The EPA's intervention is the latest development in its on-again, off-again involvement in Dimock, where the Pennsylvania Department of Environmental Protection blamed drilling by Cabot Oil & Gas Corp. of Houston for contaminating water wells of 19 homes three years ago.
The controversy has pitted neighbor against neighbor, with some residents engaged in a federal lawsuit against Cabot, which denies its drilling contaminated the wells. Relations between state and federal regulators have become increasingly tart over the response.
Cabot halted water deliveries to the homes on Dec. 1 after the state said it had met the terms of a 2010 settlement and federal regulators declared the water posed no health risk.
But EPA reopened its investigation in December after seeing new test results, and then signaled this month it would supply water to households, before backing away from that decision the next day.
Michael L. Krancer, the secretary of Pennsylvania's DEP, sent EPA a letter on Jan. 5 calling the EPA's knowledge of Dimock "rudimentary."
Anti-drilling protesters demanded a federal intervention when EPA Chief Lisa Jackson visited Philadelphia on Jan. 13.
EPA toxicologist Dawn A. Ioven, in a memo posted on the agency's website, said that well-test results from eight homes showed that four "contained contaminants at levels of potential concern."
The well water of one house whose occupants include two toddlers contained arsenic at levels that would pose a long-term cancer risk.
Three other houses contained excessive levels of manganese and sodium.
Tests also found glycol, which is used in antifreeze, at safe levels, and 2-methoxyethanol, a solvent, which does not have an established toxicity level. Those houses are not receiving shipments of water.
Industry supporters said the contaminants were naturally occurring compounds common in many Pennsylvania drinking water wells.
"If this thing wasn't about politics, EPA would be providing drinking water to the tens of thousands of Pennsylvania residents who actually do have legitimate issues with their well water, issues that have nothing to do with oil and natural gas development," said Chris Tucker, a spokesman for Energy in Depth, an industry group.
Actor Mark Ruffalo, who founded a group called Water Defense, said the EPA's move rescues Dimock residents who "have been robbed of their clean drinking water by Pennsylvania's state government in collusion with Cabot Oil and Gas."
Contact staff writer Andrew Maykuth at 215-854-2947, @Maykuth on Twitter or email@example.com.