When I was in Delaware, an electrician was working on a sump pump in a house in a relatively new subdivision. He threw his cigarette into the sump and if flared up. Anyway, the authorities were called in and soon the EPA was there in "moon suits" and all. It seems that the developer buried a lot of organic waste (grubbing stuff) in the yards. The stuff decayed creating methane which followed the gravel base of the sewer lines into the french drains. Everything had to be dug up and hauled. The developer went belly up and the county now has regulations on where waste materials can be buried.