Burning natural gas for electricity is much cleaner than coal. But there's a problem - leaking methane, a potent greenhouse gas. Nearly two years ago Colorado implemented rules to try to limit methane leaks from natural gas infrastructure. Now the EPA is proposing to model federal rules on Colorado's. Still finding and plugging leaks remains a challenge nationwide. In Pennsylvania, where thousands of gas wells and pipelines are working the Marcellus Shale, researchers are trying to figure out how much is leaking. For our Inside Energy project, The Allegheny Front’s Reid Frazier tagged along.