Manganese, a metal, exceeded drinking water standards in more than 7 percent of wells tested, and radon exceeded the Environmental Protection Agency’s proposed standards in 33 percent of the wells. Additionally, they found uranium to be a significant predictor of the presence of other forms of radioactivity that can cause health problems.

The study released this week by the US Geological Survey found that arsenic exceeded federal safety standards for public drinking water at 13 percent of about 2,000 sites tested — nearly double the national rate.

In the most extensive study yet of groundwater in New England, federal scientists have found potentially harmful levels of naturally occurring arsenic, uranium, ­radon, and other contaminants in water supplying wells across the region.


The study is the first of its kind to enumerate the array of contaminants affecting well water, the authors said.

Nearly 5,000 wells were tested overall, although not every well was tested for each contaminant.

“This study, like some national studies, confirms that far and away these natural contaminants are the biggest threat to human health,” said Joseph D. Ayotte, chief of groundwater investigations and research for the Geological Survey in New Hampshire. “The results underscore that everyone should test their wells for these and other contaminants. Our work supports advice from state agencies and the EPA.”

The health consequences of ingesting water with elevated levels of arsenic, uranium, and other contaminants depends on the concentrations and how long someone drinks the tainted water. Scientists have long been aware of high concentrations of arsenic in the region’s groundwater, which is produced by the passage of water with low oxygen content through the type of crystalline rocks in much of New England.

The potential health issues include various types of cancer, reproductive and developmental problems, kidney and blood diseases, diabetes, and a weakened immune system.


“The real risk comes from long-term exposure,” said ­Andrew Smith, state toxicologist at the Center for Disease Control in Maine, where about half the population relies on private wells for water, a higher share than any other state.

He said tests by his agency found 1 in 10 wells in Maine have arsenic levels that exceed federal standards, and 1 in 20 have unsafe levels of uranium.

“With long-term exposure, the risks of cancer range from 1 in 100 to 1 in 1,000 people,” he said.

In the study, scientists examined water-quality data from nearly 4,800 public-supply wells sampled by the EPA between 1997 and 2007, as well as 117 private wells sampled by the Geological Survey from 1995 to 2007. The samples included only well water from crystalline rock aquifers found in most of New England and small portions of northern New Jersey and southern New York State.

The study follows a federal report last year that found 13 percent of 478 private wells in Central Massachusetts had arsenic concentrations that exceed drinking water standards.

Ayotte and others noted that while public water supplies are treated to ensure that tap water meets federal requirements, there are no such requirements for private wells, which are used by 2.3 million people in New England. They said all private wells should be tested and treated, noting that all of the contaminants identified in the study can be reduced or eliminated with filtration devices.

“The well-to-well variability of water quality from bedrock aquifers in the region underscores the importance of testing public and private wells individually,” said Sarah Flanagan, a Geological Survey scientist and lead author of the study.


She said the study also revealed, for the first time, excessive levels of naturally occurring fluoride in aquifers in the White Mountain region of northern New Hampshire.

In Massachusetts, state officials said they test and treat all public drinking water systems to ensure contaminants are at safe levels.

But they said homeowners who use private wells should consult the state Department of Environmental Protection’s website to see whether they live in an area at risk for excessive contaminants.

“We have advised private well owners in certain areas to test their drinking water,” said Kenneth Kimmell, the department’s commissioner. Most of the state’s at-risk areas are in Central Massachusetts.

In addition to natural sources of contamination, human activities have affected the quality of the groundwater from crystalline rock aquifers.

The study found sodium and chloride in water sources, both naturally occurring as well as from road salt. Researchers also found elevated nitrates from septic systems or fertilizer; a gas additive known as methyl tert-butyl ether; pesticides; and chloroform, which can come from chlorine in sewerage systems.

The concentrations of those contaminants were not considered dangerous to people, but some, such as chloride, can affect aquatic organisms.

“The study gives us a very comprehensive picture of the potential risks of groundwater in New England,” said Joshua Hamilton, a senior scientist at the Marine Biological Laboratory in Woods Hole. “This is something we should be paying close attention to.”


David Abel can be reached at dabel@globe.com. Follow him on Twitter @davabel.