[Enrichment Levels and Source Apportionment of Heavy Metals in Grassland Soil of the One River and Its Two Tributaries in Xizang].

An understanding of the spatial distribution， accumulation levels， and sources of heavy metals in grassland soils on the Qinghai-Tibet Plateau has significant implications for protection of the local ecology. This study focused on the grassland soils of the "One River and Its Two Tributaries" region， where 561 soil samples were collected to analyze the concentrations and spatial distribution of heavy metals （Cr， Ni， Cu， Zn， Pb， Cd， As， and Hg） within the top 40 cm of soil. Three methods were used to evaluate heavy metal enrichment and accumulation： the enrichment factor， single factor pollution index， and pollution load index. In addition， the positive matrix factorization （PMF） model was used to identify the sources of these heavy metals. The results showed that the mean concentration of heavy metals in the grassland soil was below the pollutant risk screening threshold in the study area， indicating that the soil was not polluted. However， the concentrations of Cu， Zn， and Cd exceeded the background levels of Xizang and China， indicating the presence of distinctive regional characteristics. The soil heavy metal evaluation suggested that the enrichment and accumulation levels of Hg and Cd in the grassland soils were relatively higher than those of the natural background values of Xizang. This finding warrants further attention and investigation. Spatially， the enrichment levels of Hg were higher in the southern counties along the Yarlung Zangbo River， and Cd enrichment levels were higher in the counties of Lhasa. Soil Hg and Cd showed an increasing trend in enrichment and accumulation with increasing elevation. The PMF results of the source apportionment demonstrated that the concentration of heavy metals in the grassland soils was influenced by a combination of natural and anthropogenic factors. The concentration of Cr was derived from the lithology， with a contribution rate of 68.2%. Soil Cd and As were primarily influenced by soil parent material， with contribution rates of 47.3% and 75.7%， respectively. The concentrations of Ni， Pb， Cu， and Zn were mainly influenced by mineralization zones and mining activities， with contribution rates of 73%， 49.1%， 50.7%， and 66%， respectively. The majority of the Hg （64.3%） was derived from atmospheric deposition. These findings provide a scientific basis for the prevention and control of grassland soil pollution and ecological protection in the "One River and Its Two Tributaries" region.