Source Apportionment and Ecological Risk of Heavy Metals in Cultivated Soils in Karst Area of Southeastern Guangxi

A total of 4,152 surface soil samples were systematically collected from the Gangbei District of Guigang city, Guangxi, China, to determine the characteristics, sources, and formation mechanisms of heavy metals in the cultivated soils of this karst area. The contents of heavy metal (As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn) and the pH values of the soil were quantified. Statistics, the Nemerow index method, the geo-accumulation index, and principal component analysis (PCA) were employed to study the heavy metal content and spatial distribution in the surface soil, identify pollution sources, and assess the ecological risk posed by these heavy metals. The concentrations of all eight heavy metals varied and generally exceeded the Guangxi and national background values for surface soils. The level of As was identified as the primary indicator for monitoring the ecological risk associated with heavy metals in the soil; while Cd and Pb served as secondary indicators. The levels of As, Cd, and Pb exceeded the limits specified in the GB 15618—2018 Soil Environmental Quality Risk Control Standard for Agricultural Land Soil (Trial) in 46.32%, 20.3%, and 14.6% of the samples, respectively, by 9.15%, 0.3%, and 0.2%. The main sources of heavy metal pollution included carbonate rock strata, agricultural activities, industrial emissions, domestic pollution, and mining activities. The Devonian-Carboniferous high-background parent rocks fundamentally influenced the baseline distribution of heavy metals. Mining and smelting activities contributed to the localized enrichment of As, Cd, and Pb in the soil. The ecological risks posed by these metals exhibited spatial variability within the study area: clean areas, comprising 53.11%-58.53% of the region, were predominantly located in the northwestern and southeastern clastic rock areas. Polluted areas, accounting for the remaining 41.47%-46.89% of the area, were characterized by three pollution belts classified as light, moderate, and severe, situated in the central-southwestern carbonate rock and northeastern clastic rock regions. As, Cd, and Pb were identified as the primary factors contributing to ecological risk. This study offers guidance for implementing targeted and region-specific measures aimed at mitigating the ecological risks posed by heavy metals in cultivated soils within karst areas.