The Environmental Protection Agency announced, in a draft report, that an aquifer in Wyoming was found to contain compounds associated with hydraulic fracturing, the process used to get natural gas out of underground shale deposits.
Wenonah Hauter, executive director at advocacy group Food and Water Watch, said the EPA's findings illustrate the risks of rushing ahead with shale development.
"It is also a huge blow to the oil and gas industry, who continues to insist that fracking is safe," she said in a statement. "This latest development exacerbates the industry's existing credibility problem as it tries to sell this risky technology to the American public."
EPA analysis of deep monitoring wells in the Wyoming aquifer found levels of glycols and other synthetic chemicals associated with hydraulic fracturing, or fracking, fluid "well above" standards that are considered safe for drinking water.
In actual drinking water, however, the EPA said those chemicals were "generally below established health and safety standards."
Energy companies and some state regulators state that if done correctly, fracking poses little risk to the environment.
Nevertheless, the EPA said it was "concerned" about the movement of the contaminants from the deep portions of aquifer to drinking water wells.