UC Riverside

New federal drinking water standards on per- and polyfluoroalkyl substances (PFAS) were recently established in the United States; however, the occurrence and speciation of PFAS in public drinking water systems nationwide is poorly understood. New federal drinking water standards on per- and polyfluoroalkyl substances (PFAS) were recently established in the United States; however, the occurrence and speciation of PFAS in public drinking water systems nationwide is poorly understood. This letter provides an urgently needed comprehensive analysis on PFAS occurrence in American public drinking water systems, via data mining on the most updated US EPA 5th round of Unregulated Contaminant Monitoring Rule database as of May 2026, representing 95% of eventual data entries and more than 1.9 million water samples from >10 000 public water systems. Data analysis showed that the most widely occurring PFAS compounds were PFPeA, PFHxA, PFBS, PFBA, PFOS, PFOA, PFHxS and PFHpA, detected by 3.3–13% of public water facilities nationwide. 90th percentile concentrations of these compounds ranged from ca. 7 to 17 ng L −1 , with PFBA, PFOS and PFOA exhibiting the highest concentrations. 6.7–6.8% of all drinking water samples had either PFOA or PFOS concentration exceeding 4 ng L −1 . Eastern U.S. and Pacific Southwestern regions have the highest percentages (4.6–19%) of public water systems with PFOA or PFOS level exceeding 4 ng L −1 . The occurrence of PFAS was also modestly associated with the dependence of groundwater as the drinking water source, with higher chances of PFOA or PFOS at more elevated levels in drinking water when associated with a groundwater source. This study provides new insights into PFAS occurrence nationwide, and can assist public water systems proactively prepare for PFAS treatment prioritization and future regulatory compliance.