Spatial variability of per- and polyfluoroalkyl substances in surface waters across clean, agricultural, and suburban zones in Thailand

Per- and polyfluoroalkyl substances (PFAS) are persistent, bioaccumulative contaminants of emerging concern. However, their spatial distribution across land-use watersheds in Thailand remains poorly characterized. Understanding these patterns is essential for identifying sources and developing mitigation strategies. This study evaluated the occurrence, distribution, and ecological risks of 12 PFAS in upstream surface waters, typically considered pristine, and downstream areas impacted by human activities. Surface water samples were collected from 15 sites along the Chao Phraya River Basin (CPRB), the Lop Buri River (LBR), and an irrigation canal (IRR) supplying critical domestic and agricultural water. PFAS were extracted via solid-phase extraction and quantified using high-performance liquid chromatography-tandem mass spectrometry. Total PFAS concentrations ranged from below detection limits to 79.4 ng/L in CPRB (CP5), 30.3 ng/L in LBR (LB7), and 7.5 ng/L in IRR. Perfluorobutane sulfonic acid (PFBS) and perfluoropentanoic acid (PFPeA) were the most frequently detected compounds. These results indicate that land use patterns, including industrial discharge, agricultural runoff, and domestic inputs, strongly influenced spatial distributions of PFAS. These findings underscore the need for systematic PFAS monitoring to identify contamination sources, assess ecological risks, and develop targeted mitigation strategies to protect the surface and irrigation waters. • First simultaneous detection of PFAS in both the Chao Phraya and Lop Buri Rivers. • Upstream surface waters previously considered clean were contaminated with PFAS. • Short-chain PFAS (≤8 carbons) predominated in surface water. • Perfluorobutane sulfonic acid (PFBS) was the most frequently detected PFAS. • Land use was a key PFAS source, affected by population density and human activity.