Monitoring limitations and challenges in implementation of drinking water PFAS standards in US public water systems

The presence of polyfluoroalkyl substances (PFASs) in US drinking water has generated concerns about their toxicity and ability to be monitored. In 2024, the US Environmental Protection Agency (USEPA) set Maximum Contaminant Levels (MCLs) for six PFAS compounds and introduced a Hazard Index (HI) for four others under the Safe Drinking Water Act. In May 2026, the USEPA proposed revisions to several of these parameters. However, these revisions had not been approved at the time of publication of this manuscript. The main objective of this study was to examine PFAS occurrence in US. public water systems (PWSs), focusing on identifying monitoring limitations and challenges in meeting MCL regulations. This study analyzed 1.98 million records from 10,777 PWSs through February 2026 and examined how PFAS monitoring unfolds across public water systems nationwide. The focus was on exceedances, the quality of the data, frequency, locations, types of water sources, and the environmental risk impact assessments of the size of the population served. Results show that 8838 exceedances were recorded based on the 2026 proposed MCL, impacting between 37.2 to 81.3 million users and 11.54 to 25.22 million cubic meters/day of water supply. The proposed 2026 MCL revisions result in only marginal reductions in the number of affected PWS (1.4 to 1.7%) and population served (0.5 to 0.7%) relative to the 2024 standards. About 38% of samples for several PFAS compounds collected between 2013 and 2016 had Minimum Reporting Levels (MRL) above current MCLs, limiting their regulatory value. The analysis, based on the environmental quality standards, created accurate HI assessments for PWSs. That said, many exceedances remain undetected due to low sample quality and frequency. Larger PWSs exhibit higher exceedance rates relative to current MCL thresholds, likely due to greater urban contamination and the increased complexity of their water supply systems. Groundwater influenced by surface water shows higher exceedance rates for PFOA and PFOS MCLs.