Shoreline and seafloor fluxes of water and short-lived Ra isotopes to surface water of San Pedro Bay, CA

Quantifying Ra inputs to the coastal ocean is an important constraint for using Ra isotopes as a tracer of nearshore mixing in the water column. A study of the short-lived Ra (223Ra, 224Ra) inputs from the seafloor and shoreline to the surface water of San Pedro Bay is presented. The Ra flux is a function of the seawater flux through sediments. Three different water exchange mechanisms were examined. At the shoreline, tides generated water table fluctuation and pump 3.3 m3 d− 1 (m shoreline)− 1 of water through the beach, with a residence time of about a day based on the ingrowth of Rn and Ra. Waves pump a model-derived estimate 6.4 m3 d− 1 (m shoreline)− 1 of water through the beach face. On the seafloor, waves and currents interacting with ripples generate pressure gradients that drive seawater circulation through sediments. A model fit to a Rn profile indicated that the top 28 cm of pore water exchanges 25% per day with the overlying water. Applying these parameters, the model also produced a reasonable fit to Ra pore water profiles. Extrapolating this rate across the seafloor in contact with the mixed layer, 32 m3 d− 1 (m shoreline)− 1 of water is pumped through the seafloor. Combined, a total of 42 m3 d− 1 (m shoreline)− 1 of surface water is pumped through sediments. Using these water exchange rates and pore water measurements of 223Ra and 224Ra, the Ra flux from the shoreline and seafloor were computed. Only 5% of the short-lived Ra originates at the shoreline, while the seafloor accounts for 95% of the inputs to the surface mixed layer. A Ra budget for San Pedro Bay surface mixed layer was developed. For 224Ra, inputs were balanced by radioactive decay. But for 223Ra, an onshore flow of low-Ra water is required to balance the isotope budget. Using the short-lived Ra isotope data, the maximum residence time of water in San Pedro Bay is 18 ± 10 days.