Ecological risk assessment of soil heavy metals in the middle of the “One River and Two Tributaries” basin in Tibet

This study focused on the characteristics of soil heavy metal pollution in the middle area of the "One River and Two Tributaries" basin in Tibet. Four layers of soil, namely S1(0~10 cm), S2(10~20 cm), S3(20~30 cm), and S4(30~40 cm), at 41 points in the farmland soil were sampled; then, the concentrations of Cd, Cr, Cu, Hg, Pb, Zn, Ni, and Mn were determined. The sources of heavy metals were analyzed using the multivariate statistical method, and the soil heavy metal ecological risk was evaluated based on the geo-accumulation index, enrichment factor, and potential ecological risk index. The results showed that the concentrations of Cd, Cr, Cu, Pb, Zn, Ni, and Mn in soil layer S1 were higher, with mean values of 0.21, 82.95, 34.67, 35.81, 75.31, 49.99 mg·kg-1, and 697.39 mg·kg-1, respectively. The average concentration of each element in all four layers was lower than the risk screening values of soil contamination in farmland. The concentration of soil heavy metals decreased as the soil layer increased. Thus, the lowest concentration was observed in layer S4. Source analysis showed that Cd, Cu, and Pb in the surface(S1, S2, and S3)soil were mainly affected by human activities, while Cr, Ni, Zn, and Mn were mainly controlled by natural sources. Layer S4 was less affected by human activities, and the sources of its heavy metal elements were mainly natural. The ecological risk analysis showed that the geo-accumulation index, enrichment factor, and single ecological risk parameter of Cd in soil layer S1 were higher than those of the other elements with mean values of 0.790, 1.973, and 77.782, respectively. This indicated that layer S1 had significant contamination and a moderate ecological risk level. All the parameters of each element in the other soil layers were lower than those in layer S1, thereby implying that slight contamination and lower ecological risk. The average value of the comprehensive potential ecological risk index of the four soil layers was 103.959, thereby indicating that the soil in the study area was characterized by slight ecological risk. Generally, the quality of the soil in the study area was good, but the concentrations of Cd and Pb showed serious accumulation