u.s. federal regulations under subtitle d of rcra formed in october 1979 regulate the siting, design, construction, operation, monitoring, and closure of msw landfills. subtitle d now requires controls on the migration of methane in landfill gas. monitoring requirements must be met at landfills during their operation, and for an additional 30 years after. the landfills affected by subtitle d of rcra are required to control gas by establishing a way to check for methane emissions periodically and therefore prevent off-site migration. landfill owners and operators must make sure the concentration of methane gas does not exceed 25% of the lel for methane in the facilities' structures and the lel for methane at the facility boundary.