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 Dec. 1 after the state said it had met the terms of a 2010 settlement and after 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 that 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 Jan. 5 calling the EPA's knowledge of Dimock "rudimentary."
Anti-drilling protesters demanded a federal intervention last Friday when EPA chief Lisa Jackson visited Philadelphia.
EPA toxicologist Dawn A. Ioven, in a memo posted on the agency's website, said 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.
Cabot said the contaminants were naturally occurring compounds common in many Pennsylvania drinking-water wells.
DEP spokeswoman Katherine Gresh said that "EPA does not seem to have presented any new data here" and that contaminants such as arsenic and manganese could be removed by home water-treatment systems, which had been offered to the residents under the agency's 2010 settlement with Cabot.
Cabot installed the treatment systems in a few Dimock homes, but some of the 11 residents who are involved in the federal lawsuit declined the offer.
Anti-drilling activists Thursday hailed the EPA's "rescue" of the Dimock residents and planned a news event for Friday to greet the first water shipment.
Contact staff writer Andrew Maykuth at 215-854-2947, firstname.lastname@example.org,
or @Maykuth on Twitter.