Distribution and risk assessment of heavy metals in the upper, middle, and lower reaches of the Yellow River wetlands

Wetlands serve as significant sinks and sources of heavy metals. In this study, surface sediment samples (0-25 cm) were collected from 15 sampling sites across the wetlands in the Ningxia reach, Henan reach, and delta reach of the Yellow River to investigate the contents, distributions, and ecological risks of heavy metals such as As and Cd in the wetland sediments. Our assessment revealed that the sediments were alkaline in upper and lower reaches compared to the middle reach and that the alkaline sediments were more conducive to heavy metal enrichment. There was no significant spatial distribution pattern of the heavy metals across the wetlands in the whole Yellow River basin. The contents of the heavy metals decreased with increasing sediment depth at each sampling site. Geoaccumulation index (I-geo) analysis revealed that heavy metals had a negative I-geo value at each sampling site, except for the following metals and sampling sites: in the upper reaches, the I-geo values for Cd, Mn, and Ni were 0.71, 0.17, and 0.04, respectively; in the middle reaches, the I-geo value for Cd was 0.28; and in the lower reaches, the I-geo value for Sb in the delta wetlands was 0.21. Pollution load index analysis and enrichment factor (EF) analysis revealed the occurrence of severe Cd contamination in Ningxia reach, with an EF greater than 3, indicating a high degree of anthropogenic impact. There was a strong correlation (r > 0.8) among the various heavy metals in the wetlands of both the Ningxia and delta reaches, suggesting that these two reaches have the same source for such heavy metal pollutions.