Evaluation and Migration Path Analysis of Soil Heavy Metal Pollution in a Metal Mining Area of Qinling Mountain

The Qinling Mountains are rich in metal mineral resources, but the problem of mine ecological environment damage left over from long-term high-intensity and large-scale mining has seriously affected the regional ecosystem in the Qinba area. Therefore, it is essential to objectively assess the characteristics and ecological risks of heavy metal pollution in the soil of the mining area, and to find out the migration path of heavy metals. In recent years, domestic and foreign scholars have carried out a lot of research work from the aspects of pollutant distribution characteristics^7-8, risk assessment^9-12, source analysis¹³, etc. The distribution characteristics, risk assessment and migration path analysis of heavy metal pollutants in metal mine soil in the Qinling Mountain area need to be supplemented.

To understand the pollution characteristics, migration path and pollution degree of soil heavy metal in metal mines of the Qinling Mountains.

A total of 50 soil samples and 19 surface water samples were collected in the study area. AFS, ICP-MS/OES were used to determine the contents of 8 heavy metals (Cd, Hg, As, Pb, Cr, Cu, Ni, Zn). The pollution degree and spatial distribution characteristics of heavy metals in the soil of the mining area were studied, and the migration path of heavy metals in the mining area was analyzed. The level of contamination and ecological risks was analyzed using the individual pollution index, Nemerow comprehensive pollution index and the potential ecological risk index.

The variation coefficient of As, Cd, Hg, Cu and Ni in the study area was large, and indicated that human intervention was more significant than initially thought. The pollution of As and Cd was the most serious; all the detection results exceeded the background value. The single Nemerow pollution index of Cd, Hg, As and Cu in the soil of the tailings pond area were all severe pollution grade, which were 18.60, 3.91, 24.62 and 6.96 times, respectively. The pollution level of Cd and As in agricultural land reached the severe pollution level, 5.36 and 4.99 times respectively. No detection index of industrial site reached the severe pollution level.

　　From the perspective of spatial content distribution, the tailings pond was the most seriously polluted by heavy metals, among which As, Cd, Cr, Cu and Pb exceeded the background value by 28.9, 22.6, 1.41, 8.69 and 0.345 times, respectively. Agricultural land and the industrial site followed, which was consistent with the analysis results of the single index method, Nemerow comprehensive pollution index method and potential ecological risk index method. Weighting the data by Nemerow composite pollution index of the 8 heavy metals, the tailings pond was 19.11, reaching the level of severe pollution; agricultural land was 2.86, which was moderately polluted; the industrial site was 0.96, which was the warning value. The analysis results of potential ecological risk assessment index of heavy metals showed that the comprehensive risk index of tailings pond was 255.9, presenting a medium risk grade. Agricultural land was 130.23, which was low ecological risk grade. The industrial site was 152.66, which was medium risk.

According to the analysis of pollution path, it can be seen that mining engineering is an important cause of ecological environmental damage around the study area, among which water seepage and water gushing at the tailings pond are particularly significant pollution sources, and polluted surface water is an important cause of soil pollution on agricultural land. It is suggested that heavy metal pollution in the tailings pond should be the focus for later restoration and treatment process of soil ecological environment in mining areas, and heavy metal indicators such as As, Cd, Hg and Cr should be continuously monitored.