Distribution Characteristics of Heavy Metals in Soils Affected by Different Land Use Types in a Superimposed Pollution Area with High Geological Background

Abstract

The high geological background and the residual pollution of historical zinc smelting in the karst area of northwest Guizhou has led to serious soil heavy metal pollution. In order to understand the characteristics of soil heavy metal pollution in agricultural land, wasteland, and forestland, 126 surface and profile soil samples were collected from a typical zinc powder factory-polluted small watershed in Weining County, Northwest Guizhou Province. The concentrations and distribution characteristics of Cd, As, Pb, Cu, and Zn in soils were studied. The results showed that the average ω(Cd), ω(As), ω(Pb), ω(Cu), and ω(Zn) in the surface soil were 8.14, 45.07, 263.97, 41.36, and 716.98 mg·kg-1, respectively, which were 1.29-12.33 times higher than the background values. The content of heavy metals in soil of different land uses in descending order was agricultural land, wasteland, and forestland. From the perspective of spatial distribution, the soil with high heavy metal concentration was concentrated around the zinc smelting plant and slag, and the accumulation of heavy metals in soil was greatly affected by the pollution sources of mining activities but less affected by the land use mode. In soil profile, heavy metals in the surface layer were generally higher than those in bottom layers, showing a significant difference (P<0.05) in wasteland. Five heavy metals in 80-100 cm soil were higher than the background value, reflecting the existence of a high geological background value. Correlation analysis and principal component analysis showed that Pb, Zn, and As in the surface soil were mainly affected by human activities such as mining and transportation, whereas Cd and Cu were both affected by high geological background and human activities. The single ecological risk coefficient of Cd in soil was the highest, which reached very strong or very strong risk, whereas As, Zn, Cu, and Pb reached slight or medium risk. The comprehensive potential ecological risk index indicated strong ecological risk, which in agricultural land was higher than that in wasteland and forestland. In the karst high geological background area, the influence of lead and zinc smelting on the distribution and accumulation of heavy metals in soils was greater than that of land use; soil heavy metal pollution was serious, and ecological risk was high in this area. Therefore, strict control of heavy metal-contaminated soil should be strengthened.