Distribution Characteristics and Risk Assessment of Heavy Metals in Soils of the Typical Karst and Non-Karst Areas

To investigate the distribution characteristics and hazard levels of eight heavy metals (As, Cd, Cr, Cu, Hg, Ni, Pb, Zn) in karst soil with a high geological background of heavy metals, 32 and 40 surface soil samples were collected from limestone and clastic rock areas, respectively, in the northern part of Mashan County, Guangxi Province, a typical mountainous county dominated by primary industries in China. Geostatistical methods, Pearson’s correlation analysis, the geo-accumulation index, and the potential ecological hazard index were applied to explore the influencing factors of those heavy metals and evaluate their potential contamination risks. The results show that (1) the levels of the eight heavy metal elements in the surface soils of karst areas exceeded the background values of soil for Mashan County, the background value of soil (layer A) in China, and abundance value of upper crust. According to the soil pollution risk screening values specified in the Soil Environmental Quality: Risk Control Standard for Contamination of Agricultural Land, the proportions of heavy metals in the soils of karst areas were ranked as Cd (100%) > As (90.6%) > Cr (84.4%) > Zn (68.8%) > Ni (37.5%). Meanwhile, the heavy metals in the soils of non-karst areas did not exceed the overall values for Mashan County, and Ni, Pb, and Zn did not exceed the overall national soil values. One-quarter of Cd in non-karst samples exceeded the risk-threshold screening value. There was a high degree of variation and a significant difference in the contents of heavy metal elements between karst and non-karst areas. (2) The element combinations of As-Cd-Cu-Hg-Ni-Pb-Zn and Cr in karst areas were characterized by the influence of carbonate rock parent material. The non-karst areas were characterized by Ni-Cu-Pb-Zn, As-Cr-Hg, and Cd assemblages, which were mainly influenced by the mixture of laterite parent materials, sand shale parent materials, and basic-rock residual materials, and that may be affected by element migration caused by soil erosion and anthropogenic activities. (3) Analysis of the geo-accumulation index showed that karst areas were generally found to be at the clean to light pollution level, except for in the areas whose samples exhibited medium/high pollution levels for Cd and Cr, with the Cd pollution being the more serious of the two. Small amounts of Cd and Cu were present in the non-karst areas at a light contamination level, while other elements were at the level of no pollution. (4) The results of the potential ecological risk index showed that Cd and Hg were the main ecologically hazardous heavy metal elements in the soils of the study areas. The potential ecological risk level in karst areas was much higher than in non-karst areas, especially for Cd, and was mainly influenced by the carbonate rock parent material.