| Non-ferrous metal mine tailings pond occupied a large amount of land, was also potential pollution source. Heavy metals released from sulphides-rich tailing oxidation would discharge into surrounding environment with water and wind. This research was financially supported by the Special Fund of Ministry of Land and Resources of People’s Republic of China for Scientific Research in the Public Interest. Two tailings ponds at Hongtou Mountain (HTM) copper mine and Chaihe Lead-Zinc mine were selected in this research. Tailings samples were collected in 0～500 cm depth to investigate the dispersal characteristic of heavy metal pollution in tailings pond. Base on the multidisciplinary perspective of soil, environmental and ecological, put forward an "Ecological closure and land resue" remediation model for the current problems. Laboratory column leaching experiment and field application were conducted to evaluate the performance of different topsoil constructions:CT (30 cm soil), SLS (soil+limestone), SL(soil+lime) and SC (soil+ clay). Results showed:The pH values of the tailings in Hongtou Mountain were lower in 0-40 cm (pH< 4.0). The pH value of the tailings was from 3.01 to 8.11, which increased with the depth in 0-220 cm, and varied between 7.0 and 7.5 in 220-500 cm. Sulphides oxidation cause low pH in surface tailings. Cu, Zn and Cd were the major pollutants in the HTM tailings pond. Three distinct zones (oxidised zone, cemented layer and unoxidised zone) were found in HTM tailings pond. The depth of the oxidation front was 40 cm corresponded to weathering for 20 years, elements in oxidised tailings were obviously lower due to the sulphides oxidation.The amount of Cu, Zn and Cd leaving the tailings pond with drainage water was 8.08%, 21.60%and 72.93% of the amount released from oxidation. Most of the Cu released from oxidation was retained in the cemented layer, and the total mass of S, Zn and Cd released from the tailings to surrounding ecosystems were estimated about 762.75 t,6997.5 kg and 86.06 kg in the past 20 years.In Chaihe Pb-Zn mine 1# tailings pond, the pH values of the tailings in 0-500 cm were from 6.83 to 7.97, which increased with depth in 0-120 cm. Pb, Zn and Cd are the major contaminants in Chaihe Pb-Zn mine tailings pond and the mean content of Pb is 1372.79 mg kg-1, Zn is 4077.15 mg kg-1 and Cd is 49.74 mg kg-1. In Chaihe Pb-Zn mine 2# tailings pond,the total contents of Zn, Pb and Cd in reclaimed soil were 1538.55 mg kg-1,748.53 mg kg-1 and 7.1 mg kg-1, and the acid-soluble Zn, Pb, Cd were 10.32 mg kg-1,9.9 mg kg-1,0.65 mg kg-1. That Indicating heavy metals in tailings could migrate to topsoil, Single-topsoiling method used in Chaihe Pb-Zn tailings pond did not prevent the migration of heavy metals.Base on the strategy of pollution source control and the multidisciplinary perspective of soil, environmental and ecological, put forward an "Ecological closure" remediation model for the current problems in current measures and existing standard. Top-soil reconstruction is the major content of ecological closure remediation strategy, different topsoil reconstructions were designed to satisfy the need of Ecological closure remediation model.210-day laboratory column leaching experiment and 3-year field application were conducted to evaluate the performance of different topsoil constructions.During the 210-day laboratory column leaching experiment, the pH values of the leachates from the columns increased with time, to 7.8～8.0 in the 180th day. The topsoil effectively reduced the concentrations of Pb, Zn and Cd in the leachate. The presence of a limestone or lime interlayer reduced the mobility of Zn and Cd, and a clay interlayer was effective in preventing the migration of Pb and Zn but had less impact on Cd.During the 3-year field experiment, the pH values of the topsoil increased slightly with time, from 7.15 to the range of 7.67 to 8.20. At the end of the experiment, Pb, Zn and Cd concentrations in the topsoil at the SLS, SL and SC plots showed little change compared with the initial value in 2012. Acid-soluble Pb, Zn and Cd concentrations of the upper soil (0-10 cm) at CT increased from 0.19,1.95 and 0.11 mg kg-1 in 2012 to 1.82,32.87 and 0.56 mg kg-1 in 2015. Acid-soluble Cd in topsoil and in the interlayer showed a small increase at plot SC, indicating that Cd may migrate through the clay layer to topsoil with pore water. The Pb contents in leaves at SLS and SL were 2.38 and 3.04 mg kg-1, which were appreciably lower than at CT and SC, and the Zn and Cd contents in leaves at CT were 45.78 and 0.26 mg kg-1, which were higher than at SLS, SL and SC. Topsoiling with an interlayer could be a reliable technique for preventing toxic elements from moving downward or upward during ecological restoration of tailings ponds. According to analysis the results, Topsoiling with a lime layer was the best method used in Chaihe Pb-Zn mine tailings pond land reclamation.Tailings pond land reclamation should be aimed at the prevention of heavy metal pollution, safety of tailings dam, tailings resource conservation and land eco-utilization. A new technical model for tailings pond reclamation should be established base on the systemic and overall situation viewpoint, that is useful for the establishment of tailings pond reclamation plan and the determination of treatment measure. The results could also improve the standards of environmental and land for tailings pond reclamation. |
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