A study published Tuesday has found that hydraulic fracturing, or fracking, had a "clear impact" on the groundwater in Pavillion, Wyoming, raising new concerns over the drilling method.

"This is a wake-up call," said Dominic DiGiulio, lead author of the Stanford University study, published in Environmental Science and Technology. "It's perfectly legal to inject stimulation fluids into underground drinking water resources. This may be causing widespread impacts on drinking water resources."

Residents of Pavillion, an oil and gas boom town, first began complaining of tainted water in the 1990s after fossil fuel companies began conducting more than 180 drilling operations in the state's Wind River Basin. The Environmental Protection Agency (EPA) released a draft study in 2011 which found that oil and gas activities may have contaminated the town's water supply, but the agency shut down its preliminary research in 2013 after criticism from the industry and the state's fossil fuel regulators.

After the EPA handed over its investigation to Wyoming, state regulators said they had no plans to continue the research, even as the federal Agency for Toxic Substances and Disease Registry advised Pavillion residents not to bathe in, cook with, or drink water from their taps.

Stanford's study picks up where the EPA left off—and goes a step beyond, researchers said.

Fracking involves injecting chemically infused water into the ground at high velocity to release shale gas trapped deep underground. Energy companies often use "proprietary blends that can include potentially dangerous chemicals such as benzene and xylene," the study states. "When the wastewater comes back up after use, it often includes those and a range of potentially dangerous natural chemicals."

Rob Jackson, a co-author of the study, said the decades of drilling in Pavillion "put people at risk."

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"These are not best practices for most drillers," Jackson said. And, he added, "there are no rules that would stop a company from doing this anywhere else."

The study, based on publicly available records and documents obtained through the Freedom of Information Act (FOIA), tracks the occurrence of dangerous fracking chemicals in underground wells as well as their impact on the allegedly potable drinking water that supplies the residents of Pavillion. The impact reaches far beyond the small town of 231 people, the researchers said.

"When you look at everything as a whole, it seems implausible that all this is due to natural conditions," DiGiulio told InsideClimate News. "When you look at the compounds, it's a virtual fingerprint of chemicals used in the field."

"Geologic and groundwater conditions at Pavillion are not unique in the Rocky Mountain region," he said. "This suggests there may be widespread impact to underground sources of drinking water as a result of unconventional oil and gas extraction."

Wenonah Hauter, executive director of the environmental group Food & Water Watch, said the study "echoes what hundreds of other scientific investigations across the nation have already shown: drilling and fracking contaminates drinking water."

"Had the EPA finalized its own study with similar findings on Pavillion years ago, we might have already turned the corner toward a clean energy future," Hauter said. "But sadly the EPA continues to look the other way, much to the delight of the oil and gas industry. It's time for the Obama administration to deal with reality and address contamination from fracking in places like Pavillion, Wyoming; Dimock, Pennsylvania; and Parker County, Texas."