Untreated groundwater in Maryland and almost a dozen other states carries a “very high” risk of being so corrosive that it could contaminate drinking water, according to the U.S. Geological Survey.

Officials warn that the groundwater being pumped to homes through wells could leach lead from pipes. About 1 million people in Maryland rely on wells for their water — about one in six state residents.

While water that flows through county or municipal treatment plants or pipes is subject to lead testing, water from wells is not.

The review is the first nationwide examination of the corrosivity of groundwater, prompted by the lead contamination of drinking water in Flint, Mich., the study’s lead author said.

The study found the highest risk of corrosive water along the Atlantic coastal plain, said Kenneth Belitz, chief of groundwater assessment for the geological survey’s National Water Quality Assessment Program. That plain includes Maryland’s Eastern Shore and parts of the western shore that are close to the Chesapeake Bay, to the south and east of Interstate 95.

In addition to Maryland, researchers found the highest risks of corrosive groundwater in Maine, New Hampshire, Rhode Island, Connecticut, New Jersey, the District of Columbia, Delaware, South Carolina, Georgia and Alabama.

Corrosive water could cause lead to leach into water from lead pipes or fittings used in homes built before 1930, lead solder used as recently as the late 1980s, and some brass and steel plumbing components, according to the study.

Maryland Department of the Environment officials declined to comment on the findings Wednesday, saying they had not yet had time to thoroughly review the report.

Surveys indicate that many Americans who drink from wells aren’t aware the water should be tested. About 44 million people across the country get their water from wells.

Researchers emphasized that there are many other factors that affect tap-water quality, but urged residents to check for lead.

“It is an essential factor that should be carefully considered in testing for water quality in both public and private supplies nationwide,” Stephen Moulton II, chief of the USGS National Water-Quality Assessment Program, said in a statement.

The federal government does not regulate private wells for water quality. In Maryland, the state Department of the Environment requires permits to dig or install wells, but there is no statewide water-testing mandate.

Residents in Anne Arundel and Washington counties must have water tested to obtain certificates of potability after wells are dug, but after that, there is no continuing requirement for water-quality testing, according to state officials.

The National Ground Water Association urged well users to have their water tested and emphasized that no level of lead concentration is safe.

If lead is found, residents should find and remove sources of lead or have their water treated before drinking it.

The Maryland Geological Survey conducted a study of lead levels in Harford County wells in 2013 and found unsafe levels of the toxic heavy metal in 7.5 percent of “first draw” samples, taken after water was left sitting in pipes overnight. Samples taken after leaving faucets running for 30 seconds contained levels below an Environmental Protection Agency threshold of concern.

In Flint, municipal water became contaminated with lead when officials switched the city’s supply from Lake Huron and the Detroit River to the Flint River, which contains corrosive water.

— Baltimore Sun