Spatial distribution of heavy metals in the surface sediments of Laizhou Bay and their sources and pollution assessment

Surface sediment samples were collected in 2016 at 32 stations in Laizhou Bay. Studies are devoted to the concentrations, spatial distribution patterns and interrelations of seven heavy metals (Cu, Pb, Zn, Cr, Cd, As and Hg). Comparison is made with previous investigations. The pollution status is evaluated systematically with potential ecological risk index (PERI), geoaccumulation index (I_geo) and sediment quality guidelines (SQGs). Our results show that the heavy metals in the surface sediments are mainly concentrated in the north of the Yellow River estuary, Xiaoqing estuary (the southwest) and the northeast of Laizhou Bay. Cr, Cd and Hg are obviously related with fine grain sediments, indicating the control of sediment grain size over heavy metal contents. The distribution patterns of sediment types are mostly consistent with the mean circulation in Laizhou Bay, and are significantly related to the hydrodynamics of surrounding estuaries and tidal currents. Results of heavy metal pollution assessment suggest that heavy metal contents at all stations are lower than the possible effect level (PEL), and the integrated potential ecological risk is low. Geoaccumulation index suggests that about 60% of the study area are polluted by Hg, lightly polluted by Cd (37.5%) and As (12.5%). Cu, Pb, Zn, Cd, As and Hg show significant positive correlations; Cu, Pb and Zn are mainly controlled by natural crust weathering process, while Cu and Zn are affected by TOC as well; Cd and As are presumed to be related to human activities; and Cr came mainly from the natural weathering process of the crust, in addition to some from human activities. Accordingly, the Yellow River and Xiaoqing River were likely to be the main sources of surface sediments in the west and southwest of Laizhou Bay, which is basically consistent with previous studies.