Poor well construction, the proximity of fracked wells to drinking water sources and the prevalence of unlined disposal pits, where diesel-oil based drilling muds and other production fluids were stored for decades, bolsters EPA’s initial contention that natural gas operations were responsible for a polluted aquifer east of Pavillion, they say.

The researchers poured over more than 1,000 well files, including drilling and completion reports, regulatory actions and cement bond logs. The files provide a link between the chemicals listed in frack treatment reports and compounds later discovered in two EPA monitoring wells, they say.

“We documented impact to a water resource as a result of hydraulic fracturing for the first time,” DiGiulio said in an interview Monday.

The researchers say the findings support calls to limit fracking at shallower depths where well stimulations are more likely to contaminate drinking water supplies.

Unlike many unconventional oil and gas plays, where fracking is conducted at deep intervals underground, Pavillion’s gas wells were drilled to the same depth as nearby water wells.

“No state has any restrictions on how shallowly you can frack a well,” Jackson said. “That needs to change.”

The report comes as Wyoming regulators contemplate the next step in their own investigation into the Pavillion field and amid a growing barrage of criticism over the state’s inquiry. A draft study released by the Department of Environmental Quality in December found little evidence of oil and gas pollution in water samples taken at 13 domestic water wells.

But the EPA, in a review of Wyoming’s investigation released earlier this month, said the state lacked the data to support its claim that much of the pollution was naturally occurring and not attributable to gas production. Wyoming officials have called for additional study, but have yet to provide specific details on what that work would entail.

A spokesman for Encana Corp., the operator of the Pavillion Field, said numerous rounds of testing by state and federal regulators have produced “no evidence that the water quality in domestic wells in the Pavillion Field has changed as a result of oil and gas operations; no oil and gas constituents were found to exceed drinking water standards in any samples taken.”

That DiGiulio and Jackson’s study was released under the banner of Stanford University and published in the peer-reviewed journal Environmental Science and Technology is telling in its own right.

The draft Pavillion report DiGiulio authored for the EPA in 2011 created a national firestorm, attracting criticism from Congressional Republicans, Wyoming officials and industry representatives, who said the study was based on shoddy science.

EPA dropped its investigation in 2013. DiGiulio, who worked at the agency for 31 years and served as lead investigator in Pavillion, retired the next year. He has been a visiting scholar at Stanford ever since.

A Star-Tribune investigation later showed EPA brass worried it could not successfully defend the study and chose to abandon it.

Many of the criticisms directed at EPA over the years went unaddressed by the agency, enabling misconceptions about the investigation to linger, DiGiulio said.

“When I retired the technical story wasn’t fully told,” he said.”I think it’s important for someone to sort through all the data and draw conclusions. And unfortunately that had not be done. That’s what motivated me to try and review everything, including comments from industry.”

DiGulio and Jackson analysis offer a competing narrative to the criticism directed at EPA by state regulators and industry representatives.

Industry critics once argued samples from EPA’s groundwater monitoring wells should be discounted because of faulty construction. But the compounds found in those monitoring wells are more commonly associated with fracking – not the cements used to encase a well, DiGiulio and Jackson say.

State regulators have said nearby natural gas wells were properly constructed. DiGiulio and Jackson documented five cases where wells failed after being fracked. The failures raise the possibility that fracking fluid could escape a well.

And water samples taken from EPA’s monitoring wells more closely resembled produced water from gas operations than drinking water found throughout the Wind River Formation in Fremont County, belying the claim that the contamination was naturally occurring.

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 the difference in Wyoming’s and Stanford’s findings.

Wyoming considered 13 drinking water wells, but did not review the findings from EPA’s 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.

“We’ve shown clear evidence of contamination to the aquifer itself,” Jackson said. “Contamination to domestic water wells may have happened. We don’t know. If you continue to do this, if you continue to allow this, you will have more problems, even with domestic water wells in the future.”

And that points to what the researchers say is the wider problem. Frack jobs in Pavillion were often completed at depths ranging from 750 to 1,050 feet, or in close proximity to water wells. Unlike many horizontal wells, which can travel to depths beyond 15,000 feet, no layer of rock exists to keep frack fluid and water separate, increasing the risk of contamination overtime.

“The paper documents issues with well integrity at Pavillion,” Jacksons said. “You don’t have to have a problem with well integrity though, when the hydraulic fracturing is within a few hundred feet of domestic water wells, and when your surface casing is to shallow and you don’t have cement in others.”