• • PE release rate (26.1±1.8%) significantly exceeds PP (16.4±1.3%) under 6-month simulated rainfall at 0.2% abundance and 20–40 μm size, indicating polymer-specific mobility that dictates environmental exposure risk.
• • MPs of 40–60 μm size exhibit the highest release and vertical migration, while excessive abundance triggers aggregation and suppresses release, highlighting a critical size-abundance threshold for transport prediction.
• • Vertical migration peaks between 14–18 months under 24-month simulated rainfall, with PE migrating deeper than PP, underscoring the long-term contamination potential of sludge-amended soils.
• • Environmental factors exert decisive control: migration increases with pH but decreases with electrolyte concentration; sludge leachate promotes transport at low organic matter concentrations but inhibits at high concentrations, necessitating site-specific risk assessments.