What about the folks near the well’s who can now light their tap water on fire?
http://www.youtube.com/watch?v=qYJj-1jNOxE
We could do that in my neck of the woods forever.
If the water well taps or passes through a water saturated coal seam, it can occur. The process that occurs is this:
The naturally occurring methane is trapped in the pores by the water or is saturated in the water (similar to CO2 in a soda bottle). Pumping the water reduces the pressure and allows methane to be released, and methane and water come to the surface. Whole new natural gas fields have been developed in CO and NM by dewatering coal seams and pumping the gas to the surface.
However, there are cases where improper oil/gas well construction can cause migration into drinking water. I know because I investigated and wrote a professional paper on it in another lifetime. Most of the cases occurred because old, deeper oil/gas wells drilled to deeper formations passed through the gas-containing coal seams and the cement was either porous or not present allowing the coal seam gas to pass upwards along the pipe into the drinking water zone above. This is due to poor well construction back 40-50 years ago and would not be allowed today. It can be resolved by working over the old well or plugging it which may not solve the problem if the gas is already in the drinking water. Then a gas/water separation system must be installed at the well head to remove the methane. Hydraulic fracturing itself is not to blame for these occurrences.