The Associated Press reports that the latest federal study on hydraulic fracturing is corroborating other recent studies showing no evidence that chemicals used in the drilling process for natural gas have contaminated drinking water supplies.

The current study was undertaken by the National Energy Technology Laboratory in Pittsburgh for the Department of Energy. At a drilling site in western Pennsylvania, drilling fluids were tagged with unique markers that allowed scientists to monitor possible percolation up to the shallow regions, near the water table.

The markers were carried by the drilling fluids to depths of more than 8,000 feet below the surface. No traces of the markers could be detected at a “monitoring zone” set up by the study team at a depth of 5,000 feet.

Thus, after a year of monitoring, the researchers are finding that the drilling fluids used in fracking stayed thousands of feet below the shallow areas that supply drinking water, Department of Energy geologist Richard Hammack reports.

While scientists are optimistic about this landmark study and its unique approach to tagging the drilling fluids, they remain cautious about the myriad factors affecting the safety of fracking, most notably industry practices.

“This is good news,” said Duke University scientist Rob Jackson, who was not involved with the study. He called it a “useful and important approach” to monitoring fracking. But he cautioned that the single study does not prove that fracking cannot pollute, since geology and industry practices vary widely in Pennsylvania and across the nation.