Fracking.

A recent study from Yale University points toward evidence that contamination of drinking water from the Hydraulic Fracturing process is most likely related to surface spills and cracked wells, not migration from gas wells or deep shale layers. At least in Pennsylvania, that is. The study was conducted in the Marcellus Shale region of Pennsylvania, and the researchers make note that the geography of any given area must be taken into account when considering these dynamics. Whether or not this holds true in other regions remains to be seen. Read more about the study here.