In Pennsylvania, the closer you live to a well used to hydraulically fracture underground shale for natural gas, the more likely it is that your drinking water is contaminated with methane. This conclusion, in a study published in the Proceedings of the National Academy of Sciences USA in July, is a first step in determining whether fracking in the Marcellus Shale underlying much of Pennsylvania is responsible for tainted drinking water in that region.

Robert Jackson, a chemical engineer at Duke University, found methane in 115 of 141 shallow, residential drinking-water wells. The methane concentration in homes less than one mile from a fracking well was six times higher than the concentration in homes farther away. Isotopes and traces of ethane in the methane indicated that the gas was not created by microorganisms living in groundwater but by heat and pressure thousands of feet down in the Marcellus Shale, which is where companies fracture rock to release gas that rises up a well shaft.

Most groundwater supplies are only a few hundred feet deep, but if the protective metal casing and concrete around a fracking well are leaky, methane can escape into them. The study does not prove that fracking has contaminated specific drinking-water wells, however. “I have no agenda to stop fracking,” Jackson says. He notes that drilling companies often construct wells properly. But by denying even the possibility that some wells may leak, the drilling companies have undermined their own credibility.

The next step in proving whether or not fracking has contaminated specific drinking-water wells would be to figure out whether methane in those wells came from the Marcellus Shale or other deposits. Energy companies claim that the gas can rise naturally from deep formations through rock fissures and that determining a source is therefore problematic. Yet some scientists maintain that chemical analysis of the gas can reveal whether it slowly bubbled up through thousands of feet of rock or zipped up a leaky well. Jackson is now analyzing methane samples in that way.

Another way to link a leaky fracking well to a tainted water well is to show that the earth between them provides pathways for the gas to flow. Leaky wells have to be identified first, however. Anthony Ingraffea, a fracking expert at Cornell University, is combing through the inspection reports for most of the 41,311 gas wells drilled in Pennsylvania since January 2000. Thus far, he says, it appears that “a higher percentage” of Marcellus Shale fracking wells are leaking than conventional oil and gas wells drilled into other formations. Stay tuned.