Methane, Nitrous Oxide, and Trace Volatile Organic Compound Emissions and Gas Collection System Efficiencies in California Landfills
The California Air Resources Board (CARB) and CalRecycle contracted with researchers at California Polytechnic State University, San Luis Obispo (Cal Poly) to perform a detailed assessment of greenhouse gases (GHG) (methane, nitrous oxide, and carbon dioxide), carbon monoxide, and 78 non-methane volatile organic compound (NMVOC) emissions from landfills in California. In extensive field testing, the researchers determined methane emissions using aerial measurements at 16 selected landfills and measured emissions of all 82 GHGs and NMVOCs using ground-based static flux chambers at a subset of five landfills. Researchers used more than 65,000 individual gas concentration measurements in the ground-based tests to determine the surface flux from 31 individual cover types over California’s wet and dry seasons.
Researchers determined that flux and emissions of methane, nitrous oxide, and NMVOCs are highly variable both within a given landfill and between landfills: the variations of positive flux of a given chemical at a given landfill was up to 6 orders of magnitude, and up to 7 orders of magnitude between landfills. Seasonal flux variations for a given site were low and generally were within one order of magnitude between the dry and wet seasons. The minimum and maximum measured methane fluxes were -3.73 to 96.2 g/m2-day. The fluxes for nitrous oxide and trace NMVOCs ranged from -0.00410 to 0.145, and -0.00193 to 1.81 g/m2-day, respectively.
Analyses included examining correlation of flux to various landfill characteristics and operational conditions. Researchers identified cover characteristics as the main factor that controlled surface flux. The fluxes generally decreased with the order daily, intermediate, and final covers; high to low permeability covers; and thin to thick covers. For daily covers, locations with autofluff or green wastes had the highest surface fluxes. For intermediate covers, fluxes were generally higher when green wastes were layered or mixed with soils than when soils were used alone for intermediate covers. For final covers, soil conventional covers were more effective for impeding methane flux compared to the alternative cover. The report includes specific guidelines to provide thresholds for landfill operations based on geotechnical characteristics of soil covers, such as using long-term cover thickness of at least 150 cm (methane) and 75 cm (NMVOCs), and using soil with at least 60 percent fine content and 12 percent clay content for long-term covers.
Researchers observed differences between aerial and ground measurements, which may be attributable to emissions from the active waste placement surface at the landfills (not measured in the ground tests) and the high uncertainties in the aerial measurements. Researchers also found that collection efficiency at a given landfill may be higher or lower than previously estimated. To calculate landfill gas collection efficiencies, researchers compared multiple approaches to determine total methane generation, i.e., collection efficiency was calculated using the quantity of gas collected divided by measured generation (as the sum of gas collected and ground- or aerial-measured emissions) versus modeled generation. Collection efficiencies at the five landfills ranged between 23.2 and 91.4 percent using aerial measurements; between 38.9 and 100 percent using ground-based measurements; and between 24.5 and 75.9 percent using the U.S. Environmental Protection Agency’s Landfill Gas Emissions Model. Due to large uncertainty in modeling gas generation, researchers advised that the use of collection efficiency as a measure of emissions may not be reliable.
On March 4, 2021, Cal Poly researchers presented these findings at a CARB research webinar with more than 350 attendees.
The documents below provide the complete report as well as individual sections of the report to allow for ease of downloading. For questions, including available data, contact LMR staff.