Radium isotopes in San Pedro Bay, California: Constraint on inputs and use of nearshore distribution to compute horizontal eddy diffusion rates

Coastal resource management requires predictive models that track the fate of nearshore solutes. Short-lived Ra-223 (11 days half-life) and Ra-224 (3.5 day half-life) are potentially useful for evaluating cross-shelf dispersion rates required to develop these models. Long-lived Ra-226 and Ra-228 provide further constraint on mixing and hydrologic budgets. 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 the ocean include: (1) wave- and tide-driven circulation of water through permeable beach sands, (2) benthic inputs due to molecular diffusion, bioturbation, circulation of bottom water through surficial sands, and seaward groundwater advection, and (3) flow of Ra-rich water from estuaries. The magnitude of these inputs to San Pedro Bay, CA, was determined from concentrations in waters collected from each of these potential sources. Benthic inputs supplied 90% of the short-lived Ra to surface water, although estuaries may become dominant locally as coastal morphology and the Ra emanation rate varies in the longshore direction. Longshore changes in the magnitude of each source and a persistent onshore flow create significant longshore short-lived Ra concentration gradients in this region. Variations in the shoreline inputs generated by tidal oscillations in the hydraulic gradient between the beach water table and sea level produced temporal variations in shoreline concentration on scales of 6--8 hours. Over periods longer than 10 days, the surface water short-lived Ra inventory fluctuated by as much as 30%. Little variation was observed at shorter periods. No correlation with environmental parameters that may impact source functions was found. The mean summer surface water short-lived Ra distribution was best fit with a two-dimensional advection-diffusion model with an eddy diffusivity of 1.3 +/- 0.2 m2 s -1. Scale dependent mixing may occur beyond 270 m offshore and is apparent in the Ra-228 distribution, 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.