Assessment of Soil Heavy Metal Pollution in the Dump of a Western Inner Mongolian Coal Mine

Abstract

Exploring the status and sources of heavy metal pollution in the soil of the dump in Wuhai City, western Inner Mongolia, is of great significance. This study selected a sunny slope, a half-sunny slope and shady slope with plots of (A) Astragalus adsurgens + Agropyron desertorum + Elymus, (B) Caragana korshinskii+Astragalus adsurgens + Agropyron desertorum + Lolium, and (C) Medicago sativa + Artemisia ordosica+Astragalus adsurgens + Brassica juncea, and a naturally repaired slope for comparison, yielding a total of 10 types of sample plots. The soil heavy metal pollution levels and the potential ecological harm were assessed by the process of measuring the contents of seven heavy metal elements (As, Hg, Pb, Cr, Cd, Cu and Zn) in dump soil as a single factor pollution index, and then the comprehensive pollution index, potential ecological risk index and the mine soil heavy metal pollution sources were explored by correlation analysis and principal component analysis. The results showed three important aspects of the pollution levels, impacts and sources in this dump. (1) The heavy metal contents in Qifeng dump had little influence on the plant community composition type and no clear relationship with the slope direction, but the Cr content in the manually configured sample was significantly reduced compared with that in the naturally restored slope. The contents of Hg and Pb exceed their soil grade I standards, and Cd exceeds the National Grade II standard compared with the soil background values of Inner Mongolia. The As, Hg, Pb and Cd levels of the dump exceed their standards, so there is a certain degree of heavy metal accumulation in the soil of the dump. (2) The single factor pollution index in descending order is Cd > As > Pb > Cr > Hg > Cu > Zn, and all the dump samples are polluted by Cd. According to the Nemerow composite index, the heavy metals in the soil of the dump are at middlingor moderate pollution levels. The potential ecological risk index values of the individual heavy metals were in the order of Cd > As > Hg > Pb > Cr > Cu > Zn, so Cd was the most important potential ecological risk factor ranging from 108.650 to 180.600. The comprehensive potential ecological risk index ranged from 114.665 to 188.792, indicating that 50% of the plots were at slight or moderate potential ecological risks, respectively. According to the different evaluation methods, Qifeng dump is polluted by heavy metals, and the pollution degree and ecological risk associated with Cd are much higher than those of the other heavy metals. Therefore, timely control measures should be taken for Cd. (3) The correlation analysis and principal component analysis showed that Hg, Pb and Cd came from road coal dust diffusion and exhaust emissions, while Cu, Zn and Cr came from transportation and agricultural production activities, and As came from coal combustion pollution.