Risk assessment and influencing factors of heavy metals in the weathered crust of black rock series from a mountainous area at the source of the Pearl River

　　This paper is the result of geological survey engineering.
　　Objective Black rock series are usually rich in heavy metals. Extreme rainfall can cause the weathered crust of the black rock series in the mountainous area of the Pearl River Source to break into fine particles on the ground surface, thereby threatening food security and human health. Therefore, it is of great significance to clarify the potential risks and influencing factors of the heavy metals therein. Methods In May 2024, a total of 9 sets of weakly weathered - semi-weathered - completely weathered matrix samples were collected in a mountainous area where black rock series are widely distributed in the Pearl River Source area. The major elements, S, pH, SOC, As, Cd, Cr, As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn were analyzed. The pollution load index method, potential ecological risk index method, and health risk assessment model method were used to evaluate the heavy metal pollution status, potential ecological risks, and human health risks. The influencing factors were discussed through correlation analysis and redundancy analysis. Results The pH range of the weathered crust is from 3.18 to 9.02, with an average value of 6.08, mainly acidic. The proportions of Cu, Cd, Ni, Cr, As, and Zn exceeding the risk screening values are 44.4%, 40.7%, 22.2%, 14.8%, 11.1%, and 7.4% respectively, and the proportion of Cd exceeding the risk control value is 16.7%. The evaluation results of the Pollution Load Index Method show that the weathered crust as a whole presents light pollution. The evaluation results of the Potential Ecological Risk Index Method indicate that it is mainly characterized by light and moderate ecological risks. The human health risk assessment reveals that the proportions of non-carcinogenic hazards and carcinogenic risks exceeding the limits for children are 37.04% and 40.74% respectively, and the proportion of carcinogenic risks exceeding the limit for adults is 11.11%. The top two heavy metal elements contributing the most to the ecological risks are Hg and Cd, while the ones to human health risks are Cr and As. The contents of MgO, Na₂O, K₂O, pH, Fe₂O₃, S, and P₂O₅, as well as the vegetation amount, slope aspect, and erosion degree, have a relatively significant impact on the potential heavy metal pollution and risks in the fully weathered layer and the semi-weathered layer. SIC, SOC, and S, as well as slope position and dip angle of the rock layer, have a relatively close relationship with the potential heavy metal pollution and risks in the weakly weathered layer. Conclusions Although compared with anthropogenic pollution, the potential heavy metal pollution and risks of the weathered crust of black rock series are only moderate or light, however, factors such as extreme rainfall, land leveling for cultivation, and excavation and construction may cause it to generate more severe pollution and risk situations.