Applications Of Radionuclides To Trace The Sources, Transport Pathways And Depositions Of Sediments In The Changjiang Estuary,East China Sea And Coastal Environment Of Eastern Hainan Island

The Changjiang Estuary/East China Sea is one of the highest turbidity estuaries/coasts in the world, and the coastal environment of east Hainan is characterized with tropical monsoons and tropical oceans climate. The source. transport pathways and depositions of particles/sediments in the Changjiang Estuary, East China Sea and coastal environment of east Hainan was examined by particulate radionuclides (7Be,210Pb,137Cs and234Th). The potential of applying radionuclides measurements in particulate pollutants studies and the underlying mechanism have also been discussed.The seasonal variations in depositional fluxes for7Be and210Pb during the years2006and2010were observed, the precipitation-normalized depositional fluxes of7Be and Pb are generally higher during spring and winter, but lower during summer. Based on the activities distribution patterns of7Be,210Pbxs and234Thxs in the particulate and dissolved phases in the Changjiang Estuary, the average distribution coefficients (Kd, cm3g-1) are7.6×lO4for7Be,1.18×105for210Pb and2.80×105for234Th, with a range of suspended particle concentrations of5.1-288mg L-1and salinities of0-33.6. Most of the nuclides can be scavenged to the particles with the high suspended particle concentration. However, the advection input/out process was dominant factor for particulate nuclides removal from water column in the Changjiang freshwater endnumber to river mouth areas. A two-dimensional model was used to evaluate the settling or "resuspension" flux of particles/sediments in the water column in the Changjiang Estuary. The scavenging times of7Be,210Pb and234Th in the estuary ranged2.7-11.1,9.0-85.4and3.9-11.2days and the removal times of7Be,210Pb and234Th ranged1.0-13.3,1.6-20.2and1.2-4.6days. The exchange of removal and resuspension was intensity, but the removal fluxes and the resuspension fluxes were comparable. In the coastal region, the removal times of7Be,210Pb and234Th by the material balance of one-dimensional model were around1.1-25.5,1.2-27.1and0.7-23.0days, respectively.Based on the variation distribution of activities of7Be,210Pbxs and137Cs in surface sediments collected in the East China Sea, the routes and budgets of these radionuclides was elucidated. Mass balance calculations suggest that the main source of7Be was local direct atmospheric deposition and most of7Be in the estuary and the inner shelf decayed and exported to offshore. The East China Sea was the sink for particle-reactive210Pb and137Cs, about70%of210Pb and95%of137Cs was transported from outer shelf to the estuary and inner shelf.The residence times of particulate7Be and210Pbxs had ranges of0.4-1.6and1.65-5.15days, respectively, in the Wenjiao/Wenchang River Estuary; and ranges of0.02-3.2and0.61-4.44days, respectively in the Wanquan River Estuary. All residence times of suspended particles for the two nuclides increased in the seaward direction. It was found that most of the Cd was deposited in the estuary in both the Wenjiao/Wenchang and Wanquan River Estuaries, while much of the Cu and Zn were transported to the coast.The ranges of activities of7Be, Pbxs,234Thxs and137Cs in surface sediments collected offshore of eastern Hainan Island were0.14-12.7,37.4-199,2.24-176and0.02-1.06Bq kg-1, with averages of3.93±4.75,110±10.5,62.1±8.89and0.52±0.22Bq kg-1, respectively. The activities of the radionuclides increased from coast to offshore in the northern section. The upwelling may cause high particle fluxes with high activities of210Pbxs and234Thxs. A comparison of the source and transport of the suspended sediments with river discharge along the coast shows that the coastal currents and offshore upwelling are the dominant factors for the transport and sources of surface sediment in the study region. The sediments were transported from south to north by the coastal current, and sediments with a large grain size may be deposited via the north loop current. The ratios of the nuclide activities indicated that the suspended particles need approximately one year to be removed from the water column into the seabed and that the main source of the sediments off eastern Hainan Island in the study regions was terrigenous deposition.