- A wide variety of consumer products that are treated with poly- and perfluoroalkyl substances (PFASs) and related formulations are disposed in landfills. Landfill leachate has significant concentrations of PFASs and acts as secondary point sources to surface water. Here, we model how PFASs enter leachate using four lab-scale anaerobic bioreactors filled with municipal solid waste (MSW) and operated over 273 days. Duplicate reactors were monitored under live and abiotic conditions to evaluate influences attributable to biological activity. The biologically-active reactors simulated the methanogenic conditions that develop in all landfills, producing ~140 mL CH₄/dry g refuse. The average total PFAS leaching measured in live reactors (16.7 nmol/kg dry-refuse) was greater than the average for abiotic reactors (2.83 nmol/kg dry-refuse), indicating biological processes were primarily responsible for leaching. The low level leaching in the abiotic reactors was primarily due to PFCAs ≤C8 (2.48 nmol/kg dry-refuse). Concentrations of known biodegradation intermediates, including methylperfluorobutane sulfonamide acetic acid and the n:2 and n:3 fluorotelomer carboxylates, increased steadily in concentration after the onset of methanogenesis, with the 5:3 fluorotelomer carboxylate becoming the single most concentrated PFAS observed in live reactors (9.53 nmol/kg dry-refuse).