Spatial and Temporal Distribution and Risk Assessment of Dissolved and Particulate Heavy Metals in the Middle and Lower Reaches of the Yellow River During the Water and Sediment Regulation Period

Water-sediment regulation Scheme (WSRS) is a crucial artificial control method for water and sediment in the lower reaches of the Yellow River (YR). During this period, the impact of high flow discharge and high sediment concentration on the distribution and transport of heavy metals in the Yellow River warrants particular attention. This study analyzes heavy metals in water and suspended sediments across different phases of the WSRS. During the WSRS, the lower Yellow River showed decreased levels of dissolved heavy metals. Evaluations of the ecological risk posed by Cd, Cu, Ni, Pb, Zn, and Cr in suspended sediments suggested an absence of risk or minimal risk levels. A greater percentage of Cd and Pb was found in bioavailable forms. And RAC analysis further revealed that Cd poses a relatively higher migration risk. Compared to the water regulation stage (WRS), the sediment regulation stage (SRS) was characterized by lower dissolved heavy metal concentrations but higher particulate heavy metal contents. Over 85% of heavy metals were transported by SPM in WSRS. During the sediment regulation phase, the mean particulate transport proportion for heavy metals surpassed 96%. This phase accounted for more than 50% of the total heavy metal flux delivered to the sea throughout the entire regulation period. These findings offer valuable insights into controlling and managing heavy metal risks during WSRS in the YR.