Assessing methods to estimate emissions of non-methane organic compounds from landfills

The non-methane organic compound (NMOC) emission rate is used to assess compliance with landfill gas emission regulations by the United States Environmental Protection Agency (USEPA). A recent USEPA Report (EPA/600/R-11/033) employed a ratio method to estimate speciated NMOC emissions (i.e., individual NMOC emissions): speciated NMOC emissions=measured methane (CH4) emission multiplied by the ratio of individual NMOCs concentration relative to CH4 concentration (CNMOCs/CCH4) in the landfill header gas.

The objectives of this study were to (1) evaluate the efficacy of the ratio method in estimating speciated NMOC flux from landfills; (2) determine for what types of landfills the ratio method may be in error and why, using recent field data to quantify the spatial variation of (CNMOCs/CCH4) in landfills; and (3) formulate alternative models for estimating NMOC emissions from landfills for cases in which the ratio method results in biased estimates.

This study focuses on emissions through landfill covers measured with flux chambers and evaluates the utility of the ratio method for estimating NMOC emission through this pathway. Evaluation of the ratio method was performed using CH4 and speciated NMOC concentration and flux data from 2012/2013 field sampling of four landfills, an unpublished landfill study, and literature data from three landfills.

The ratio method worked well for landfills with thin covers (