ederal geologists have identified a band of counties stretching from northern Massachusetts to coastal Maine where water in private wells is likely to contain potentially harmful levels of arsenic.

The heightened risk comes from a conjunction of bedrock that contains traces of arsenic, alkaline ground water that is most likely to release it and growing numbers of suburban households using private wells sunk directly into the rock, the scientists report in a study being published in the June edition of Environmental Science and Technology, a journal published by the American Chemical Society.

No studies have linked water consumption in the area with elevated cancer rates. Even so, public health officials in the affected states have recently begun urging well users to test their water.

The highest levels measured in the study were many times lower than concentrations linked to bladder cancers and other ailments in past studies overseas. Still, the researchers estimated that more than 100,000 people in the affected areas were likely to be drinking water exceeding the newly adopted federal standard for arsenic, which in 2006 drops to 10 parts per billion, from the longstanding limit of 50 parts per billion.

The Bush administration accepted the lower limit for public water supplies in 2001, after expert panels concluded that it was justified and that filtration costs were affordable.

The researchers said the potential exposure was likely to increase because of the fast rate of growth of suburban neighborhoods in the region, almost all of which rely on water from private wells.

"These communities are all getting bigger by the day," said Joseph D. Ayotte, the study's lead author and a hydrologist in the New Hampshire office of the United States Geological Survey.

The analysis was done by the Geological Survey.

Private wells are not governed by federal water rules and are subject to few state regulations, leaving it to property owners to test for potential problems. Such wells — of necessity situated directly beneath a particular bit of property — have a greater likelihood of drawing water with elevated arsenic levels because they are often drilled directly into bedrock, the authors said. In contrast, public wells tend to draw on ground water circulating in beds of loose sediment, where arsenic is rarely a problem.

The two counties with the most people with possible exposure were Rockingham, in coastal New Hampshire, and York, the southernmost county in Maine. Both have just the wrong combination of a particular metamorphic rock, high-pH ground water and lots of people drinking water from private wells, the researchers said.

Eastern New England, like western Minnesota, parts of Michigan and some parts of the Southwest, had already been identified as tending to have naturally elevated arsenic levels in groundwater.

The new study more precisely defines the area of risk in New England, experts said, and should help property owners determine whether arsenic testing is necessary.

State health and environmental officials in the region said they had not yet seen the new study, but said they were already working with the federal Environmental Protection Agency to encourage more testing for arsenic and other hazards.