From Ron Bailey:
Just as for conventional wells, it is possible that natural gas can escape into aquifers if the wells are not properly sealed using steel and cement casings. A new study in the Proceedings of the National Academy of Sciences published today finds elevated levels of natural gas in groundwater wells within 3,000 feet of active gas well sites. The researchers conclude that the source is likely leaky casings.Michael Giberson has more comments over at Knowledge Problem. For more on natural gas, see Matt Ridley's recent overview of the shale gas shock [PDF].
However, the study more reassuringly “found no evidence for contamination of the shallow wells near active drilling sites from deep brines and/or fracturing fluids.” In any case, should their findings stand up to subsequent research, the problem is not fracking, but improperly sealed well-casings. It should be noted that the wells were not tested for methane before gas drilling began. It would be interesting to repeat the study looking at conventional gas wells.