Agreed. I own a property where the water table lies under 2 coal seams. The water is super sweet and clean as it filters through all that coal, but it also picks up methane gas.
I was amazed when a neighbor showing me around went to a tap in his yard, turned it on and lit it up! fire and water at the same time! Cool!.
But the point is, methane in ground water is not that unusual. It is separated in a holding tank before using in the house, in case you wondered.