| San Diego Bay (SDB) is a typical semi-closed, low inflow estuarine. Our previous geochemical studies have shown that there are systematic differences between waters inside SDB and in the open ocean in terms of activities of particulate and dissolved particle-reactive radionuclides (210Pb, 210Po and 234Th) and suspended particulate matter (SPM). The difference in SPM concentrations is found to have caused the distinction between the two waters. Vigorous tidal exchange has caused temporal variations of the above chemical species and SPM concentrations, especially in north SDB. Sediment 210Pb inventory in north SDB was more than 12 times higher than the atmospheric input. A process we defined as "stripping" may be responsible for the extraordinarily high inventory. It is a process in which seawater with higher dissolved 210Pb activity from the open ocean mixes with particle-abundant SDB water through tidal exchange and is scavenged efficiently to the sediment.; To establish a connection between the behavior of particle-reactive radionuclides and tidal hydrodynamics, an existing 2-D hydrodynamic model is modified and integrated with geochemical and particle dynamic sub-models. It is the first study on the hydrodynamic modeling of naturally occurring, particle-reactive radioisotopes, which have been used as tracers as oceanic processes for decades. The geochemical and particle dynamic submodels include sediment resuspension, settling, and transport; particle scavenging of radionuclides, river and atmospheric input of freshwater, radionuclides and SPM, radioactive decay, and in-situ production. The scavenging of radionuclides by particles is simulated as a equilibrium process in which the distributions of radionuclides are determined by distribution coefficient Kd.; Equipped with the boundary conditions provided by the previous geochemical studies, the model was able to reproduce the observed spatial and temporal variations of SPM and radionuclides in our previous geochemical studies. Moreover, model reproduced higher inventories of 210Pb in north SDB, indicating that the model is functioning correctly over a long time-scale.; This study is a pioneering effort to the geochemical-hydrodynamic modeling of a dynamic coastal environment. The novel modeling approach improved our understanding of the behavior of non-conservative, particle-reactive radiotracers and expanded the potential application of these radiotracers in the studies of coastal environments. |
