The supply of drinking water for parts of Long Island is under threat, according to a new federal report.

The report found that the groundwater in some coastal areas of Nassau County, a major suburb of New York City, is increasingly turning salty. That shift, called saltwater intrusion, is the result of decades of pumping fresh water out of wells for homes and irrigation, creating space for saltwater from the ocean to seep into the underground aquifers once filled with freshwater.

The change could take generations to reverse, even if pumping stopped altogether, according to the report. And it could force coastal areas — including Long Beach, Great Neck and Oyster Bay — to look for new supplies of drinking water, possibly by digging wells further inland, which could put new pressure on those places as well.

Those places “are at that point of the spear,” said Frederick Stumm, a research hydrologist at the U.S. Geological Survey and the report’s lead author. “They’re the most vulnerable communities right now to intrusion.”

The findings in Long Island come as the United States faces a groundwater crisis. In an investigation last year, The New York Times examined data for tens of thousands of wells around the country. In almost half those sites, the amount of groundwater had declined significantly over the past 40 years.