Particle dynamics of the Changjiang Estuary and adjacent coastal region determined by natural particle-reactive radionuclides (⁷Be, ²¹⁰Pb, and ²³⁴Th)

The timescale of transport processes in estuarine and coastal regions can be evaluated using natural radionuclides with different half-lives. The distribution patterns of ⁷Be, ²¹⁰Pb, and ²³⁴Th in the water column from April to July 2008 were used to calculate the removal and residence times in the Changjiang Estuary. The results showed that the maximum particulate activities of ⁷Be, ²¹⁰Pb, and ²³⁴Th were observed approximately 150 km downstream (the turbidity maximum zone) of the freshwater end-number. The mean distribution coefficients (K_d, cm³ g^-1) of the high suspended particulate matter (SPM) group are higher than those of the lower-SPM group for ⁷Be and ²¹⁰Pb; for ²³⁴Th, the reverse is true. Based on a material balance in two-dimensional models of ⁷Be, ²¹⁰Pb, and ²³⁴Th in the Changjiang River mouth, the removal times of these nuclides were approximately 0.66-12, 1.6-21, and 1.2-5.4 days, respectively. The residence times increased toward the seaward side. In the coastal region, the removal times of ⁷Be, ²¹⁰Pb, and ²³⁴Th calculated by material balance in one-dimensional models were approximately 1.1-26, 1.2-27, and 0.70-23 days, respectively. Moreover, an enhanced resuspension process not only controlled the partitioning of ⁷Be, ²¹⁰Pb, and ²³⁴Th between the particulate and dissolved phases, but also, this process may play a dominant role in controlling the dynamic behavior of SPM in the water column compared with the advection input/output fluxes of the SPM in the river mouth areas. The removal and the resuspension fluxes were comparable in the estuary. Meanwhile, old composition (whose deposition into the seabed for a long time is enough for ⁷Be to decay completely) occupied only a small part in the resuspended sediment during resuspension process.