HYDRODYNAMIC MODELING OF PARTICLE SCAVENGING AND TIDAL EXCHANGE OF PARTICLE-REACTIVE RADIONUCLIDES IN SAN DIEGO BAY, CALIFORNIA

San Diego Bay (SDB) is a semi-closed, low inflow estuarine embayment that ventilates with outer sea mainly through vigorous tidal exchange. To better understand the relationship between the behavior of particle-reactive radionuclides 210Pb, 210Po, and 234Th and the hydrodynamics of the tides, we upgraded a well-verified 2-D hydrodynamic model (TRIM) to simulate the following processes for the radionuclides: advection, diffusion, decaying, particle scavenging, river and atmospheric input and in-situ production. Advection and diffusion were simulated by the original semi-implicit, finite difference model that employs a Eulerian-Lagrangian method (ELM) for advection terms. Resuspension due to tidal currents is calculated based on bottom shear stress. Particle-producing processes such as storm runoff and ship movements are modeled as random processes that statistically fit the observation. The coupling of sediment and water column is established by tracking the transport, resuspension and settling of particles in each grid cell via ELM. Particulate matter is divided into 2 typical size groups and treated differently for resuspension, settling, transport and scavenging of radionuclides, which is modeled as a first-order irreversible reaction. Our findings include: 1) Shipping traffic should be the dominant and continuous source of particles to the entire SDB; river input and tidal resuspension are less important; 2) Fine particles are far more important than coarse particles in terms of scavenging and transport of radionuclides; 3) Tidal exchange between two distinct waters of SDB and outer sea has resulted in a 3-5 times larger inventory of particle-reactive radionuclides in north SDB sediments than adjacent regions through a ‘stripping’ process.

The hydrodynamic modeling has provided important insight to our previous studies. Time-series data of TSS and 210Pb, 210Po, 234Th in north SDB showed good correlation with tidal phases, which was verified by the model. At the same time, prolonged running of the model has translated short-term, tidal-induced variations of TSS and radionuclides into long-term, steady-state patterns as discovered by previous geochemical studies. Our approach also provides a novel framework for the studies of chemical tracers in coastal environments.