Approximately 71 to 95 million people in the Lower 48 states—more than 20% of the country's population—may rely on groundwater that contains detectable concentrations of per- and polyfluoroalkyl substances, also known as PFAS, for their drinking water supplies. These findings are according to a U.S Geological Survey study published in the journal Science.

The predictive model results can help members of the public, water suppliers and regulators understand the potential for PFAS contamination, guide future studies and inform strategic planning for water resources.

USGS scientists are the first to report national estimates of PFAS occurrence in untreated groundwater that supplies water to public and private wells. This research also provides the first estimate of the number of people across the country who are potentially affected by PFAS-contaminated groundwater.

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Along with a scientific report, the USGS published an interactive, online map so users can see probability estimates of PFAS occurrence.

Note that predictive results are intended to be evaluated at state, regional and national scales rather than at individual household levels. Probability estimates are for the presence of PFAS in groundwater and do not account for any subsequent actions taken by states, municipalities or individuals to treat drinking water. The model does not include estimates of PFAS concentrations; testing is the only way to confirm the presence of contaminants.

Exposure to certain PFAS may lead to adverse health risks in people, according to the U.S. Environmental Protection Agency. PFAS are a group of synthetic chemicals used in a wide variety of common applications, from the linings of fast-food boxes and non-stick cookware to fire-fighting foams and other purposes. PFAS are commonly called "forever chemicals" because many of them do not easily break down and can build up over time, making them a concern for drinking water quality.

"This study's findings indicate widespread PFAS contamination in groundwater that is used for public and private drinking water supplies in the U.S.," said Andrea Tokranov, USGS research hydrologist and lead author of this study.

"This new predictive model can help prioritize areas for future sampling to help ensure people aren't unknowingly drinking contaminated water. This is especially important for private well users, who may not have information on water quality in their region and may not have the same access to testing and treatment that public water suppliers do."

The EPA has established legally enforceable levels, called maximum contaminant levels, for six types of PFAS in drinking water. The EPA regulates public water supplies, and some states have additional regulations for drinking water.

Some homes use private water supplies, where residents are responsible for the maintenance, testing and treatment of their drinking water. Those interested in treatment processes and testing options can read EPA's guidance or contact their state officials or water supplier.

The states with the largest populations relying on public water supplies with potentially contaminated groundwater sources are Florida and California. Regarding private wells, Michigan, Florida, North Carolina, Pennsylvania, New York and Ohio have the largest populations relying on potentially contaminated groundwater.

The study also presents data according to population percentage. In Massachusetts, for example, the source water for 86 to 98% of people who rely on groundwater from public water supplies could be contaminated with PFAS. In Connecticut, the source water for 67 to 87% of the people who rely on groundwater from private wells could be affected. Details by state can be seen in the report's tables S6 through S8.

"To derive these estimates, the team analyzed 1,238 groundwater samples collected by USGS scientists and determined how factors such as urban development and well depth can impact PFAS occurrence," continued Tokranov.

"With that information, a detailed machine learning model was developed and used to identify which geographic areas have a higher likelihood for contamination. That information was combined with existing USGS research on the number of people in a given area who rely on groundwater for drinking water to establish population estimates."

Scientists present separate estimates for public and private wells because they typically receive water from different groundwater depths. Public wells using groundwater as the primary water source are usually deeper than private wells.

There are more than 12,000 types of PFAS, not all of which can be detected with current tests; the USGS study tested for the presence of 24 common types. The USGS estimates consider the presence of at least one of those 24 types of PFAS. The most frequently detected compounds were perfluorobutane sulfonate known as PFBS, perfluorooctane sulfonate known as PFOS and perfluorooctanoate known as PFOA.

This research provides a broad outlook for the Lower 48 states and presents state-level estimates. Scientists did not look in detail at specific cities or provide estimates for the types of PFAS present or PFAS concentrations.