Spatial Differentiation of Heavy Metals in a Landfill Site in a High-Altitude Cold Region Driven by Freeze-Thaw Cycles

• • Cd is the primary pollutant, with all evaluation methods (Pi, Igeo, PN, RAC) consistently indicating severe contamination; Cd concentrations exceed background levels by up to 10-fold, necessitating priority remediation. • • Ni exhibits high migration potential: its weak-acid-extractable fraction reaches 5.55%, the highest among the seven metals, despite moderate total concentrations, indicating a significant risk of leaching and bioavailability under freeze-thaw conditions. • • Freeze-thaw cycles (>180 cycles per year) drive vertical redistribution: Cd, Cu, Pb, and Zn accumulate in the top 0-10 cm, while Ni migrates to depths of 30-50 cm, forming a unique 'freeze-thaw-driven' contamination pattern that challenges conventional landfill management. • • The study provides critical data for designing remediation strategies in high-altitude cold regions, emphasizing the need for impermeable barriers and long-term monitoring of Ni mobility in active freeze-thaw layers.