Spatial-temporal variability and transportation mechanism of polychlorinated biphenyls in the Yangtze River Estuary.

Although the variability of polychlorinated biphenyls (PCBs) is strongly dependent on the hydro-sediment pattern, the quantification of this interaction is still not well described, especially for estuary areas. In this study, both chemical analyses and numerical simulation were conducted to explain the temporal-spatial variability and transportation mechanism of PCBs in the Yangtze River Estuary (YRE). The impacts of the upstream Three Gorges Dam (TGD) on estuarine PCBs were also addressed with a simulated scenario. The results showed that the PCBs levels in the YRE were relatively low or moderate and the highest levels were related to the maximum turbidity zone. The spatial variability of PCBs is strongly dependent on the hydrological circulation, which resulted in a declining trend from the inner YRE to the adjacent sea. The seasonal variability of PCBs could be due to the joint influence of the current and the erosion/deposition environment. The opposite temporal trends of the overlaying water and sediment are driven by the seasonal characteristics of hydro-sediment patterns. The simulated results also indicated that the distribution, fluxes and transport ability of PCBs in the South Branch changed as a result of the sediment discharge reduction after construction of the TGD.