Field-scale evidence of PFAS pollution potential from municipal landfills

Leachate from municipal landfills is a major source of per- and polyfluoroalkyl substances (PFAS) to groundwater. This study presents field-scale evidence of PFAS generation, release, and downward migration in a closed landfill comprising a 15 m waste body overlying a 16 m sandy unsaturated zone. PFAS composition and transport were monitored using a vadose zone monitoring system, with monthly leachate and porewater sampling over six months (November 2022–April 2023). Analysis of leachate samples shows short-chain perfluoroalkyl carboxylic acids (PFCAs), particularly PFBA, PFPeA, and PFHxA, dominate leachate and are preferentially released from decomposing waste, subsequently migrating through the unsaturated zone. In contrast, long-chain PFAS (PFOS, PFNA) and the precursor 6:2 FTS persist in shallow waste (<5 m) but are absent in deeper waste profiles, indicating in-waste transformation. Deeper waste layers and the underlying unsaturated zone show attenuation of long-chain compounds and enrichment of mobile short-chain species. Hydrological data indicate a transition from preferential flow in the waste to matrix-dominated flow in the sandy vadose zone, with episodic infiltration pulses governing PFAS transport. Overall, these findings demonstrate that landfills act as long-term, depth-stratified PFAS sources, where coupled hydrological and biogeochemical processes control the mass pollution potential of PFAS and associated groundwater risks.