Using Radium Isotopes to Evaluate Cross-shelf Dispersion in the Coastal Ocean: Application to San Pedro Bay, CA

The short-lived radium isotopes Ra-223 (11 day half life) and Ra-224 (3.6 day half-life) are potentially useful for evaluating cross-shelf dispersion rates in the coastal ocean. A requirement for this application is that their source function and its variability in time and space must be defined. The primary mechanisms for introducing radium into coastal surface waters include: (1) wave and tide-driven circulation of water through permeable beach sands, (2) input from the seafloor due to molecular diffusion and circulation of bottom water through surficial sands, (3) flow of water rich in Ra from marshes and estuaries, and (4) net advection of groundwater. The importance of these inputs to San Pedro Bay was determined from concentrations in waters collected from each of these potential sources. In most of the region, mechanism (2) supplies 90% of the input, although mechanisms (1) and (2) may become dominant locally as the coastal morphology varies in the longshore direction. Longshore variations in the composition of beach sand and the presence of persistent coastal eddies create longshore gradients in Ra concentration that are significant in this region. Temporal variations in shoreline concentrations on time scales 6-8 hours reflect variations in mechanism (1) as tides rise and fall, with drainage of water from the beach face creating higher concentrations during the falling tide. Despite these complications in characterizing the source function, the distribution of short-lived Ra isotopes is useful in constraining the rate of horizontal mixing. A two-dimensional advection-diffusion model was best fit with an eddy diffusivity of 1.3+/-0.2 m2/s over length scales of several km offshore, with a value about 50% smaller in the littoral zone. The scale dependence of eddy diffusivity is also apparent in the distribution of Ra-228, which requires lower eddy diffusivities in the nearshore than in the offshore region. A budget for Ra-226 indicates that little groundwater directly enters the ocean in this region, although some groundwater may enter marshes and estuaries that are adjacent to the coast.