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 Oct. 24. 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.