Heavy metals in estuarine surface sediments of the Hai River Basin, variation characteristics, chemical speciation and ecological risk

The Hai River Basin (HRB) is considered to be one of the most polluted areas in China due to the high regional population density and rapid economic development. The estuaries of the HRB, which receive pollutants from terrestrial rivers, may subsequently suffer potential pollution and result in ecological risk of heavy metals. Six heavy metals (Cd, Cr, Cu, Ni, Pb, and Zn) were measured in estuarine surface sediments from 10 estuaries of the HRB to investigate their variation characteristics and ecological risks. The spatial difference of Cr, Ni, Pb, and Zn in sediments was higher than that of the rest two elements. The Yongdingxin Estuary (YDX) and Ziyaxin Estuary (ZYX) in the Northern Hai River System (NHRS) were the most severe in terms of heavy metal contamination. According to the Risk Assessment Code (RAC) classification, Cd associated with the exchangeable and carbonate fraction (the average of 21.3 %) indicated medium risk to high risk. More than 50 % of Cr, Cu, Ni, and Zn on average were associated with the residual fraction. Based on the sum of the first three fractions (exchangeable and carbonate + reducible + oxidizable), the mobility order of these heavy metals was Cd >Pb > Zn ≈ Cu > Ni > Cr. Compared to the background values of cinnamon soil, the potential ecological risk index (RI) values ranged from 25.6 to 168, with an average of 91.2, indicating a low ecological risk in estuarine sites of the HRB. Cd and Pb were the dominant contributors to the toxic-response factor (45.8 and 25.5 %, respectively). The results give insight into the different control measures pertaining to heavy metal pollution and risk for both relatively clean estuaries and urban seriously polluted areas, respectively, for the formation of protect strategies of aquatic environment in the HRB.