A deal to sell a controversial central Wyoming natural gas field has fallen apart amidst allegations that drilling there has caused water pollution.

Texas-based Legacy Resources backed out of a $45 million deal to buy the field near Pavillion, Wyom., from EnCana last week, soon after the Environmental Protection Agency said it had detected cancer-causing benzene at 50 times the level safe for humans and other carcinogenic pollutants during its latest round of sampling.

The cancelled sale could signal difficulty for companies trying to turn over aging gas fields if there are environmental or health concerns related to their operations.

“Although Encana retained responsibility for any outcome resulting from the ongoing groundwater investigation undertaken by EPA, due to the continued attention surrounding the investigation, and uncertainty regarding further development, Legacy is not prepared to go forward with the transaction,” said EnCana spokesman Doug Hock, in an email to ProPublica.

Legacy Resources did not respond to a call requesting comment.

Legacy Resources announced it had agreed to buy EnCana’s Pavillion-area wells, which produce an estimated 13 million cubic feet of gas a day, on Nov. 1. At the time, the company also said it planned to drill new wells in Pavillion to tap the 45 billion cubic feet of gas it believes lies underground.

But the prospects for future development have dimmed.

Residents had long complained of widespread water contamination and alleged that fracking was to blame. EnCana had trucked in replacement drinking water to some residents. The company faced increasing controversy when the EPA announced in late 2009 that it had found hydrocarbon contaminants in residents’ drinking water wells. The agency advised residents not to drink their water and to ventilate their homes when they showered or washed dishes. ProPublica began reporting on concerns about water contamination in Pavillion in 2008.

On Nov. 9 the EPA announced more test results from samples taken in Pavillion, this time from two water monitoring wells drilled to 1,000 feet – far below most drinking water wells in the area. It found benzene, along with acetone, toluene, naphthalene and traces of diesel fuel. It also detected a solvent called 2-Butoxyethanol (2-BE) that is commonly used by the drilling industry to fracture wells. It also can be used for cleanup at well sites.

EnCana has maintained that the pollutants found in Pavillion-area wells occur naturally, and that drilling is not to blame. “Nothing EPA presented suggests anything has changed since August of last year – the science remains inconclusive in terms of data, impact, and source,” Hock wrote to ProPublica.

Hock said that the EPA’s monitoring wells were drilled into a zone known to contain methane gas, and suggested the pollutants would have been expected to be there. He said that the 2-BE was only detected in one sample and could have leached from the plastics used to drill many drinking water and monitoring wells. In previous statements to ProPublica, he has said that the 2-BE might have come from household cleaning agents, which can contain the chemical. Hock did not reply to questions about whether EnCana had used 2-BE in fracking or any other processes in Pavillion.

The EPA’s latest findings are consistent with previous samples taken from water wells at 42 homes in the area since 2008.

The agency has so far been careful not to draw conclusions about the cause of the pollution. EPA officials had said they planned to release a detailed report analyzing possible causes of the pollution by the end of November, but now say it will be at least a few more weeks.