From California dreaming to California data: Challenging historic models for landfill CH4 emissions

Authors: Spokas, Kurt; BOGNER, JEAN - University Of Illinois; CORCORAN, MEG - Oak Ridge Institute For Science And Education (ORISE); WALKER, SCOTT - Department Of Resources Recycling And Recovery (CALRECYCLE)

Submitted to: Elementa: Science of the Anthropocene
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/23/2015
Publication Date: 6/16/2015
Publication URL: http://handle.nal.usda.gov/10113/61095
Citation: Spokas, K.A., Bogner, J., Corcoran, M., Walker, S. 2015. From California dreaming to California data: Challenging historic models for landfill CH4 emissions. Elementa: Science of the Anthropocene. DOI: 10.12952/journal.elementa.000051.

Interpretive Summary: This article details the recent evaluation of a seasonal model for the evaluation of methane emissions and compares the result to the current state inventory methodology as well as actual field measurements from 10 sites. This is the first effort at quantifying the impact that soil methane oxidation can have on reducing the magnitude of the resulting greenhouse gas flux (GHG) over an annual cycle. The model results compare very favorably with actual site emission measurements. The differences demonstrate the need to utilize models that account for an entire annual cycle when forecasting GHG emissions from a particular site or operation. These results have profound implications for developing more realistic, science-based urban and regional scale GHG inventories that involve soil microbial reactions. These results are significant to farmers and policy makers and will assist scientists and engineers in developing improved models for predicting net greenhouse gas emissions. These types of annual tools will be critical for improving soil carbon management.

Technical Abstract: Improved quantification of diverse CH4 sources at the urban scale is needed to guide local greenhouse gas (GHG) mitigation strategies in the Anthropocene. Herein, we focus on landfill CH4 emissions in California, challenging the current IPCC methodology which focuses on a climate dependency for landfill CH4 generation but does not explicitly consider site-specific climate or soil dependencies for emissions. Relying on a new comprehensive California landfill database, a field-validated process-based model for landfill CH4 emissions (CALMIM), and field measurements at 10 California sites, we validate the contrary position: no climate dependency for CH4 generation with a strong climate dependency for CH4 emissions. Contrary to the historic IPCC first order model for methanogenesis with kinetic constants related to climate, we demonstrate a robust linear empirical relationship (r2 = 0.85; n=128) between waste mass and biogas recovery [@242 Nm3 biogas hr-1 at 50% CH4 per million Mgwaste-1], with no statistically significant relationships with climate [mean annual temperature (MAT) and mean annual precipitation (MAP)], site age, or status (open/closed). For emissions, the current IPCC methodology does not consider soil or climate drivers for gaseous transport or seasonal methanotrophy in daily, intermediate, and final cover soils, allowing only 10% annual oxidation. On the other hand, we confirm strong climate and soil dependencies for landfill emissions—e.g., average intermediate cover emissions below 20 g CH4 m-2 d-1 when the site’s MAP >500 mm y-1. Cover-specific fractional oxidation can realistically range from negligible to 100%. Thereby, the predicted highest-emitting sites shift from landfills containing the largest mass of waste in the current inventory to sites with large areas of thinner intermediate cover and sites with periodically reduced rates of soil CH4 oxidation during the annual cycle (too hot/dry/cold). These differences have profound implications for developing more realistic, science-based urban and regional scale GHG inventories for landfill CH4 while reducing uncertainties for this important anthropogenic source.