Scientists in Pennsylvania say they've found a clear link between natural gas drilling in the Marcellus Shale region and local groundwater contamination. Samples of drinking water near three houses in Bradford County showed traces of a drilling chemical used in the fracking process, suggesting the compound had leaked from poorly built natural gas wells into household water supplies, the scientists said in an analysis this week.

Despite the small sample size, “This is the first case published with a complete story showing organic compounds attributed to shale gas development found in a homeowner’s well,” Susan Brantley, one of the study’s authors and a geoscientist from Pennsylvania State University, told the New York Times.

Scientists studied water samples collected from household wells in 2012. Bradford County, in northeastern Pennsylvania, is the epicenter of the state's natural gas fracking boom. In 2011, the three homeowners sued drilling company Chesapeake Energy Corp. after they reported traces of natural gas and sediment in their outdoor spigots. That year, Pennsylvania environmental officials cited Chesapeake for violating the state’s Oil and Gas Act and Clean Streams Law by allowing natural gas to enter the wells, though Chesapeake did not admit to wrongdoing. In 2012, the homeowners settled the lawsuit and Chesapeake bought the three houses, the Times noted.

The study, published this week in the Proceedings of the National Academy of Sciences, showed that water in one of those three houses contained 2-Butoxyethanol, or 2BE. The compound is used frequently in oil-and-gas drilling fluid as well as in common household products like latex paints, liquid soaps, cosmetics and herbicides; 2BE is not known to cause cancer in humans, though it has been shown to cause lesions and neurological damage in mice and rats.

The report’s authors said the amount of 2BE found in the drinking water was within federal safety guidelines and did not pose a health risk. They suggested the chemicals likely came from nearby gas wells and migrated laterally through miles of rocks.

Garth Llewellyn, a hydrogeologist with Appalachia Hydrogeologic and Environmental Consulting and the lead author of the report, said the team did not find any traces of 2BE in water wells that were farther away from drilling sites, the Times reported. “When you include all of the lines of evidence, it concludes that that’s the most probable source,” Llewellyn told the newspaper.

Energy in Depth, an advocacy group for the Independent Petroleum Association of America, disputed the Pennsylvania fracking study, saying it has “major research gaps.” Katie Brown, a spokeswoman for the group, said the authors arrived “at a set of conclusions in the end that, based on our reading, don’t quite square with the facts of the case as they demonstrably exist.”

In the study, scientists urged researchers and regulatory agencies to continue studying the potential effects of fracking on local water resources. “More such incidents must be analyzed and data released publicly so that similar problems can be avoided through use of better management practices,” they wrote.

Update: 5/12/15 An initial version of the Bradford County study did not note that one of the authors, Garth Llewellyn, was a consultant for the homeowners who sued Cheseapeake Energy. The report has since been amended to clarify Llewellyn's involvement with the families.