In late 2011, the Environmental Protection Agency published a draft report on an investigation of groundwater contamination near Pavillion, Wyoming, where fracking had jump-started an oil and natural gas field that includes the Wind River Reservation. It's an unusual geologic setting, with little separation between the drinking water aquifer and the rocks being fracked for gas. Add poorly sealed gas wells, the draft report concluded, and you get fracking contamination that appeared to have reached the drinking water aquifer.

Controversy ensued, and the EPA withdrew from the investigation before the report was ever finalized, giving the state of Wyoming control.

One of the EPA scientists leading the investigation, Dominic DiGiulio, subsequently took a job at Stanford University. Along with Stanford colleague Rob Jackson, DiGiulio has tabulated all the EPA data that was sitting in scientific limbo—DiGiulio even went as far as using a Freedom of Information Act request to access EPA data from a couple of water samples that weren’t published.

All that data now has a home in the form of a paper published in Environmental Science & Technology. The end result? The researchers stand by the EPA team’s original conclusions.

During the investigation, the EPA drilled a pair of sampling wells between 200 and 300 meters deep—roughly the depth of the deepest water well in the area and the shallowest fracking activities. They were hoping for insight that couldn’t necessarily be gleaned from sampling shallower water wells alone, where an array of possible contamination sources can confuse things.

As before, the researchers argue that samples from these wells indicate the presence of the chemical-laced water that's pumped down gas wells to hydraulically fracture rock, releasing the gas it holds. That conclusion had drawn intense criticism from the industry, which argued that the EPA samples were likely just picking up chemicals from the mud used to drill and seal their monitoring wells, which combined with naturally occurring petroleum compounds. In this paper, the researchers do some new work to test that hypothesis and conclude that it just doesn’t fit the data, providing the response to criticisms that the EPA never really supplied.

In most areas where fracking has been employed, cases of possible water contamination have been limited to methane bubbling up into wells. Contamination from the fracking fluids themselves is, for the most part, unlikely. In Pavillion, though, the researchers are making the case that fracking fluid is able to enter the drinking water aquifer. (Though that doesn’t necessarily mean it will make it into anyone’s water well.)

Most fracking sites resemble a neat layer cake of rock types, with oil and gas in deeper layers that are tightly sealed off. The geology beneath Pavillion is a little patchier, and the gas is not so deep. So fracking fluid that gets pumped into the gas-bearing rocks need not necessarily stay in the gas-bearing rocks.

That’s especially true because of the way some of these gas wells were constructed. The researchers again dug through the available well documentation, noting that half of them lacked cement seals around the middle section of well pipe. This “intermediate casing” between the bottom and surface sections is now required in some states, but Wyoming is not one of them. In addition to that missing safeguard, at least a few wells have failed tests for leaks in the well pipe.

Put it all together and the researchers say it’s not hard to imagine that some fracking fluid leaked where it wasn’t supposed to.

The other half of the Pavillion story was contamination above ground, from pits used to hold fluids or drilling mud. Prior to the recent fracking revolution, a number of these pits were used without any sort of lining to keep the contents from seeping into the ground. Petroleum-contaminated water from the gas wells went into those pits, as did diesel-fuel-laden drilling mud, so it comes as no surprise that contamination has been found around them.

Some pits have already been through cleanups run by the state of Wyoming, but low-level contamination was found in a few neighboring water wells during recent tests. The researchers write that those results “suggest impact from unlined pits and the immediate need for further investigation including installation of monitoring wells.”

And that’s the thing—while Wyoming is doing some investigation of its own, the EPA’s study was aborted with just two monitoring wells in the ground. The situation in Pavillion is complex, and useful answers aren’t possible without more of the right kind of data in just the right places. So while the researchers are pretty sure that fracking activities have impacted an aquifer that legally qualifies as protected by the Safe Drinking Water Act, there are still a lot of unknowns about what that really means for the people who live there.

A state report on tests of about a dozen drinking water wells concluded that “it is unlikely that hydraulic fracturing fluids (injected into the deeper production zone(s)) have risen to the depths utilized by water-supply wells.” The report also found that more investigation of shallow contamination around those pits was warranted, though.

DiGiulio told the Casper Star-Tribune, “Cumulatively over time, that’s a lot of fluid going into those pits. 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.”

Environmental Science & Technology, 2016. DOI: 10.1021/acs.est.5b04970 (About DOIs).