeled and measured emissions for this project focused on 29 international sites with multiple cover types in North & South America, Europe, Africa, Asia, and Australia.  The base data for the comparisons was derived from published literature and from collaborations with U.S. and international research groups.

We conclude that, using default parameters, CALMIM provides a conservative order-­of-­ magnitude estimate for “typical annual emissions” from site-­specific landfill cover materials which is suitable for inventory purposes.  Importantly, through the use of 30-­ year average climate data, CALMIM replicates the typical annual variability which would be expected for GHG inventory purposes with respect to the site-­specific soils and temperature/moisture-­dependent CH4 oxidation rates.  Thus CALMIM can provide an improved estimate for annual emissions based on the major processes which directly control emissions-namely, the thickness and physical properties of cover materials, the presence of engineered gas extraction, and seasonally-­variable CH4 oxidation rates for each cover.

The use of site-­specific “custom” data for soil gas profiles, annual weather, and other inputs can improve comparisons with field data for more specialized applications, including critical science questions relating landfill CH4 emissions to various operational, design, and climatic considerations (including future climate change). Those questions include:

  • How would an increase or decrease in the existing cover thickness at a specific location affect emission and oxidation rates?
  • What is the relative impact of gas recovery vs. methanotrophic CH4 oxidation with respect to reducing net CH4 emissions to the atmosphere?
  • What design and operational strategies could be employed at specific sites to reduce emissions to negligible values?
  • When should field measurement campaigns be scheduled to quantify typical annual variability in emissions and oxidation?
  • How would CH4 oxidation and emissions change over the longer term for current covers under future climate change scenarios?

As part of the EREF project, in collaboration with Waste Management, Inc. and Purdue University, we also completed a field project at a central Indiana landfill to provide recommendations for developing field-­based “custom” soil gas profiles for CALMIM modeling.

As a final product for this project, we completed a new 2010 GHG inventory for landfill CH4 emissions for the state of California (see Figure 1). This was the first application of CALMIM to a revised regional inventory, enabling direct comparison with the current California Air Resources Board methodology based on the IPCC model with a fixed 10% oxidation.  Although the total state emissions were similar, the regional distribution of emissions for specific sites was very different, primarily due to the regional and seasonal variability of CH4 oxidation.  Unlike the current inventory, where the sites with the largest quantity of waste-­in-­place are the highest emitters, the CALMIM-­based California inventory more realistically relates higher emissions to soil temperatures and moisture conditions which are less optimum for oxidation at seasonally dry, hot, and cold (high elevation) sites.  Representing the largest % of the waste footprint at individual sites, intermediate covers were responsible for >90% of the state emissions. Modeling results and field data indicated that intermediate covers are characterized by significantly lower emission rates for thicker covers.  However, modeling results also suggested that there can be an “optimum” thickness for a specific cover soil and specific soil gas profile at a specific site due to increasing limitations for O2 diffusion in soils thicker than the optimum.  Overall, California cover soils exhibit strong seasonal trends for oxidation over an annual cycle, with temporal variability in % oxidation for intermediate covers over the entire state ranging from <20% to >90%.  The lowest values were associated with late summer/early autumn months which are characterized by hotter, drier soils over much of the state. Detailed comparisons for modeled emissions vs. measured emissions at 10 California sites support recent literature by a number of investigators that the assumed 10% oxidation rate, based on seasonal modeling for one northeastern U.S. site in the mid-­1990’s, needs to be replaced with a site-­specific tool.

In general, CALMIM provides a user-­friendly tool for improving GHG inventories for landfill CH4 emissions consistent with current understanding of the major controls on emissions, addressing research questions related to site-­specific design and operational practices, determining timing of field campaigns to address seasonal variability, and simulating future emissions under climate change scenarios.