Spatial distribution, source, and risk assessment of soil toxic metals in the coal-mining region of northwestern China

Soil toxic metal pollution is one of the most prominent environmental problems in the rapid industrialization of societies because of the considerable harm caused to human existence and the surrounding environments. Soil samples from 80 sampling sites around the coal-mining region of northwestern China were collected, and the geo-accumulation index (I-geo), pollution index (PI), and potential ecological risk index (PRI) were calculated, with the objective of assessing the soil toxic metal pollution level. The results showed that the average concentrations of Cr, Hg, and As exceeded the regional background values and the national soil environmental quality standards of China, while those of Zn, Cu, and Pb were below both soil-quality standards. The I-geo of toxic metals was ranked as Hg > As > Cr > Pb > Cu > Zn. The I-geo of Zn, Cu, and Pb indicated low pollution; the soils were moderately polluted by Hg and slightly moderately polluted by As, while other elements presented low pollution levels. The PI values of both As and Hg were higher than 3, indicating heavy pollution of these two metals. Zn and Cu originated from parent material, while Cr, As, and Hg originated from human activities such as coal burning, chemical industry, and traffic. Pb was influenced by both natural factors and human activities. The results of ecological risk assessment in the region showed that Zn, Cu, Cr, and Pb in all sample sites presented a low ecological risk, while Hg presented a high ecological risk. Therefore, Hg is the most hazardous toxic metals in the region. The spatial distribution trends revealed that the high-risk regions were found to be the industrial region of the study area. The research results provide a scientific basis and technical support for monitoring and early warning of soil pollution in arid regions.