Philadelphia Enquirer editorial. Published: Mar. 22, 2010.

...Representatives of the Marcellus Shale Coalition, an industry trade group, point out that not a single case of groundwater contamination has been linked to their drilling technique, called hydraulic fracturing or "fracking."

It involves pumping up to three million gallons of water (per well), combined with sand and chemicals, more than a mile underground to shatter the rock and release the gas.

But fracking does carry potential risks to the environment. Those concerns are causing government officials elsewhere to proceed cautiously on Marcellus drilling. New York state has imposed a moratorium on Marcellus wells until it completes an environmental-impact assessment.