Seasonal variation and ecological risk assessment of heavy metals in sediments of the Yellow River estuary

To explore the seasonal variation patterns of heavy metals in sediments and their potential ecological risk effects in the coastal areas of the Yellow River estuary, we conducted four quarterly surveys in March, June, September, and November 2021. The spatial distribution characteristics, seasonal dynamics, and potential ecological risk levels of seven heavy metals (Zn, Pb, Cu, Cr, As, Hg, and Cd) in surface sediments of the study area were systematically analyzed. Results show that the average contents of the seven heavy metals in the sediments followed the order of Cr 66.92, Zn 62.98, Cu 20.59, Pb 20.35, As 11.67, Cd 0.14, and Hg 0.03 mg/kg. Evaluation based on the China’s national marine sediment quality standard revealed that heavy metal contents at most stations fell into Class I. Only some heavy metals at a few stations fell into Class II (occupying 4.86%). The distribution of heavy metals in surface sediments exhibited seasonal differences except for Hg and Cd. The contents of the other five heavy metals showed a similar seasonal variation trend of increasing in spring, decreasing significantly in summer, rebounding in autumn, and decreasing again in winter. The temporal and spatial variations of heavy metals in sediments were generally regulated by the distribution of fine-grained sediments dominated by estuarine hydrodynamic conditions and the pulsed material input caused by the Water-Sediment Regulation Scheme (WSRS). Using the heavy metal background values of Chinese loess as the evaluation benchmark, the potential ecological risk degree of single heavy metals in the study area was ranked as: Hg ＞ Cd ＞ As ＞ Pb ＞ Cu ＞ Cr ＞ Zn. Among them, the average ecological risk indices of Hg and Cd reached the medium-risk level. The average comprehensive ecological risk index of the seven heavy metals was at the low-risk level but close to the upper limit of low risk, with the highest risk index in autumn and the lowest in winter. This suggests that marine environmental monitoring after the conclusion of the WSRS requires our attention.