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Massachusetts is at greater risk than all but five other states from ground water that’s potentially corrosive enough to cause toxic metals in household pipes to leach into drinking water, according to a new report by the US Geological Survey.

As much as 90 percent of the state’s ground water is potentially corrosive, but the dangers are mainly for 534,000 Massachusetts residents who draw their water from private wells, the report found. Unlike public water systems, wells are not subject to state and federal testing and treatment requirements.

“Our findings suggest that people who use private wells for their water supply should really have their water tested at the tap,” said Ken Belitz, the lead author of the report and the agency’s chief of ground water studies. “The more corrosive their water, the more likely they are to have lead in their water.”

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The study comes less than a year after the drinking water crisis in Flint, Mich., which caused lead poisoning among children and required residents to drink bottled water. That was the result of corrosive surface water from the Flint River, a public water supply.

Naturally corrosive water isn’t dangerous to consume on its own. The danger comes when it reacts with pipes and plumbing fixtures in homes. Signs that metals have leached into water include bluish-green stains in sinks, a metallic taste, and small leaks in plumbing fixtures.

The ground water in Massachusetts, as well as much of the Northeast, is more acidic than other parts of the country, in part because of the composition of the region’s rocks and minerals. Its proximity to the ocean, which adds chloride to the rain, also adds to its corrosivity.

Nearly all ground water in the eastern part of the state is considered potentially corrosive, Belitz said.

On Cape Cod, where about 20 percent of homes draw water from private wells, local officials recommend that those residents flush their pipes before drinking the water by letting their faucets run for as much as five minutes, especially after the water hasn’t been used for several hours.

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“I advise folks to collect a gallon of water, after everyone has showered, and leave that in the refrigerator to drink for the day,” said George Heufelder, director of Barnstable County’s Department of Health and Environment. “Given the risks, that’s prudent.”

He advised residents with private wells to switch to the public supply, if possible.

“It’s safer,” he said, although how much safer depends on how well and often the water is tested and treated.

Lead in the water supply can be dangerous to children in even tiny amounts. Four years ago, federal health officials lowered by half the amount of lead in a child’s blood that they said warrants medical attention.

In 2014, nearly 5,000 children in Massachusetts tested positive for elevated lead levels, according to the state Department of Public Health. Lead poisoning can cause learning disabilities, behavioral problems, and even death, public health advocates say.

In Massachusetts, environmental officials recommend that prospective home buyers test well water as a precaution. Testing for lead should be conducted every 10 years, along with annual testing for bacteria and other contaminants.

More frequent tests should be done in heavily developed areas where hazardous chemicals are used, after construction is done on wells, or if there’s a noticeable change in the color or taste of the water, officials say. The tests are best done after a heavy rainstorm, which can expose construction problems and poor filtration of the soil, they say.

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About 44 million people in the United States get their drinking water from private wells, yet many homeowners are unaware of the need for testing, Geological Survey officials said.

Before 1930, lead-coated pipes and fittings were frequently used in homes. But significant amounts of lead were still used through the 1980s as part of the solder connecting pipes and fittings. Some brass and galvanized steel used lead until 2014.

State officials declined to answer questions about ways to address the risks of private wells.

In a statement, Edmund Coletta Jr., a spokesman for the state Department of Environmental Protection, said the department “remains dedicated to addressing corrosiveness in public drinking water sources and ensuring clean and safe drinking water for all Massachusetts residents.”

Environmental advocates said they worry whether the department is up to the job of regulating the water supply.

Since 2000, the department has lost about 35 percent of its staff to budget cuts, and shed nearly 100 employees in the past year alone.

Those concerns are magnified as the department seeks to increase its responsibilities. Earlier this year, department officials sought authority to regulate pollution in state waterways, tasks now performed by the US Environmental Protection Agency.

“Their responsibilities are increasing, the threat to the public is increasing, and their budget is decreasing,” said Jack Clarke, director of public policy for the Massachusetts Audubon Society. “The DEP really needs to beef up its ground water monitoring.”

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