In the tailings pond of the Hongtou Mountain (HTM) copper mine in northeastern China, four sampling points were set at intervals of 80 m along the discharge direction of the tailings slurry to investigate the vertical distribution characteristics of heavy metals in shallow layer (0-5 m) tailings. In addition, 18 sampling points were set by a grid method (50 m 9 50 m) to estimate the historical loss amount of contaminants from the surface layer (0-1.2 m) of the HTM tailings pond. Tailings samples were taken at every 0.2 m, and the pH value, the total content and acid-soluble fraction of heavy metals and chemical compositions were determined following the standard methods. The results showed that the pH values of tailings ranged from 3.01 to 8.11, which increased with depths from 0 to 2.2 m, and varied between pH values of 7.0 and 7.5 in 2.2-5 m. Cu, Zn, Cd and S were the major contaminants in the tailings. The total Cu content was higher at the downstream sampling points near the dam. Zn and Cd were higher at the upstream sampling points near the discharge. Three distinct zones (oxidised zone, cemented layer and unoxidised zone) were found in the HTM tailings pond. The depth of the oxidised zone was 0.4 m, and the elements in the oxidised tailings were obviously lower due to the sulphides oxidation and acid mine drainage. Cu was enriched in the cemented layer, and the Cu content reached values between 1200 and 3000 mg/kg, which were 1.5-3 times higher than the average value. Zn, Cd, Ni and Fe were also slightly elevated in the cemented layer. Below the groundwater table, the acid-soluble fraction of Cu, Zn and Cd was relatively stable at a lower level and showed insignificant variations, as the cemented layer and groundwater could efficiently prevent the oxidation process. The amount of Cu, Zn and Cd leaving the tailings pond with drainage water was 8.08, 21.60 and 72.93%, respectively, of the amount released from oxidation. Most of the Cu released from oxidation was retained in the cemented layer. The total mass of S, Zn and Cd released from the tailings to surrounding ecosystems was estimated approximately 762.75 t, 6997.5 and 86.06 kg, respectively, in the last 20 years.
