Another house had elevated sodium levels, but Cabot said the EPA cited data from a 2008 test when the residence had in place a water-softening system, which treats water by adding sodium. Cabot said more recent tests showed less sodium than in the public water the EPA started delivering to the four residents last month by truck.
"Based on this re-examination, it appears that EPA selectively chose data on substances it was concerned about in order to reach a result it had predetermined," the company said in a statement posted Tuesday on its website.
Terri White, a spokeswoman for the EPA's regional office in Philadelphia, said the agency was reviewing Cabot's response and "will respond accordingly."
Cabot's statement raises the stakes in a legal and public-relations battle over Dimock, which anti-drilling activists describe as ground zero in their effort to halt shale-gas development and hydraulic fracturing.
The EPA's intervention in Dimock last month cast doubt on the judgment of the Pennsylvania Department of Environmental Protection, which had cited Cabot for contaminating water wells of 19 Dimock homes three years ago but said the company had met the terms of a 2010 settlement and allowed it to halt water deliveries to residents Dec. 1.
Federal regulators at first declared that the water posed no health risk, but some residents engaged in a federal lawsuit against Cabot said their water was still unsafe. The EPA reversed course Jan. 19, saying it would deliver water to four houses and conduct its own water sampling of 61 homes in the rural Susquehanna County town.
"We believe that the information provided to us by the residents deserves further review, and conducting our own sampling will help us fill information gaps," EPA regional administrator Shawn M. Garvin said at the time.
Kate Sinding, a Natural Resources Defense Council lawyer, said Cabot's previous water tests were in dispute, and she endorsed the EPA's decision to conduct its own tests.
Cabot has denied contaminating the wells. "There's been a ton of misinformation out there, and we're trying to straighten the record out," Cabot spokesman George Stark said.
Cabot said the residents whose water showed elevated levels of sodium and manganese declined its offers to install $30,000 water-treatment systems that could have removed most of the contaminants.
Under a settlement with the state, Cabot was required to offer the treatment systems to 19 residents, along with payments up to $399,000, based on twice their houses' assessed values.
For more Marcellus Shale coverage, including an interactive graphic that tracks gas production and waste
by county, visit www.philly.com/deepdrill
Contact staff writer Andrew Maykuth at 215-854-2947, @Maykuth on Twitter, or email@example.com.