Brief descriptions of the U.S. Greenhouse Gas Inventory (GHGI) categories are provided below. For further discussion of these terms as used in the GHGI, refer to the respective section in the latest national inventory report (EPA, 2017b).
Abandoned coal mines: Methane emissions released from coal mines after their closure.
Coal mining (active): Methane emissions resulting from activities at underground coal mines and surface mines, as well as post-mining activities. The largest fraction of methane emissions comes from underground coal mines through ventilation and degasification systems.
Enteric fermentation: A natural by-product of microbial fermentation of carbohydrates and amino acids in the rumen and the hindgut of farm animals (e.g., cattle, buffalo, sheep, goats, swine, horses, and mules and asses). These emissions are considered biogenic methane.
Landfills: Methane emissions from the anaerobic decomposition of biodegradable waste fractions (e.g., food waste, garden waste, and paper) in municipal solid waste and industrial landfills. The mass of methane that is subsequently emitted to the atmosphere is a function of site-specific (1) operational practices (engineered biogas recovery and thickness and composition of cover materials) and (2) seasonal climate, which drives cover-specific methane transport and (aerobic) oxidation rates. These emissions are considered biogenic methane.
Manure management: Includes methane emissions from the anaerobic decomposition of manure. The release of methane is largely a function of the type of treatment and storage of manure, with solid systems (e.g., stacked or composted manure or manure directly deposited on pasture, range, and paddock) resulting in less methane than liquid systems (e.g., in lagoons, ponds, tanks, or pits), as well as the chemical characteristics of the manure (e.g., amount of volatile solids and pH), and environmental conditions (e.g., temperature and wind speed). These emissions are considered biogenic methane.
Natural gas systems: Includes fugitive and vented emissions from production (including gathering and boosting stations), processing, transmission, storage, and distribution facilities.
Petroleum systems: Covers emissions from both onshore and offshore operations, including the production of crude petroleum, transportation, and refining operations. Emissions may be a result of fugitive leaks, vents, operational upsets, and/or uncombusted methane.
Rice cultivation: Methane generated from methanogenesis occurring in flooded rice fields. The water management regime as well as soil properties (soil carbon) are primary factors determining the amount of methane generated.
Wastewater treatment and composting: Methane can be formed during wastewater transport, treatment, and post-treatment stages at domestic and industrial operations. Composting of organic waste, such as food waste, garden and park waste, and wastewater treatment sludge can also lead to methane emissions. The amount of methane released is a function of the amount of degradable organic material in the waste. These emissions are considered biogenic methane.