PITTSBURGH (AP) — Hydraulic fracturing for natural gas hasn’t contaminated drinking water wells in Arkansas, according to a new study, but researchers said the geology there may be more of a natural barrier to pollution than in other areas where shale gas drilling takes place.

The most passionate critics and supporters of hydraulic fracturing, or fracking, often describe the process in extremes, suggesting it is either inherently dangerous for the environment or that it poses virtually no risk at all. But Avner Vengosh, a Duke University professor of geochemistry and water quality, said making generalizations about fracking in Arkansas, Pennsylvania and Colorado doesn’t make scientific sense.