The Stanford researchers stop short of linking fracking to contaminated drinking water samples. Contamination found in domestic wells is more likely connected to 44 unlined pits, where production fluids were disposed between the 1960s and 1990s, they say.

“Cumulatively overtime that’s a lot of fluid going into those pits,” DiGiulio said. “If I lived in the Pavillion oilfield, I would be much more concerned about those pits. I would view hydraulic fracturing as a long term potential risk in need of further investigation. But if I actually lived out there, I’d be focused on those pits right now.”

There is evidence, however, that fracking contaminated the aquifer underlying the Pavillion field. The distinction helps explain the difference in Wyoming’s and Stanford’s findings.

Wyoming considered 13 drinking water wells, but did not review the findings from EPA monitoring wells. In all, state investigators examined roughly a third of the well files in their review of the Pavillion field, DiGuilio and Jackson say. The Stanford researchers also examined the remaining two-thirds of the documents.

The history of fracking, well failures and monitoring well results, among other factors, tell the wider story of a contaminated aquifer, they say.