The EPA opened its inquiry in Dimock, Pa., after residents provided it with private water tests detecting contaminants and complained that state regulators weren't doing enough to investigate the cause.

When an elderly woman's water well exploded on New Year's morning in 2009, Pennsylvania officials discovered pervasive methane contamination in the well water of 18 homes and linked it to bad casing and cementing in gas company wells. In 2010, they took a series of steps against the drilling company involved, citing it for regulatory violations, barring it from new drilling until it proved its wells would not leak and requiring it to temporarily supply water to affected homes.

But residents said state officials hadn't investigated whether the drilling was responsible for the chemicals in their water. The EPA stepped in to find out if residents could trust the water to be safe after the drilling company stopped bringing replacement supplies.

Starting in early 2012, federal officials tested water in more than five dozen homes for pollutants, finding hazardous levels of barium, arsenic and magnesium, all compounds that can occur naturally, and minute amounts of other contaminants, including several known to cause cancer.

Still, the concentration of pollutants was not high enough to exceed safe drinking water standards in most of the homes, the EPA found (in five homes, filtering systems were installed to address concerns). Moreover, none of the contaminants – except methane -- pointed clearly to drilling. The EPA ended its investigation that July.

Critics pointed to the Dimock investigation as a classic example of the EPA being overly aggressive on fracking and then being proven wrong.

Yet, as in Pavillion, the agency concluded its inquiry without following through on the essential question of whether Dimock residents face an ongoing risk from too much methane, which is not considered unsafe to drink, but can produce fumes that lead to explosions.

The EPA also never addressed whether drilling – and perhaps the pressure of fracking – had contributed to moving methane up through cracks in the earth into their water wells.

As drilling has resumed in Dimock, so have reports of ongoing methane leaks. On June 24, the National Academy of Sciences published a report by Duke University researchers that underscored a link between the methane contamination in water in Dimock and across the Marcellus shale, and the gas wells being drilled deep below.

The gas industry maintains that methane is naturally occurring and, according to a response issued by the industry group Energy In Depth after the release of the Duke research, "there's still no evidence of hydraulic fracturing fluids migrating from depth to contaminate aquifers."