The head of Southern California Gas Co said it would take at least three more months to plug a massive underground leak of natural gas that has been seeping into the air since mid-October and now accounts for a quarter of the state's entire methane emissions. The utility's president and chief executive officer, Dennis Arriola, also said on Tuesday the company would begin this week drilling a relief well designed to intersect the damaged pipeline hundreds of feet beneath the surface and inject it with fluids and cement.