Distribution, historical variations, and geochemical fractions of toxic trace metals and their ecological risks in sediments of the Nanliu River Estuary, South China

In this study, the historical variation (Pb-210 dating) and distribution of toxic trace metals in sediments of the Nanliu River Estuary (NRE) were studied to understand the spatiotemporal variation therein and related risks. Geochemical fractions of trace metals (Pb, Cu, Cr, Zn, and Ni) were also analyzed using the fractionation technique. The results suggested that the concentrations of trace metals are higher in the western part of the estuary, and that Pb was significantly enriched. The NRE has experienced rapid economic development during the past four decades, but a decreasing trend of trace metals content after 2000 was observed. This may be due to effective implementation of national environmental control measures. Zn, Ni, Cr, Cu, and Pb in the surface sediments of the NRE exist mainly in residual form. The order of trace metals based on the risk assessment code (RAC) is Zn > Cu > Ni > Pb > Cr. According to enrichment factor (EF) and geo-accumulation index (I-geo) values, Hg, As, and Pb in the sediment of the NRE cause moderate levels of pollution and require attention. Based on the geochemical method, anthropogenic contributions of Cd, Zn, Pb, and Cu in the sediments of the NRE exceeded 50 % after 1980, possibly corresponding to the rapid industrial development and economic growth in the NRE resulting from the "reform and opening up".