Encana spokesman Doug Hock said there was much to question about the draft study.
The compounds EPA said could be associated with fracking, he said, could have had other origins not related to gas development.
"Those could just have likely been brought about by contamination in their sampling process or construction of their well," Hock said.
The low levels of hydrocarbons found in local water wells likewise haven't been linked to gas development and substances such as methane itself are naturally occurring in the area.
"There are still a lot of questions that need to be answered. This is a probability and it is one we believe is incorrect," Hock said.
Sen. James Inhofe said the study was "not based on sound science but rather on political science."
"Its findings are premature, given that the Agency has not gone through the necessary peer-review process, and there are still serious outstanding questions regarding EPA's data and methodology," the Oklahoma Republican said in a statement.
Wyoming last year became one of the first states to require oil and gas companies to publicly disclose the chemicals used in fracking. Colorado regulators are considering doing the same.
The public and industry representatives packed an 11-hour hearing on the issue in Denver on Monday. They all generally supported the proposal but the sticking point is whether trade secrets would have to be disclosed and how quickly the information would have to be turned over.
And while the EPA emphasized the Wyoming findings we're highly localized, the report is likely to reverberate.
The issue has been highly contentious in New York, where some upstate residents and politicians argue that the gas industry will bring desperately needed jobs while others demand a ban on fracking to protect water supplies. New York regulators haven't issued permits for gas drilling with high-volume hydraulic fracturing in the Marcellus Shale since they began an extensive environmental review in 2008.
Kate Sinding, an attorney with the Natural Resources Defense Council in New York City, said in an e-mail Thursday that the EPA in Wyoming is now recognizing what other experts and families in fracking communities have known for some time: "Fracking poses serious threats to safe drinking water."
David Neslin, director of the Colorado Oil and Gas Conservation Commission, said the Wyoming case is being watched.
"Protection of drinking water in Colorado is of the highest importance to us and therefore we look forward to reviewing EPA's draft analysis, as well as feedback from other parties, including Wyoming, on this matter," he said.
Associated Press writers Colleen Slevin in Denver and Mary Esch in Albany, N.Y. contributed to this report.