This link is to a very good, and informative paper on Hydraulic Fracturing of Natural Gas Wells. It concentrates on the Marcellus Shale region of the Northeast US, but, it covers many general topics about shale gas (including a map of deposits in the US).
The Marcellus shale region which stretches from the Middle of NY State, through much of West Virginia and Pennsylvania, and on into the middle of Ohio is one of the largest natural gas reservoirs in the US. Covering an area of about 95,000 square miles, it is estimated to contain 225 to 500 Trillion cubic feet of natural gas. Getting that gas OUT of shale, however, involves some what are called unconventional methods. Horizontal drilling being one, and hydraulic fracturing of the rock formation (fracking) being the most controversial.
Hydraulic fracturing involves pumping large amounts of water under high pressure into the wells. This causes the rock to fracture, and allows the gas to escape up the well. Of course, it is not just water, that would be too easy. Mixed with the water are various chemicals and additives designed to make the process work better. Also included are "propping agents" designed to keep the fractures open. Many of the concerns about this process revolve around these additives, and their potential to contaminate our drinking water. This paper describes some of those additives. There is currently no requirement for drilling companies to disclose what chemicals they are using (under the guise of being "proprietary"). Fracking has become a common way to improve recovery (or allow recovery) from wells.
The potential is certainly there, and in a widespread area. The studies indicate a need for up to 16 wells for every square mile (640 acres) of the development. Each of these wells will have to be fraced individually. Each well could use from 500,000 to 1,500,000 gallons of fracturing fluid. And it may have to be done on an ongoing basis.
Much of the area in question receives 40 inches of rain a year. This amounts to 731,000,000 gallons of water falling on each square mile. (http://mo.water.usgs.gov/outreach/rain/index.htm). 16 wells could contribute at least 24,000,000 gallons of fracturing fluid to the groundwater. This is 3% of total rainfall. A lot? No worries? Will it escape to the groundwater? Therein lies the debate.
Read the paper. For a view from another perspective, read these :-)
Finally, here is Natural Gas dot org's article on wells.
Time will tell.
More recent articles about fracking: While I have not made up my mind one way or the other, the only information I can find on this process is either about the process itself, or negative.