Pollution degree and characteristics of heavy metals in soil of a coal mine area in western Chongqing

【Objective】 To study the degree of soil heavy metal pollution caused by coal mining in western Chongqing, and analyze its characteristics so as to provide scientific basis for environmental pollution prevention and control and ecological restoration in the local coal mining area.【Method】 The content of carbon, nitrogen, phosphorus and major heavy metals in the soil of an abandoned coal mine area in western Chongqing was determined by national standard method. The single factor pollution index method, Nemero comprehensive index method and soil pollution load index method were used to evaluate the pollution degree of heavy metals in the soil of the coal mine area.【Result】 The contents of total nitrogen, total phosphorus and organic matter in the soil in the coal mine area in western Chongqing ranged from 8.400 to 126.000 g/kg, 1.508 to 6.033 g/kg and 18.006 to 216.070 g/kg, respectively. The contents of total nitrogen and organic matter were higher in the central area of the coal mine(both sides of main coal transport lines, coal mine entrance, hillside of mine, and waste dump) and lower in the area around the coal mine(farmland, residential areas, parks, and schools), and the contents of total phosphorus were lower in the central area of the coal mine than in the area around the coal mine. The mining of coal mine in western Chongqing has resulted in the excessive of heavy metalscadmium(Cd), chromium(Cr), copper(Cu), nickel(Ni), lead(Pb), zinc(Zn) in the soil in this region to varying degrees. The content of Cd and Cu in various places has exceeded the Chongqing soil background value and the national soil background value, among which the minimum value of Cd has reached 6.37 times of the screening value of soil pollution risk in agricultural land. The content of Cr and Cu in soil on both sides of coal trunk line and Pb in soil of waste dump were the highest. The evaluation of pollution degree showed that soil Cd pollution in Yuxi coal mine area reached the serious pollution level, and the single factor pollution index ranged from 25.811 to 60.023. Both Nemerow composite pollution index and soil pollution load index showed heavy pollution in the coal mine.【Conclusion】 The soil in a coal mine area in western Chongqing has been polluted by multiple heavy metals caused by coal mining, among which Cd and Cu are the main heavy metal elements that threaten the health of local soil ecological environment.Compared with Nemerow index method, soil pollution load index method can better reflect the pollution characteristics of coal mine area in this study.