Methane is the largest component of natural gas. Since it evaporates out of drinking water, it is not considered toxic, but in the air it can lead to explosions. When methane is found in water supplies, it can also signal that deeply drilled gas wells are linked with drinking water systems.
In many cases the methane seepage comes from thousands of old abandoned gas wells that riddle Pennsylvania's geology, state inspectors say. But other cases, including several this year and the 2004 disaster that left three people dead, were linked to problems with newly drilled, active natural gas wells.
The issue came to the forefront in January when methane was found in the water at 16 homes in the small town of Dimock [2], in northeastern Pennsylvania. State officials cited Cabot Oil & Gas for several violations they say allowed the gas to seep out of the well structures and into water supplies there. The Department of Environmental Protection asked the company to encase its lower well pipes completely in concrete — a process known in the industry as "cementing" — and assured the public that the contamination in Dimock was rare.
But according to a department spokeswoman, there have been at least 52 separate cases of what the state calls "methane migration" in the past five years.
http://www.propublica.org/article/water-problems-from-drilling-are-more-frequent-than-officials-said-731