The ranking Republican on the Senate Environment and Public Works Committee is asking the head of the U.S. Environmental Protection Agency for more information about an EPA investigation into groundwater contamination in a Wyoming gas field.

Oklahoma Sen. James Inhofe asked EPA Administrator Lisa Jackson in a letter Tuesday to explain a recent comment she made about the contamination in the Pavillion area in central Wyoming. Jackson told a Bloomberg news program last month that a petroleum industry practice called hydraulic fracturing could have affected nearby areas containing groundwater.

"This statement appears to contradict statements by you and other members of the Federal Government about hydraulic fracturing and drinking water contamination," Inhofe wrote.

Hydraulic fracturing involves pumping pressurized water, sand and chemicals underground to open fissures and improve the flow of oil or gas. Environmentalists say "fracking" is a threat to groundwater, but the petroleum industry says there isn't a single documented case of fracking contaminating groundwater.

After several decades of widespread fracking in Wyoming, Oklahoma and other oil- and gas-producing states, no widely accepted, definitive cases of groundwater pollution due to fracking have been put forth by the EPA or anybody else.

Inhofe asked Jackson for the basis of her comment. "Because of these contradictory statements, I am concerned that EPA has pre-determined that hydraulic fracturing is the cause of contamination in their Pavillion investigation and the Agency is trying to make the data conform to that conclusion, instead of engaging in an open scientific inquiry," he wrote.

An EPA spokeswoman said the agency was reviewing the letter, which followed an EPA report last month that documented high levels of benzene and other pollutants in two wells drilled to check for pollution.

Other EPA studies in the Pavillion gas field over the past two years have documented hydrocarbon pollution in 17 local water wells. Some Pavillion residents say their water reeks of pollutants and has made them ill. Health officials have advised them not to drink their well water and to ventilate their bathrooms while showering.

The EPA has been planning to follow up the November report with one that gets into possible sources of the pollution. That report is due any day now.

"EPA scientists are continuing to complete their analysis of those data," said the EPA spokeswoman, Betsaida Alcantara, by email. "We are working hard to complete a report interpreting the findings very soon."

Inhofe spokesman Matt Dempsey said the senator wrote the letter because he "is the leading advocate for hydraulic fracturing in the United States Senate and has had concerns about the Obama administration's war on natural gas. And so he therefore takes his oversight responsibility seriously and that's why he's been looking closely at the actions of the EPA in Wyoming."

Another member of the Environment and Public Works Committee, Sen. John Barrasso, R-Wyo., also has had an eye on the Pavillion contamination and what Jackson has said about it. His primary concern "is that the people of Pavillion have clean drinking water," spokeswoman Laura Mengelkamp said by email. "They deserve to have answers about what's in their water. He also believes we need to identify the source of the problem. Whoever or whatever is responsible for this problem, folks there need to have answers."

Wyoming became one of the first states to require petroleum companies or their contractors to disclose the ingredients in their specially formulated fracking fluids.

Drilling has been going on in the Pavillion area for more than 40 years. One environmentalist said pinpointing fracking as the cause of contamination there has been difficult in part because petroleum companies in years past didn't publicly disclose the chemicals they pumped underground.

"We're forced to try to prove a negative here. Fluids that have been discovered and the chemicals and compounds that have been discovered in Pavillion certainly point to natural gas production," said Richard Garrett with the Wyoming Outdoor Council. "It's up to industry to tell us historically what they've used in Pavillion and help the EPA reach the correct conclusion."

Calgary, Alberta-based Encana owns the Pavillion gas field. Midland, Texas-based Legacy Reserves announced this fall it was buying the gas wells, pipelines and other assets from Encana for $45 million, but the deal fell through two weeks ago. Legacy expressed reservations about buying the gas field amid the ongoing EPA investigations.