Abstract Recent studies suggest per‐ and polyfluoroalkyl substances (PFAS) are ubiquitous in rivers worldwide. In the Asia‐Pacific region, the frequency of PFAS detection in rivers is increasing. However, the overwhelming majority of studies and data represent high population and urbanized river catchments. In this study, we investigate PFAS occurrence in major Philippines river systems characterized by both high and low population densities. In the Pasig Laguna de Bay River, which drains a major urban conurbation, we detected PFAS at concentrations typical of global rivers. Unexpectedly, we did not detect PFAS in river water or sediments in low population density river catchments, despite our instrument detection limits being lower than the vast majority of river concentrations reported worldwide. We hypothesize that septic tanks, as the dominant wastewater treatment practice in Philippines catchments, may control the release of PFAS into groundwater and rivers in the Philippines. However, no groundwater PFAS data currently exist to validate this supposition. More broadly, our findings highlight the need for more representative PFAS sampling and analysis in rivers to more accurately represent regional and global detection frequencies and trends.
