Centennial-Scale Characteristics and Pollution Assessment of Heavy Metals in Sediments from the Tuo Lake Nature Reserve

To explore the characteristics and pollution levels of typical heavy metals in the columnar sediments of the Tuo Lake Nature Reserve, based on the construction of a ²¹⁰Pb chronological framework, the content characteristics of eight heavy metals, including As, Cd, Cr, Pb, Hg, Cu, Ni, and Zn, in the study area since 1908 were investigated. The geo-accumulation index (<i>I</i> _geo), enrichment factor (EF), and potential ecological risk index (PER) were used to assess the risks of heavy metal elements in the sediments. Combined with principal component analysis and correlation analysis, the sources of heavy metals in the Tuo Lake sediments were explored. The results indicate that, except for As and Pb, the average values of the other six heavy metals in the study area are higher than the background values, with Cd exhibiting a higher degree of variation in content across stratified sediments compared to other heavy metals. Over the past century, the overall pollution level in the research area has been relatively low, with only some heavy metals showing significant changes locally, especially in the upper part of the columnar samples. The geo-accumulation index indicates that Cr, Pb, and Ni have been in an unpolluted state over the past century, with only two layers reaching moderately polluted levels for Cd. In the evaluation of enrichment factor, the trends in the enrichment factors of five heavy metals (Cr, Pb, Cu, Ni, and Zn) over the past century have been basically consistent, with only a small portion of Cd (12.5%) and Hg (5%) reaching moderate enrichment. The potential ecological risk index indicates that As, Cu, Ni, Pb, Cr, and Zn all belong to the low ecological risk category, with only a few layers of Cd (7.5%) and Hg (5%) reaching considerable ecological risk levels. The comprehensive potential ecological risk index (RI) shows that most layers (87.5%) are of low ecological risk, and samples with moderate risk levels (12.5%) are concentrated in sediments formed since 2000. The heavy metals in the sediments of the protected area mainly come from two sources: Cr, Pb, Cu, Ni, and Zn are mainly from natural geological backgrounds; As, Cd, and Hg are mainly from a combination of agricultural, transportation, and industrial sources.