The documentary "Gasland" famously captured images of people near natural gas extraction sites lighting the water from their kitchen faucets on fire.

Although not as dramatic, a new study finds that some homes near natural gas fracking sites in the Marcellus shale region of Pennsylvania have elevated levels of "stray gases" such as methane. The study found traces of methane, ethane and propane in 141 wells, with methane levels six times higher for homes that were located less than one kilometer from a natural gas drilling site.

The study, published June 24 in the Proceedings of the National Academy of Sciences, does not conclusively prove causality, but lead author Robert Jackson told NBC News that this is "strong evidence for gas leaking into drinking water in some cases." Jackson, a professor at Duke University, said "We think the likeliest explanation is leaky wells."

"Fracking" is short for "hydraulic fracturing," a process in which millions of gallons of water and chemicals are injected into a drill site at high pressure to fracture the surrounding shale rock, releasing natural gas. The Marcellus shale formation in Pennsylvania has become heavily developed in recent years by natural gas extraction wells. The fracking process, which employs a long list of undisclosed chemicals, has been linked to a host of environmental concerns.

Energy groups have already tried to discredit the new study, criticizing its methodology and conclusions. Although methane may occur naturally in the soil and water in the region, Jackson says the preponderance of ethane and propane near shale gas wells indicates a problem. "Those are gases that are not generated by microbes," he told NBC. In an interview with USA Today, Jackson said the problem may be linked to the ways in which individual wells are constructed.

Jackson and his co-authors write in their paper that "Future research and greater data disclosure could improve understanding of these issues in several ways. More research is needed across the Marcellus and other shale gas plays where the geological characteristics differ." The also suggest that more data taken before sites are drilled would be helpful, as would a public database disclosing each area's gas compositions on an annual basis.

The most important thing, though, is finding out why some areas become contaminated after natural-gas extraction. "We need to understand why, in some cases, shale gas extraction contaminates groundwater and how to keep it from happening elsewhere," the authors write.

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