The United States Geological Survey has published a study of fracking and possible water impacts in the Fayetteville Shale in Arkansas. USGS sampled 127 water wells and found no impact whether the water well was located within 2 miles of gas drilling or more than 2 miles from gas drilling.
www.usgs.gov/newsroom/article.asp?ID=3489#.UO24zHf-J6Y. About 4,000 gas wells are in the general area where gas drilling was done.
The USGS study is noteworthy, because the USGS was able to compare water quality today to water samples taken from the area in 1951 and 1983. The USGS also tested both for frack fluids coming back from depth and methane migrating from gas wells. Again, it found no impacts.
While this study is limited in geography and is at a single point in time, it is good news and makes the study a good fact for the day.